20: other technologies used at the link layer
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20: Other Technologies used at the Link Layer. Last Modified: 9/20/2014 2:41:53 AM. Token Passing: IEEE802.5 standard. 4 Mbps max token holding time: 10 ms, limiting frame length. SD, ED mark start, end of packet AC: access control byte: - PowerPoint PPT PresentationTRANSCRIPT
5: DataLink Layer 5a-1
20: Other Technologies used at the Link Layer
Last Modified: 04/21/23 06:51 AM
5: DataLink Layer 5a-2
Token Passing: IEEE802.5 standard 4 Mbps max token holding time: 10 ms, limiting frame
length
SD, ED mark start, end of packet AC: access control byte:
token bit: value 0 means token can be seized, value 1 means data follows FC
priority bits: priority of packet reservation bits: station can write these bits to prevent
stations with lower priority packet from seizing token after token becomes free
5: DataLink Layer 5a-3
Token Passing: IEEE802.5 standard
FC: frame control used for monitoring and maintenance
source, destination address: 48 bit physical address, as in Ethernet
data: packet from network layer; checksum: CRC
FS: frame status: set by dest., read by sender set to indicate destination up, frame copied OK from
ring
limited number of stations: 802.5 have token passing delays at each station
5: DataLink Layer 5a-4
Point to Point Data Link Control one sender, one receiver, one link:
easier than broadcast link:no need for explicit MAC addressingfull-duplex simultaneous bi-directional
operation = no need for media access control
e.g., dialup link, ISDN line popular point-to-point protocols:
PPP (point-to-point protocol)HDLC: High level data link control
5: DataLink Layer 5a-5
PPP Design/Features
packet framing: encapsulation of network-layer datagram in data link frame carry network layer data of any network
layer protocol (not just IP) at same time ability to demultiplex upwards
bit transparency: must carry any bit pattern in the data field
error detection (no correction) connection liveness: detect, signal link failure
to network layer network layer address negotiation: endpoint
can learn/configure each other’s network address
5: DataLink Layer 5a-6
PPP non-requirements
no error correction/recovery no flow control no need to support multipoint links
(e.g., polling)
Error recovery, flow control, data re-ordering all relegated to higher layers!|
5: DataLink Layer 5a-7
PPP Data Frame
Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future
possible multiple control fields Protocol: upper layer protocol to which
frame delivered (eg. IP, PPP-LCP, IPCP, etc)
5: DataLink Layer 5a-8
PPP Data Frame
info: upper layer data being carried check: cyclic redundancy check for
error detection
5: DataLink Layer 5a-9
Byte Stuffing “data transparency” requirement: data field
must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag?
Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte
Receiver: two 01111110 bytes in a row: discard first
byte, continue data reception single 01111110: flag byte
5: DataLink Layer 5a-10
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5: DataLink Layer 5a-11
PPP Data Control ProtocolBefore exchanging
network-layer data, data link peers must
configure PPP link (max. frame length, authentication)
learn/configure network layer information
for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address
5: DataLink Layer 5a-12
IP over Other Wide Area Network Technologies ATM Frame Relay X-25
5: DataLink Layer 5a-13
ATM architecture
Adaptation layer (AAL): only at edge of ATM network roughly analogous to Internet transport layer
ATM layer: “network” layer Virutal circuits, routing, cell switching
physical layer
5: DataLink Layer 5a-14
ATM Layer: ATM cell
5-byte ATM cell header 48-byte payload
Why?: small payload -> short cell-creation delay for digitized voice
halfway between 32 and 64 (compromise!)
