5-1 link & physical layers (2-89-90) link & physical layers computer networks

5-5-11Link & Physical LayersLink & Physical Layers (2-89-90)(2-89-90)

Link & Physical Layers

Computer NetworksComputer Networks


Chapter 5 outlineChapter 5 outline

5.1 Introduction and services5.2 Error detection and correction 5.3 Links and Access Protocols5.4 Ethernet5.5 Ethernet Model5.6 Ethernet Frame Structure5.7 LAN addresses and ARP5.8 Ethernet Technologies5.9 Hubs, bridges, and switches5.10 Point to Point Protocol


Some terminology: hosts and routers are nodes (bridges and switches too) communication channels that

connect adjacent nodes along communication path are links wired links wireless links LANs

PDU: frame, encapsulates datagram

link layer has responsibility of transferring datagram from one node to adjacent node over a link.

Link Layer: IntroductionLink Layer: Introduction

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

networkdata linkphysical

networkdata linkphysical


LayersLayers

Application SoftwareApplication Software

Application protocols (softwares) Transport Protocols (softwares) Network(Internetwork)

Protocols(softwares)

Link & Physical Protocols (Software + Hardware)

TCP/IPProtocol

Stack Logical Link ControlProtocols (software)

Medium Access ControlProtocols (Hardware)

Physical Protocols(Hardware)

Ethernet, Token Ring, Token Bus, FDDI, ...,


Datagram transferred by different link protocols over different links: e.g., Ethernet on first link,

frame relay on intermediate links, 802.11 on last link

Each link protocol provides different services e.g., may or may not

provide reliable data transfer (rdt) over link

tourism analogy: trip from Tehran to Toos

taxi: Tehran to Mehrabad Airport

plane: Mehrabad to Mashhad bus: Mashhad to Toos

tourist = datagram taxi, plane, bus =

communication link transportation mode = link

layer protocol tour agent = routing

algorithm

Link layer: contextLink layer: context


IP Add: CILAN Add: CL

ppp, 33.3kbps

IP Add: R1ILAN Add;R1L

IP Add: R1I’LAN Add: R1L’

client

server

Src IP Add=CIDst IP Add=SI

Src LAN Add=CLDst LAN Add=R1L

client to router1

Frame:router1

router2

router3

router4

Ethernet, 10Mbps

IP Add: R2ILAN Add: R2L

IP Add: SILAN Add: SL


Src LAN Add=R1L’Dst LAN Add=R2L

router1 to router2

Frame:


Src LAN Add=R4L’Dst LAN Add=SL

router4 to server

Frame:

IP Add: R4I’LAN Add: R4L’

Fast ethernet

LAN Add: 48 bit(6 × 8bit)(example: 74:29:9C:E8:FF:55)

Physical Frame TransferPhysical Frame Transfer


Framing, link access: encapsulate datagram into frame, adding header,

trailer channel access if shared medium ‘physical addresses’ used in frame headers to

identify source, destination. 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

Q: why both link-level and end-end reliability?

Link layer header Link layer trailerNetwork layer datagram

Layer 2 PDU: FrameLayer 2 PDU: Frame

Link Layer ServicesLink Layer Services


Link Layer Services (more)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:

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


in each and every host link layer implemented

in “adaptor” (aka network interface card NIC) Ethernet card, PCMCI

card, 802.11 card implements link, physical

layer

attaches into host’s system buses

combination of hardware, software, firmware

controller

physicaltransmission

cpu memory

host bus (e.g., PCI)

network adapter

card

host

applicationtransportnetworkLink(LLC)

Link(MAC)physical

Where is the link layer Where is the link layer implemented?implemented?


Adaptors CommunicatingAdaptors 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

frame

datagram

datagram


Error DetectionError Detection

EDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

• Error detection not 100% reliable!• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction


Internet checksumInternet checksum

Sender: treat segment contents

as sequence of 16-bit integers

checksum: addition (1’s complement sum) of segment contents

sender puts checksum value into UDP or TCP 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? More later ….

Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)


Checksumming: Cyclic Redundancy CheckChecksumming: 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

<D,R> exactly divisible by G (modulo 2= add without carry)

receiver knows G, divides <D,R> by G. If non-zero remainder: error detected!

can detect all burst errors less than r+1 bits widely used in practice (ATM, HDCL)

D: data bits to be sent R: CRC bits

d [bits] r [bits]

bit patternbit pattern

mathematical formula: [D×2r XOR R]


CRC ExampleCRC Example

want:D×2r XOR R = n×G

equivalently:D×2r = n×G XOR R

equivalently: if we divide D×2r by

G, want remainder R

R = remainder[ ]

D×2r

G

D=101110, G=1001r=3, D×2r=101110000

D×2r

G =101110000 10011001 101011 101 000 1010 1001 110 000 1100 1001 1010 1001 011

R


Ethernet CRCEthernet CRC

Polynomial Presentation Based on use of polynomial codes Message frame and Generator thought of as binary

polynomials Example: 101101101 ~ x8 + x6 + x5 + x3 + x2 + x0

Ethernet CRC It is a CRC-32: It is 33 bit code that is uses as Generator:

G(x) = x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 + x2 + x + 1


CRC PropertiesCRC Properties

Detect all single-bit errors if coefficients of xr and x0 of G(x) are one

Detect all double-bit errors, if G(x) has a factor with at least three terms

Detect all number of odd errors, if G(x) contains factor (x+1)

Detect all burst of errors smaller than r bits


ExampleExample

p is the drop probability of a packet in a router. What is the average hops for a packet to move from

server to client (N )? What is the average number of transmission of a packet

by server (M )? What is the average number of hopes for a packet

received by client (I )?

MNI

])p()p(

M

)p(p)p(pN

22

2

1 packet recievda ofy probabilit [ , 1

1

13121


Switching hub(LAN switch/L2S)

PPP for dial-up access point-to-point link between LAN switch and hosts.

point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host

Types of “links”- Point to PointTypes of “links”- Point to Point

Repeater hub

Client

Printer

Server

Client

Client

Remote Access ServerModem pools

TelephoneLines

Router

External Link

Serversmodem

modem

Client


broadcast (shared wire or medium) traditional Ethernet (coaxial bus, hub) 802.11 wireless LAN upstream Hybrid Fiber Coaxial

Efficiency: Low Latency & High Throughput [in average]

Types of “links”-BroadcastTypes of “links”-Broadcast

terminatorterminator


Cables, Connectors, TerminatorsCables, Connectors, Terminators


Ideal Broadcast Channel Access ProtocolIdeal Broadcast Channel Access Protocol

Broadcast channel of rate R bps When one node wants to transmit, it can send at rate

R.

When M nodes want to transmit, each can send at average rate R/M

Fully decentralized: no special node to coordinate transmissions

no synchronization of clocks, slots

Simple


Channel Partitioning divide channel into smaller “pieces” (time slots,

frequency, code) allocate piece to node for exclusive use

Random Access channel not divided, allow collisions “recover” from collisions

“Taking turns” (Token/Polling) tightly coordinate shared access to avoid collisions

Broadcast ClassesBroadcast Classes


Random Access ProtocolsRandom 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 -> “collision”, random access MAC protocol specifies:

how to detect collisions how to recover from collisions (e.g., via delayed

retransmissions)

Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA


ALOHA (Pure, Slotted)ALOHA (Pure, Slotted)

All frames same size

Slotted: Time is divided into equal size slots 1 slot = time to transmit 1 frame

Pure: Time remains continues

Slotted: Nodes start to transmit frames only at beginning of slots

Pure: Nodes start to transmit whenever a frame is made.

Slotted: Nodes are synchronized Pure: Nodes are not synchronized

If 2 or more nodes transmit in slot, all nodes detect collision


Suppose N nodes with many frames to send, each transmits in slot with probability p

probability that first node has success in a slot = p(1-p)N-1

probability that any node has a success is = Np(1-p)N-1

Suppose N nodes with many frames to send, each transmits in slot with probability p

probability that first node has success in a slot = p(1-p)N-1

probability that any node has a success is = Np(1-p)N-1

For max efficiency with N nodes, find pm that

maximizes Np(1-p)N-1

For many nodes, take limit of N pm(1- pm)N-1 as N

goes to infinity, gives 1/e = 0.37

For max efficiency with N nodes, find pm that

maximizes Np(1-p)N-1

For many nodes, take limit of N pm(1- pm)N-1 as N

goes to infinity, gives 1/e = 0.37

At best: channel used for useful transmissions 37% of time!

Slotted Aloha EfficiencySlotted Aloha Efficiency


Poisson model: probability of k frames transmission attempts in t time units. unit time = 1 frame time = F/R = TF

G =Offered Load [Frames/unit time] = λ·TF [ λ : Frame/sec ]infinite population model: (too many senders each with too many frames to transmit.

