5: datalink layer5-1 chapter 5: the data link layer our goals: r understand principles behind data...
TRANSCRIPT
5 DataLink Layer 5-1
Chapter 5 The Data Link LayerOur goals understand principles behind data link layer
services error detection correction sharing a broadcast channel multiple access link layer addressing reliable data transfer flow control done
instantiation and implementation of various link layer technologies
5 DataLink Layer 5-2
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link virtualization
ATM MPLS
5 DataLink Layer 5-3
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
5 DataLink Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without
resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card
80211 card implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
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- Slide 100
- Slide 101
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- Slide 103
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- Slide 105
- Slide 106
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- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-2
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link virtualization
ATM MPLS
5 DataLink Layer 5-3
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
5 DataLink Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without
resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card
80211 card implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
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- Slide 64
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- Slide 68
- Slide 69
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- Slide 117
- Slide 118
-
5 DataLink Layer 5-3
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
5 DataLink Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
5 DataLink Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without
resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card
80211 card implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
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- Slide 13
- Slide 14
- Slide 15
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- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
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- Slide 24
- Slide 25
- Slide 26
- Slide 27
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- Slide 118
-
5 DataLink Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on first
link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
5 DataLink Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without
resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card
80211 card implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
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-
5 DataLink Layer 5-5
Link Layer Services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit-error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-6
Link Layer Services (more)
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without
resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card
80211 card implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
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- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
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- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
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- Slide 83
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- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-6
Link Layer Services (more)
flow control pacing between adjacent sending and receiving nodes
error detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
error correction receiver identifies and corrects bit error(s) without
resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card
80211 card implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
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- Slide 44
- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
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- Slide 53
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-
5 DataLink Layer 5-7
Where is the link layer implemented
in each and every host link layer implemented in
ldquoadaptorrdquo (aka network interface card NIC) Ethernet card PCMCI card
80211 card implements link physical
layer
attaches into hostrsquos system buses
combination of hardware software firmware
controller
physicaltransmission
cpu memory
host bus (eg PCI)
network adaptercard
host schematic
applicationtransportnetwork
link
linkphysical
5 DataLink Layer 5-8
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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-
5 DataLink Layer 5-8
Adaptors Communicating
sending side encapsulates datagram
in frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram passes
to upper layer at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
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- Slide 71
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- Slide 73
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- Slide 80
- Slide 81
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- Slide 83
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- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
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- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
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- Slide 99
- Slide 100
- Slide 101
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- Slide 103
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- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-9
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
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- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
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- Slide 44
- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
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- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
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- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
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- Slide 79
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- Slide 110
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- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-10
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
otherwise
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
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- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
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- Slide 88
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- Slide 90
- Slide 91
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- Slide 93
- Slide 94
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- Slide 97
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- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-11
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
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- Slide 83
- Slide 84
- Slide 85
- Slide 86
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- Slide 88
- Slide 89
- Slide 90
- Slide 91
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- Slide 93
- Slide 94
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- Slide 96
- Slide 97
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- Slide 106
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- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-12
Internet checksum (review)
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed
checksum equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted packet (note used at transport layer only)
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
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-
5 DataLink Layer 5-13
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (Ethernet 80211 WiFi ATM)
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
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-
5 DataLink Layer 5-14
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-15
CRC calculation
Calculation of the polynomial codesdata 1101011011G 10011CRC
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
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- Slide 40
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- Slide 42
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- Slide 49