Cell header
Cell format
5: DataLink Layer 5a-15
ATM cell header
VCI: virtual channel IDwill change from link to link thru net
PT: Payload type (e.g. RM cell versus data cell)
CLP: Cell Loss Priority bitCLP = 1 implies low priority cell, can
be discarded if congestion HEC: Header Error Checksum
cyclic redundancy check
5: DataLink Layer 5a-16
ATM: network or link layer?Vision: end-to-end
transport: “ATM from desktop to desktop” ATM is a network
technology
Reality: used to connect IP backbone routers “IP over ATM” ATM as switched
link layer, connecting IP routers
5: DataLink Layer 5a-17
Datagram Journey in IP-over-ATM Network
at Source Host: IP layer finds mapping between IP, ATM dest address passes datagram to AAL5 AAL5 encapsulates data, segments to cells, passes to
ATM layer ATM network: moves cell along VC to destination (uses
existing one or establishes another) at Destination Host:
AAL5 reassembles cells into original datagram if CRC OK, datgram is passed to IP
5: DataLink Layer 5a-18
X.25 and Frame Relay
Like ATM: wide area network technologies virtual circuit oriented origins in telephony world can be used to carry IP datagrams and
can thus be viewed as Link Layers by IP protocol just like ATM
5: DataLink Layer 5a-19
X.25
X.25 builds VC between source and destination for each user connection
Per-hop control along patherror control (with retransmissions)
on each hopper-hop flow control using credits
•congestion arising at intermediate node propagates to previous node on path
•back to source via back pressure
5: DataLink Layer 5a-20
IP versus X.25
X.25: reliable in-sequence end-end delivery from end-to-end“intelligence in the network”
IP: unreliable, out-of-sequence end-end delivery“intelligence in the endpoints”
2000: IP winsgigabit routers: limited processing
possible
5: DataLink Layer 5a-21
Frame Relay
Designed in late ‘80s, widely deployed in the ‘90s
Frame relay service:no error controlend-to-end congestion control
5: DataLink Layer 5a-22
Frame Relay (more) Designed to interconnect corporate customer
LANs typically permanent VC’s: “pipe” carrying
aggregate traffic between two routers switched VC’s: as in ATM
corporate customer leases FR service from public Frame Relay network (eg, Sprint, ATT)
5: DataLink Layer 5a-23
Frame Relay (more)
Flag bits, 01111110, delimit frame Address = address and congestion control
10 bit VC ID field 3 congestion control bits
• FECN: forward explicit congestion notification (frame experienced congestion on path)
• BECN: congestion on reverse path• DE: discard eligibility
addressflags data CRC flags
5: DataLink Layer 5a-24
Frame Relay -VC Rate Control Committed Information Rate (CIR)
defined, “guaranteed” for each VC negotiated at VC set up time customer pays based on CIR
DE bit: Discard Eligibility bit Edge FR switch measures traffic rate for
each VC; marks DE bit DE = 0: high priority, rate compliant frame;
deliver at “all costs” DE = 1: low priority, eligible for discard
when congestion
5: DataLink Layer 5a-25
Summary
principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing, ARP
various link layer technologies Ethernet hubs, bridges, switches IEEE 802.11 LANs PPP ATM, X.25, Frame Relay
journey down the protocol stack now OVER!
A bit about physical connections
5: DataLink Layer 5a-26
Rating wide area internet connections T0, DS0 – 1 voice channel, 65 Kbps
What homes get for 1 telephone line T1 (Level 1 transmission line) or DS1
1.544 Mbps, 24 voice channels at 64 Kbps T3 or DS3 = 28 T1 lines, 44.746 Mbps OC3 = 3 DS3s OC12 = 12 DS3s OC48 = 48 DS3s, 2488 Mbps OC192 = 192 DS3s
5: DataLink Layer 5a-27
SONET and SDH Higher data rates often achieved using
synchronous optical networking (SONET) and Synchronous Digital Hierarchy (SDH) SONET in the US and Canada and SDH in the
rest of the world Transport over optical fiber using lasers/
LEDs Transporting large amounts of telephone
calls and data traffic over the same fiber without synchronization problems
5: DataLink Layer 5a-28
T0 = typical phoneline connection DS3 delivered native on a copper trunk or
converted to an optical fiber run when needing longer distances between termination points DS3 transported over SONET is encapsulated
in a STS-1 SONET channel Still analog when delivered over fiber
When delivering data over an OC3 or greater SONET is used. OC-3 SONET link contains three STS-1s, and
therefore may carry three DS3s. Likewise, OC-12, OC-48, and OC-192 may
carry 12, 48, and 192 DS3s respectively.5: DataLink Layer 5a-29
More on SONET
Designed to carry multiple real-time, uncompressed, circuit-switched voice lines encoded in Pulse-Code Modulation (PCM) format
Also multiple digital bit streams of differing origin within single framing protocol Multiplex circuit mode communications (T1, T3,
DS1, DS3,etc.) from a variety of different sources over same fiber
Emphasis is on merging many different flow into one quickly
5: DataLink Layer 5a-30
STM-1 (Synchronous Transport Module, level 1) frame is the basic transmission format for SDH.
STM-1 frame is transmitted in exactly 125 µs, therefore, there are 8,000 frames per second on a 155.52 Mbit/s OC-3 fiber-optic circuit
5: DataLink Layer 5a-31
Protocol neutral Not communications protocols in and of
themselves Generic, all-purpose transport containers for
moving both voice and data. Used to carry ATM, Ethernet, TCP/IP etc.