Packet Arrival AssumptionPacket Arrival Assumption

time

Send (Arrived) Packet

t

per Sec.) Packets SendAverage:( /)t(E

e]T,[P]Tt[P :Model lExponentia TSec

1

0

!k

e)Gt(t]P[k,times] unite t in ontransmissi [k Prob

Gtk t t

t t

t

tSec

Frame time


timet0t0-TF t0+TF

Vulnerableperiod

A frame transmitted at t0

Suppose F: the average frame length, R: bandwidth, TF=F/R: frame time

Transmit a frame at t0 (and finish transmission of the frame at t0+TF )

Suppose F: the average frame length, R: bandwidth, TF=F/R: frame time

Transmit a frame at t0 (and finish transmission of the frame at t0+TF )

Vulnerable period: if any other frames be transmitted during the vulnerable period, will collide with the frame sent at t0 .

Therefore the probability of a successful transmission is the probability that there is no additional transmissions in the vulnerable period.

Therefore the probability of a successful transmission is the probability that there is no additional transmissions in the vulnerable period.

Pure Aloha Model-1Pure Aloha Model-1

t


Pure Aloha Model-2Pure Aloha Model-2

time……

The green frame dose not experience collision if during Vulnerable period no one tries to transmit a frame:

GekP 2]2,0[

Vulnerable period


Throughput vs Offered Load- Pure AlohaThroughput vs Offered Load- Pure Aloha

The throughput S is given by:

framea of ion transmisssuccessful ofty Probabili LoadOfferedS

G2eGS

framea of ion transmisssuccessfula ofy ProbabilitGS


Slotted ALOHA ModelSlotted ALOHA Model

Synchronize the transmissions of stations All stations keep track of transmission time slots and

are allowed to initiate transmissions only at the beginning of a time slot.

Suppose a packet occupies one time slot Vulnerable period is from t0-TF to t0, i.e., TF seconds

long. Therefore, the throughput of the system is:

timet0t0-TF t0+TF

Vulnerable period

frame transmitted at t0

Ge]1,0k[Pcollision) no(P

GeGS

framea of ion transmisssuccessfula ofy ProbabilitGS


0 1 2 3 4 5 6 7 8 9 100

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

aloha

s-aloha

S-G Graphs-1S-G Graphs-1

GGeS

GGeS 2

Slotted ALOHA

(pure) Non-slotted ALOHA

0.37

0.18

0.5

G =Offered Load [Frames/Unit time]

S =

Thro

ughp

ut

[Fra

mes/

Unit

tim

e]


0 0.5 1 1.5 2 2.50

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

aloha

s-aloha


Slotted ALOHA

(pure) Non-slotted ALOHA


S =

Thro

ughp

ut

[Fra

mes/

Unit

tim

e]

S=kG


Slotted Aloha DelaySlotted Aloha Delay

Probability of 1 successful transmission after n-1 collision for a frame:

Expected value of n:

Average Delay for a frame:

1nGGn )e1(eP

1n

Gn enP)n(E

kOfTimeAverageBacB

)BdT)(1e(dTd .pr.trG

.pr.trAloha.S


Lack of collision controlLack of collision control

Collision ControlCollision Control

Offered load

Th

roughput

Controlled

Uncontrolled


CSMA (Carrier Sense Multiple Access)CSMA (Carrier Sense Multiple Access)

CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission


CSMA CSMA CCollisionsollisions

collisions can still occur:propagation delay means two nodes may not heareach other’s transmissioncollision:entire packet transmission time wastednote:role of distance & propagation delay in determining collision probability

A B C D

time

t0

t1

distance

CollisionDetection

Times

Fra

me tra

nsm

ission tim

e



collision occurs:A finishes transmitting a frame, then receives DD’s frame so A cannot detect the collision.

F: Frame length [bit]R: Transmission Rate (Bandwidth) [bit/sec] —› TF=F/Rd: A to D distance [m]V: signal propagation velocity in channel [m/sec] —› TAD= d/V

TF ≥ 2×TAD —› F ≥ 2×R×TAD —› α =TAD/(F/R) ≤ 0.5

F: Frame length [bit]R: Transmission Rate (Bandwidth) [bit/sec] —› TF=F/Rd: A to D distance [m]V: signal propagation velocity in channel [m/sec] —› TAD= d/V

TF ≥ 2×TAD —› F ≥ 2×R×TAD —› α =TAD/(F/R) ≤ 0.5

Collision detection condition:TF: Frame transmission Time [sec]TAD: A to D propagation time [sec]Then: TF≥ 2×TAD

Collision detection condition:TF: Frame transmission Time [sec]TAD: A to D propagation time [sec]Then: TF≥ 2×TAD

A B C D

time

t0t1

distance

CollisionDetection Time=0

TAD

TDATF ‹ (TAD+TDA) = 2×TAD

TF

CSMA collisionsCSMA collisions


Summary of MAC protocolsSummary of MAC protocols

What do you do with a shared media? Channel Partitioning, by time, frequency or code

Time Division,Code Division, Frequency Division Random partitioning (dynamic),

ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technologies (wire), hard

in others (wireless) CSMA/CD used in Ethernet

Taking Turns polling from a central site, token passing

5-5-4545Link & Physical LayersLink & Physical Layers (2-89-90)(2-89-90) Figure 6.11

IEEE 802 LAN standardsIEEE 802 LAN standards

MAC

LLC

Network Layer

802.2 Logical Link Control

802.3CSMA-CD

802.5Token Ring

802.11Wireless

LAN

OtherLANs

Various Physical Layers

Network Layer

Data Link Layer

Physical Layer

One LLC and several MACs, each MAC has an associated set of physical layers.