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-
5 DataLink Layer 5-16
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
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-
5 DataLink Layer 5-17
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet)
shared RF (eg 80211 WiFi)
shared RF(satellite)
humans at acocktail party
(shared air acoustical)
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
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- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
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- Slide 24
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- Slide 26
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-
5 DataLink Layer 5-18
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 19
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- Slide 26
- Slide 27
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-
5 DataLink Layer 5-19
Ideal Multiple Access Protocol
Broadcast channel of rate R bps1 when one node wants to transmit it can send
at rate R2 when M nodes want to transmit each can
send at average rate RM3 fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 simple
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
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- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
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- Slide 33
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- Slide 35
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- Slide 37
- Slide 38
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- Slide 40
- Slide 41
- Slide 42
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- Slide 46
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- Slide 49
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- Slide 117
- Slide 118
-
5 DataLink Layer 5-20
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
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-
5 DataLink Layer 5-21
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length =
pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots
256 idle
1 3 4 1 3 4
6-slotframe
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
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-
5 DataLink Layer 5-22
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle fr
equ
ency
bands time
FDM cable
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
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-
5 DataLink Layer 5-23
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
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- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
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- Slide 71
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- Slide 117
- Slide 118
-
5 DataLink Layer 5-24
Slotted ALOHA
Assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
Operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-25
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-26
Slotted Aloha efficiency
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
Max efficiency = 1e = 37
Efficiency long-run fraction of successful slots (many nodes all with many frames to send)
At best channelused for useful transmissions 37of time
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-27
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
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- Slide 12
- Slide 13
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- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
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- Slide 24
- Slide 25
- Slide 26
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- Slide 38
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- Slide 117
- Slide 118
-
5 DataLink Layer 5-28
Pure Aloha efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [p0-1p0]
P(no other node transmits in [p0-1p0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n -gt infty
= 1(2e) = 18
even worse than slotted Aloha
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
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- Slide 118
-
5 DataLink Layer 5-29
Pure ALOHA
Throughput versus offered traffic for ALOHA systems
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-30
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
human analogy donrsquot interrupt others
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-31
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
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-
5 DataLink Layer 5-32
Nonpersistent CSMA
1 If medium is idle transmit otherwise go to 22 If medium is busy wait amount of time drawn
from probability distribution (retransmission delay) and repeat 1
Random delays reduces probability of collisions Consider two stations become ready to transmit at
same time bull While another transmission is in progress
If both stations delay same time before retrying both will attempt to transmit at same time
Capacity is wasted because medium will remain idle following end of transmission Even if one or more stations waiting
Nonpersistent stations deferential
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-33
1-persistent CSMA
To avoid idle channel time 1-persistent protocol used
Station wishing to transmit listens and obeys following
1 If medium idle transmit otherwise go to step 2
2 If medium busy listen until idle then transmit immediately
1-persistent stations selfish If two or more stations waiting collision
guaranteed Gets sorted out after collision
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
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- Slide 77
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- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-34
P-persistent CSMA
Compromise that attempts to reduce collisions Like nonpersistent
And reduce idle time Like1-persistent
Rules1 If medium idle transmit with probability p and
delay one time unit with probability (1 ndash p) Time unit typically maximum propagation delay
2 If medium busy listen until idle and repeat step 13 If transmission is delayed one time unit repeat
step 1 What is an effective value of p
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
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-
5 DataLink Layer 5-35
Value of p
Avoid instability under heavy load n stations waiting to send End of transmission expected number of stations
attempting to transmit is number of stations ready times probability of transmitting n x p
If n x p gt 1 on average there will be a collision Repeated attempts to transmit almost guaranteeing more
collisions Retries compete with new transmissions Eventually all stations trying to send
Continuous collisions zero throughput So nxp lt 1 for expected peaks of n If heavy load expected p small However as p made smaller stations wait longer At low loads this gives very long delays
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
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-
5 DataLink Layer 5-36
Persistent and Nonpersistent CSMA
Comparison of the channel utilization versus load for various random access
protocols
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
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-
5 DataLink Layer 5-37
Which Persistence Algorithm
IEEE 8023 uses 1-persistent Both nonpersistent and p-persistent have
performance problems 1-persistent (p = 1) seems more unstable
than p-persistent Greed of the stations But wasted time due to collisions is short (if
frames long relative to propagation delay With random backoff unlikely to collide on
next tries To ensure backoff maintains stability IEEE
8023 and Ethernet use binary exponential backoff
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
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- Slide 17
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- Slide 19
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- Slide 22
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- Slide 26
- Slide 27
- Slide 28
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-
5 DataLink Layer 5-38
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
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- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
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- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
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- Slide 31