5: DataLink Layer 5a-32
SONET standard defined by Telcordia and American National Standards Institute (ANSI) standard T1.105 and T1.119
SDH standard specified in International Telecommunication Union (ITU) standards G.707, G.783, G.784, and G.803 SDH originally defined by the European
Telecommunications Standards Institute (ETSI)
5: DataLink Layer 5a-33
Carrier Pricing
Two simple components: local loop and port
Local loop = cost to transport the signal from the end user's central office (CO) to the point of presence (POP) of the carrier Local loop cost based on geography/distance
from CO to POP Port = cost to access the network through
the carrier's network Port cost based on access speed and yearly
commitment level
5: DataLink Layer 5a-34
Fiber cable runs
One example from the North Country
5: DataLink Layer 5a-35
Undersea cables
5: DataLink Layer 5a-36
multiple SONET signals can be transported over multiple wavelengths on a single fiber pair by means of wave length-division multiplexing, including dense wavelength-division multiplexing (DWDM) and coarse wavelength-division multiplexing (CWDM).
DWDM circuits are the basis for all modern submarine communications cable systems and other long-haul circuits.
5: DataLink Layer 5a-37
Other
Satellite Links Pros and Cons
5: DataLink Layer 5a-38
5: DataLink Layer 5a-39
Outtakes
5: DataLink Layer 5a-40
IEEE 802.11 MAC Protocol
802.11 CSMA Protocol: others
NAV: Network Allocation Vector
802.11 frame has transmission time field
others (hearing data) defer access for NAV time units
5: DataLink Layer 5a-41
IEEE 802.11 MAC Protocol: CSMA/CA802.11 CSMA: sender- if sense channel idle for
DISF sec. then transmit entire frame
(no collision detection)-if sense channel busy
then binary backoff
802.11 CSMA receiver:if received OK return ACK after SIFS
5: DataLink Layer 5a-42
IP-Over-ATMClassic IP only 3 “networks” (e.g., LAN segments) MAC (802.3) and IP addresses
IP over ATM replace “network” (e.g.,
LAN segment) with ATM network
IP addresses -> ATM addresses just like IP addresses to 802.3 MAC addresses!
ATMnetwork
EthernetLANs
EthernetLANs
5: DataLink Layer 5a-43
ARP in ATM Nets
ATM network needs destination ATM address just like Ethernet needs destination Ethernet
address IP/ATM address translation done by ATM ARP
(Address Resolution Protocol) ARP server in ATM network performs
broadcast of ATM ARP translation request to all connected ATM devices
hosts can register their ATM addresses with server to avoid lookup
5: DataLink Layer 5a-44
Access Control
802.11 working group considered 2 proposals for a MAC algorithm Distributed access protocols Centralized access protocols
5: DataLink Layer 5a-45
Distributed Access Protocols
Distribute the decision to transmit over all the notes
Like Carrier-sense mechanisms in Ethernet
Makes sense especially for an ad hoc network of peer workstations
Can also be good for busty traffic
5: DataLink Layer 5a-46
Centralized Access Protocols
Regulation of transmission by a centralized decision maker
Natural for networks with a base station Especially good if network is highly
utilized ( avoid fighting it out among peers)
Also good if some data is time sensitive/high priority
5: DataLink Layer 5a-47
Distributed Foundation Wireless MAC Compromise was Distributed
Foundation Wireless MAC (DFWMAC) Distributed Access control mechanism
with an optional centralized control layer on top of that Distributed Coordination Function (DCF) on
top of physical layer On top of that is optional Point Coordination
Function (PCF) that provides contention free service
Access Control
5: DataLink Layer 5a-49
CSMA
DCF uses Carrier Sense Multiple Access (CSMA) CSMA means listen before you send to make sure
the medium is idle No Collision Detection - Not CSMA/CD like Ethernet
CD based on listening while you send to make sure you hear only your signal
Wireless HW not made to send and listen at same time Large dynamic range of possible signals – cannot
effectively distinguish incoming weak signals from noise and the effects of its own transmission
Medium Access Control Logic
IFS = interframe space
Each time failincrease time towait before send
5: DataLink Layer 5a-51
Interframe Space (IFS) Values
Actually three different IFS values Short IFS (SIFS)
Shortest IFS Used for immediate response actions
Point coordination function IFS (PIFS) Midlength IFS Used by centralized controller in PCF scheme when using
polls Distributed coordination function IFS (DIFS)