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- Slide 118
-
5 DataLink Layer 5-39
CSMACD collision detection
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
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- Slide 40
- Slide 41
- Slide 42
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- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
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- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
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- Slide 115
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- Slide 117
- Slide 118
-
5 DataLink Layer 5-40
CSMACD (Collision Detection) With CSMA collision occupies medium
for duration of transmission Stations listen whilst transmitting
1 If medium idle transmit otherwise step 2
2 If busy listen for idle then transmit3 If collision detected jam then cease
transmission4 After jam wait random time then start
from step 1
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
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- Slide 15
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- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
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- Slide 45
- Slide 46
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- Slide 49
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- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-41
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
- Slide 11
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- Slide 38
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- Slide 45
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- Slide 49
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- Slide 51
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- Slide 53
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- Slide 118
-
5 DataLink Layer 5-42
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
typically used with ldquodumbrdquo slave devices
concerns polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
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-
5 DataLink Layer 5-43
ldquoTaking Turnsrdquo MAC protocolsToken passing control token
passed from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
T
data
(nothingto send)
T
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
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- Slide 13
- Slide 14
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- Slide 21
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- Slide 33
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- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
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- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
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- Slide 75
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- Slide 81
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- Slide 114
- Slide 115
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- Slide 117
- Slide 118
-
5 DataLink Layer 5-44
Summary of MAC protocols
channel partitioning by time frequency or code Time Division Frequency Division
random access (dynamic) ALOHA S-ALOHA CSMA CSMACD carrier sensing easy in some technologies (wire)
hard in others (wireless) CSMACD used in Ethernet CSMACA used in 80211
taking turns polling from central site token passing Bluetooth FDDI IBM Token Ring
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
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- Slide 19
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- Slide 22
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- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
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-
5 DataLink Layer 5-45
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
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- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
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- Slide 40
- Slide 41
- Slide 42
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- Slide 44
- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
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- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
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- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
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- Slide 118
-
5 DataLink Layer 5-46
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
- Slide 11
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- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
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- Slide 45
- Slide 46
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- Slide 49
- Slide 50
- Slide 51
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- Slide 53
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- Slide 55
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- Slide 118
-
5 DataLink Layer 5-47
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address function get frame from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs)bull burned in NIC ROM also sometimes software
settable
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
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- Slide 15
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- Slide 49
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- Slide 53
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- Slide 55
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-
5 DataLink Layer 5-48
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
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- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
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- Slide 93
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- Slide 95
- Slide 96
- Slide 97
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- Slide 101
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- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-49
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) analogy (a) MAC address like Social Security Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable address depends on IP subnet to which node is
attached
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-50
ARP Address Resolution Protocol
Each IP node (host router) on LAN has ARP table
ARP table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
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- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
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- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
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- Slide 37
- Slide 38
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- Slide 46
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- Slide 53
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- Slide 55
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-
5 DataLink Layer 5-51
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
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- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-52
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B
222222222220111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
walkthrough send datagram from A to B via R assume A knows Brsquos IP address
two ARP tables in router R one for each IP network (LAN)
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
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-
5 DataLink Layer 5-53
A creates IP datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos NIC sends frame Rrsquos NIC receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
R
1A-23-F9-CD-06-9B
222222222220
111111111110
E6-E9-00-17-BB-4B
CC-49-DE-D0-AB-7D
111111111112
111111111111
A74-29-9C-E8-FF-55
222222222221
88-B2-2F-54-1A-0F
B222222222222
49-BD-D2-C7-56-2A
This is a really importantexample ndash make sure youunderstand
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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- Slide 10
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- Slide 53
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- Slide 55
- Slide 56
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-
5 DataLink Layer 5-54
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM and MPLS
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
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- Slide 44
- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
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- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
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- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
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- Slide 111
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- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-55
Ethernet History
Developed by Bob Metcalfe and others at Xerox PARC in mid-1970s Roots in Aloha packet-radio network Standardized by Xerox DEC and Intel in 1978 LAN standards define MAC and physical layer connectivity
bull IEEE 8023 (CSMACD - Ethernet) standard ndash originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet
CSMACD Ethernetrsquos Media Access Control (MAC) policy CS = carrier sense
bull Send only if medium is idle MA = multiple access CD = collision detection
bull Stop sending immediately if collision is detected
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
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-
5 DataLink Layer 5-56
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for NIC first widely used LAN technology simpler cheaper than token LANs and ATM kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
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- Slide 75
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- Slide 77
- Slide 78
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- Slide 97
- Slide 98
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- Slide 101
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- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-57
Star topology bus topology popular through mid 90s
all nodes in same collision domain (can collide with each other)
today star topology prevails active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cable star
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
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-
5 DataLink Layer 5-58
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
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5 DataLink Layer 5-59
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame
Type indicates higher layer protocol (mostly IP but others possible eg Novell IPX AppleTalk)
CRC checked at receiver if error is detected frame is dropped
Destaddr
64 48 32
CRCPreamble Srcaddr Type Body
1648
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-60
Ethernet Frame Structure (more)
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
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-
5 DataLink Layer 5-61
Ethernet Frame Structure (more)
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
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-
5 DataLink Layer 5-62
Ethernet Unreliable connectionless connectionless No handshaking between sending
and receiving NICs unreliable receiving NIC doesnrsquot send acks or
nacks to sending NIC stream of datagrams passed to network layer can have
gaps (missing datagrams) gaps will be filled if app is using TCP otherwise app will see gaps
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
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- Slide 60
- Slide 61
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- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
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- Slide 73
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- Slide 111
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- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-63
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
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-
5 DataLink Layer 5-64
Ethernet CSMACD algorithm
1 NIC receives datagram from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters exponential backoff after mth collision NIC chooses K at random from
012hellip2m-1 NIC waits K512 bit times returns to Step 2
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
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- Slide 42
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- Slide 46
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- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
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- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-65
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random
wait will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
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- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-66
Exponential Backoff
If a collision is detected delay and try again Delay time is selected using binary exponential
backoff 1st time choose K from 01 then delay = K 512us 2nd time choose K from 0123 then delay = K 512us nth time delay = K x 512us for K=02n ndash 1
bull Note max value for k = 1023 give up after several tries (usually 16)
bull Report transmit error to host
If delay were not random then there is a chance that sources would retransmit in lock step
Why not just choose from small set for K This works fine for a small number of hosts Large number of nodes would result in more collisions
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-67
State Diagram for CSMACD
Packet
Sense Carrier
Discard Packet
Send Detect Collision
Jam channel b=CalcBackoff()
wait(b)attempts++
No
Yes
attempts lt 16
attempts == 16
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
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- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
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- Slide 100
- Slide 101
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- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-68
Collisions
A B
A B
Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences)
bull Both found line to be idlebull Both had been waiting to for a busy line to become idle
A starts attime 0
Message almost there at time T whenB starts ndash collision
How can we be sure A knows about the collision
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-69
Collision Detection How can A know that a collision has taken place
There must be a mechanism to insure retransmission on collision
Arsquos message reaches B at time T Brsquos message reaches A at time 2T So A must still be transmitting at 2T
IEEE 8023 specifies max value of 2T to be 512us This relates to maximum distance of 2500m between hosts At 10Mbps it takes 01us to transmit one bit so 512 bits
(64B) take 512us to send So Ethernet frames must be at least 64B long
bull 14B header 46B data 4B CRCbull Padding is used if data is less than 46B
Send jamming signal after collision is detected to insure all hosts see collision 48 bit signal
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-70
Collision Detection contd
A B
A B
A B
time = 0
time = T
time = 2T
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-71
MAC Algorithm from the Receiver Side Senders handle all access control Receivers simply read frames with
acceptable address Address to host Address to broadcast Address to multicast to which host belongs All frames if host is in promiscuous mode
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
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- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
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- Slide 75
- Slide 76
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- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
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- Slide 85
- Slide 86
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- Slide 88
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- Slide 90
- Slide 91
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- Slide 93
- Slide 94
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- Slide 97
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- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-72
CSMACD efficiency
Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA and simple cheap decentralized
transprop ttefficiency
51
1
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-73
Ethernet Performance
Efficiency of Ethernet at 10 Mbps with 512-bit slot times
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
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- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-74
8023 Ethernet Standards Link amp Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds 2 Mbps 10 Mbps 100
Mbps 1Gbps 10G bps different physical layer media fiber cable
applicationtransportnetwork
linkphysical
MAC protocoland frame format
100BASE-TX
100BASE-T4
100BASE-FX100BASE-T2
100BASE-SX 100BASE-BX
fiber physical layercopper (twisterpair) physical layer
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-75
Ethernet Technologies 10Base2 10 10Mbps 2 under 185 (~200) meters cable length Thin coaxial cable in a bus topology
Repeaters used to connect multiple segments Repeater repeats bits it hears on one interface to its other interfaces physical layer device only
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
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- Slide 117
- Slide 118
-
5 DataLink Layer 5-76
Ethernet Cabling
The most common kinds of Ethernet cabling
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
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- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
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- Slide 40
- Slide 41
- Slide 42
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- Slide 44
- Slide 45
- Slide 46
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- Slide 49
- Slide 50
- Slide 51
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- Slide 53
- Slide 54
- Slide 55
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- Slide 57
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- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-77
Ethernet Cabling (2)
Three kinds of Ethernet cabling (a) 10Base5 (b) 10Base2 (c) 10Base-T
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-78
Ethernet Cabling (3)
Cable topologies (a) Linear (b) Spine (c) Tree (d) Segmented
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
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- Slide 97
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- Slide 100
- Slide 101
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- Slide 103
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- Slide 106
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- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-79
Ethernet Cabling (4)
(a) Binary encoding (b) Manchester encoding (c) Differential Manchester encoding
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-80
Manchester encoding
used in 10BaseT each bit has a transition allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
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- Slide 77
- Slide 78
- Slide 79
- Slide 80
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- Slide 83
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- Slide 96
- Slide 97
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- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-81
Physical Layer Configurations for 8023 Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
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- Slide 61
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- Slide 65
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- Slide 93
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- Slide 97
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- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-82
Ethernet Overview Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters
Bus and Star topologies are used to connect hosts Hosts attach to network via Ethernet transceiver or hub or
switchbull Detects line state and sendsreceives signals
Hubs are used to facilitate shared connections All hosts on an Ethernet are competing for access to the
mediumbull Switches break this model
Problem Distributed algorithm that provides fair access
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-83
Switched Ethernet
A simple example of switched Ethernet
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-84
Switched Ethernet
Switches forward and filter frames based on LAN addresses Itrsquos not a bus or a router (although simple forwarding tables are
maintained) Very scalable
Options for many interfaces Full duplex operation (sendreceive frames simultaneously)
Connect two or more ldquosegmentsrdquo by copying data frames between them Switches only copy data when needed
bull key difference from repeaters Higher link bandwidth
Collisions are completely avoided Much greater aggregate bandwidth
Separate segments can send at once
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-85
Fast Ethernet
The original fast Ethernet cabling
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
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- Slide 14
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- Slide 16
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- Slide 25
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- Slide 33
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- Slide 35
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- Slide 37
- Slide 38
- Slide 39
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- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
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- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
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- Slide 61
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- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
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- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
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- Slide 83
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- Slide 85
- Slide 86
- Slide 87
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- Slide 93
- Slide 94
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- Slide 96
- Slide 97
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- Slide 101
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- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-86
Gigabit Ethernet
(a) A two-station Ethernet (b) A multistation Ethernet
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-87
Gigabit Ethernet (2)
Gigabit Ethernet cabling
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-88
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed
Gigabit Ethernet (1000Mbps) Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-89
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions
Most networks are limited to about 200 hosts Specification allows for up to 1024
Most networks are much shorter 5 to 10 microsecond RTT
Transport level flow control helps reduce load (number of back to back packets)
Ethernet is inexpensive fast and easy to administer
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-90
Ethernet Problems
Ethernetrsquos peak utilization is pretty low (like Aloha)
Peak throughput worst with More hosts
bull More collisions needed to identify single sender Smaller packet sizes
bull More frequent arbitration Longer links
bull Collisions take longer to observe more wasted bandwidth Efficiency is improved by avoiding these conditions
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-91
Physical and Data Link Features of Ethernet
Standards and Implementation
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
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- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
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- Slide 56
- Slide 57
- Slide 58
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- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
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- Slide 73
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- Slide 76
- Slide 77
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- Slide 79
- Slide 80
- Slide 81
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- Slide 83
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- Slide 85
- Slide 86
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- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-92
IEEE 8022 Logical Link Control
(a) Position of LLC (b) Protocol formats
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
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- Slide 60
- Slide 61
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- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
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- Slide 101
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- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-93