Longest IFS Used as minimum delay of asynchronous frames
contending for access
5: DataLink Layer 5a-52
Priority
Stations using SIFS have “priority” over others because they will test for idle faster find and then start transmitting
Others that wait longer will find the channel busy when they listen after PIFS or DIFSs
5: DataLink Layer 5a-53
IFS Usage
SIFS Acknowledgment (ACK) Clear to send (CTS) Poll response( for PCF)
PIFS Used by centralized controller in issuing
polls (for PCF) Takes precedence over normal contention
traffic DIFS
Used for all ordinary asynchronous traffic
5: DataLink Layer 5a-54
Contention Periods/ Contention-Free Periods The DCF and PCF respectively operate in
Contention Periods (CPs) and Contention Free Periods (CFPs)
In CPs, stations compete with each other to win channel access
In CFPs, an Access Point (AP) grants the opportunity of transmission to stations by polling
5: DataLink Layer 5a-55
Polling
Since PIFS smaller than DIFS, coordinator can seize coordinator and lock all traffic ( at least traffic that obeys the rules) while it polls and receives responses
When polling coordinator sends a poll to a station, it can respond using SIFS ( beating the next PIFS and any DIFS)
5: DataLink Layer 5a-56
Polling
In a CFP, a PC polls the first station in its polling list, and it may also piggyback some data to the polling frame.
The polled station responds either with an ACK or a data frame piggybacked to the ACK frame.
An SIFS separates the polling and responding frames.
Once the frame exchange sequence with the first station is done, the PC waits for a PIFS and then polls another station in its polling list.
5: DataLink Layer 5a-57
Superframes
CPs and CFPs alternate in a superframe A superframe is an interval between two beacon
frame transmissions. A beacon frame is broadcasted by APs in BSSs or
random stations in IBSSs. It carries management information to the stations.
5: DataLink Layer 5a-58
IEEE 802.11 MAC TimingPCF Superframe Construction
5: DataLink Layer 5a-59
Superframe
Point coordinator would lock out asynchronous traffic by issuing polls
Superframe interval defined During first part of superframe interval, point coordinator polls round-
robin to all stations configured for polling Point coordinator then idles for remainder of superframe Allowing contention period for asynchronous access
At beginning of superframe, point coordinator may seize control and issue polls for given period Time varies because of variable frame size issued by responding
stations Rest of superframe available for contention-based access At end of superframe interval, point coordinator contends for
access using PIFS If idle, point coordinator gains immediate access
Full superframe period follows If busy, point coordinator must wait for idle to gain access Results in foreshortened superframe period for next cycle
5: DataLink Layer 5a-60
Acknowledgements
When station received frame addressed directly to it ( not broadcast or multicast) it replies with an ACK after waiting SIFS
ACKs allow for recovery from collision since no collision detection
Use of SIFS allows for efficient delivery of an LLC data unit that requires multiple MAC frames Just get SIFS between ACK and then next frame No one else will gain control of the channel until the
entire LLC if over
5: DataLink Layer 5a-61
802.11 Physical Layer Standards
Op. Freq. Data RateTypical/Max(Mbit/sec)
RangeIndoor/Outdoor(meters)
Legacy802.11-1997
2.4 GHz ½ ?
802.11a (1999)
5 GHz 25/54 15-30
802.11b (1999)
2.4 GHz 5.5/11 45-90
802.11g(2003)
2.4 GHz 25/54 45-90
802.11n(2009)
5 and 2.4 GHz 144/600 91/182
5: DataLink Layer 5a-62
802.11b was the first, followed by 802.11a ( higher BW, less popular)
802.11g higher BW, directly compatible with b
802.11n – even higher BW, backwards compatible with b and g
5: DataLink Layer 5a-63
RC4 WEP uses RC4 a stream cipher
Stream ciphers are vulnerable to attack if the same key is used twice (depth of two) or more.
Say we send messages A and B of the same length, both encrypted using same key, K. The stream cipher produces a string of bits C(K) the same length as the messages. The encrypted versions of the messages then are: E(A) = A xor C E(B) = B xor C where xor is performed bit by bit.
Say an adversary has intercepted E(A) and E(B). He can easily compute:
E(A) xor E(B) However xor is commutative and has the property that X
xor X = 0 (self-inverse) so: E(A) xor E(B) = (A xor C) xor (B xor C) = A xor B xor C xor
C = A xor B