Physical and Data Link Features of Ethernet Logic Link Control ndash Connecting the Upper
Layers
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-94
Physical and Data Link Features of Ethernet Media Access Control (MAC)
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
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- Slide 118
-
5 DataLink Layer 5-95
Physical and Data Link Features of Ethernet Physical Implementations of the Ethernet
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
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- Slide 48
- Slide 49
- Slide 50
- Slide 51
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- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
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- Slide 111
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- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-96
Link Layer
51 Introduction and services
52 Error detection and correction
53 Multiple access protocols
54 Link-layer Addressing
55 Ethernet
56 Link-layer switches 57 PPP 58 Link Virtualization
ATM MPLS
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
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- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-97
Inter - Networking
Hubs Bridges Switches Routers
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-98
Hubshellip physical-layer (ldquodumbrdquo) repeaters
bits coming in one link go out all other links at same rate
all nodes connected to hub can collide with one another no frame buffering no CSMACD at hub host NICs detect collisions
twisted pair
hub
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
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- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
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- Slide 61
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- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
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- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
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- Slide 85
- Slide 86
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- Slide 88
- Slide 89
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- Slide 105
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- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-99
Switch link-layer device smarter than hubs take active
role store forward Ethernet frames examine incoming framersquos MAC address selectively
forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of switches
plug-and-play self-learning switches do not need to be configured
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-100
Switch allows multiple simultaneous transmissions
hosts have dedicated direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own collision
domain switching A-to-Arsquo and B-
to-Brsquo simultaneously without collisions not possible with dumb hub
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-101
Switch Table
Q how does switch know that Arsquo reachable via interface 4 Brsquo reachable via interface 5
A each switch has a switch table each entry (MAC address of host interface
to reach host time stamp)
looks like a routing table Q how are entries created
maintained in switch table something like a routing
protocol
A
Arsquo
B
Brsquo
C
Crsquo
switch with six interfaces(123456)
1 23
45
6
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-102
Switch self-learning
switch learns which hosts can be reached through which interfaces when frame received
switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-103
Switch frame filteringforwardingWhen frame received
1 record link associated with sending host2 index switch table using MAC dest address3 if entry found for destination
then if dest on segment from which frame arrived
then drop the frame else forward the frame on interface indicated else flood
forward on all but the interface on which the frame arrived
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-104
Self-learning forwarding example
A
Arsquo
B
Brsquo
C
Crsquo
1 23
45
6
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selective send
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-105
Interconnecting switches
switches can be connected together
A
B
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3
A self learning (works exactly the same as in single-switch case)
S1
C D
E
FS2
S4
S3
H
I
G
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
- Slide 9
- Slide 10
- Slide 11
- Slide 12
- Slide 13
- Slide 14
- Slide 15
- Slide 16
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Slide 24
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Slide 35
- Slide 36
- Slide 37
- Slide 38
- Slide 39
- Slide 40
- Slide 41
- Slide 42
- Slide 43
- Slide 44
- Slide 45
- Slide 46
- Slide 47
- Slide 48
- Slide 49
- Slide 50
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- Slide 55
- Slide 56
- Slide 57
- Slide 58
- Slide 59
- Slide 60
- Slide 61
- Slide 62
- Slide 63
- Slide 64
- Slide 65
- Slide 66
- Slide 67
- Slide 68
- Slide 69
- Slide 70
- Slide 71
- Slide 72
- Slide 73
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Slide 80
- Slide 81
- Slide 82
- Slide 83
- Slide 84
- Slide 85
- Slide 86
- Slide 87
- Slide 88
- Slide 89
- Slide 90
- Slide 91
- Slide 92
- Slide 93
- Slide 94
- Slide 95
- Slide 96
- Slide 97
- Slide 98
- Slide 99
- Slide 100
- Slide 101
- Slide 102
- Slide 103
- Slide 104
- Slide 105
- Slide 106
- Slide 107
- Slide 108
- Slide 109
- Slide 110
- Slide 111
- Slide 112
- Slide 113
- Slide 114
- Slide 115
- Slide 116
- Slide 117
- Slide 118
-
5 DataLink Layer 5-106
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
1
2
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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-
5 DataLink Layer 5-107
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domain collision domain
collision domain
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
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- Slide 118
-
5 DataLink Layer 5-108
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
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- Slide 118
-
5 DataLink Layer 5-109
More on Switches
cut-through switching frame forwarded from input to output port without first collecting entire frameslight reduction in latency
combinations of shareddedicated 101001000 Mbps interfaces
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
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-
5 DataLink Layer 5-110
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
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- Slide 118
-
5 DataLink Layer 5-111
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
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-
5 DataLink Layer 5-112
Repeaters Hubs Bridges Switches Routers and Gateways
(a) Which device is in which layer(b) Frames packets and headers
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
- Slide 2
- Slide 3
- Slide 4
- Slide 5
- Slide 6
- Slide 7
- Slide 8
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-
5 DataLink Layer 5-113
Repeaters Hubs Bridges Switches Routers and Gateways (2)
(a) A hub (b) A bridge (c) a switch
5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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5 DataLink Layer 5-114
Comparison
hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
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5 DataLink Layer 5-115
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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5 DataLink Layer 5-116
Chapter 5 Summary principles behind data link layer services
error detection correction sharing a broadcast channel multiple access link layer addressing
instantiation and implementation of various link layer technologies Ethernet switched LANS
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
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-
5 DataLink Layer 5-117
Chapter 5 letrsquos take a breath journey down protocol stack complete
(except PHY) solid understanding of networking
principles practice hellip could stop here hellip but lots of
interesting topics wireless multimedia security network management
Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
- Slide 1
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Review QuestionsSee the textbook (P527-)
R2 R7 R8 R9 R12 P5 P14 P17 P20
5 DataLink Layer 5-118
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