MAC provides connectionless transfer. Generally no error control because of relatively error free.

Ethernet consists of 802.2 + 802.3 + a physical layer


LAN Standards (IEEE)LAN Standards (IEEE)


IEEE 802 StandardsIEEE 802 Standards

The important ones are marked with *. The one marked with † gave up. The ones marked with are hibernating


“dominant” LAN technology: Cheap even for 100Mbs! First widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10, 100, 1000, 10000, 40000

Mbps

Metcalfe’s Ethernet Sketch1973, Xerox

EthernetEthernet


Ethernet uses CSMA/CDEthernet uses CSMA/CD

No slots Adapter doesn’t

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.

Adapter keeps trying to transmit, that is, multiple access.


Ethernet CSMA/CD algorithmEthernet CSMA/CD algorithm

1. Adaptor gets datagram and creates frame

2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits

3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame !

1. Adaptor gets datagram and creates frame

2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits

3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame !

4. If adapter detects another transmission while transmitting, aborts and sends jam signal

5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K×512 bit times and returns to Step 2


Frame Ready for Transmission

Sense Channel

Channel BusyNo

Frame Transmission & Channel Sense

BusyCollisionAbort Transmission;Send Jam Signal(3Bytes)

Frame successfully transmittedFrame successfully transmittedFrame successfully transmittedFrame successfully transmitted

Yes

Wait Inter-frame Gap96 bit-time

Increment AttemptsN++

Too Many Attempts?

Unsuccessful transmission, Excessive CollisionsUnsuccessful transmission, Excessive CollisionsUnsuccessful transmission, Excessive CollisionsUnsuccessful transmission, Excessive Collisions

Ethernet MAC flow in Half-Duplex Mode-Ethernet MAC flow in Half-Duplex Mode-TransmissionTransmission

Inter-frame Gap allows receivers time to settle

N=15 N<15

N<10

yes No

K=N K=10

Select A Random Integer R=(0 to 2k-1)

wait R×512 bit times

Set Attempt N=0Exponential backoff


A B C D

time

t0

t1

distance

Fra

me tra

nsm

ission tim

e


Collision detection at D

Collision detection at B

Frame transmission by D

Frame transmission by B

Jam frame from B

Jam frame from D

CSMA/CD CSMA/CD –– Jam Signal Jam Signal


Ethernet’s CSMA/CD (more)Ethernet’s CSMA/CD (more)

Bit time: 0.1 µsec for 10 Mbps Ethernet ;for K=1023, wait time is about 1023 50≈(512 ٭ 0.1)٭ msec

:Jam Signal makes sure all other transmitters are aware of ;collision; 32 bits

:Bit time µsec for 10 Mbps 0.1 ; Ethernet for K=1023, wait time is about 1023 50≈(512 ٭ 0.1)٭ msec

Jam Signal: makes sure all other transmitters are aware of collision; 32 bits;

Exponential Backoff: Goal: adapt retransmission

attempts to estimated current load

heavy load: random wait will be longer

first collision: choose K from {0,1}; delay is K x 512 bit transmission times

after second collision: choose K from {0,1,2,3}…

after next collision double K (and keep doubling on collisions until…..)

after 10 collisions, choose K randomly from {0,1,2,3,4,…,1023}

Exponential Backoff: Goal: adapt retransmission

attempts to estimated current load

heavy load: random wait will be longer

first collision: choose K from {0,1}; delay is K x 512 bit transmission times

after second collision: choose K from {0,1,2,3}…

after next collision double K (and keep doubling on collisions until…..)

after 10 collisions, choose K randomly from {0,1,2,3,4,…,1023}


Ethernet commentsEthernet comments

Exponential Back off: upper bounding at K = 1023(210-1) limits max size.

could remember last value of K when we were successful (analogy: TCP remembers last values of congestion window size)

Q: why use binary back off rather than something more sophisticated such as TCP’s additive increase/multiplicative decrease (AIMD) -> simplicity (?)

note: Ethernet does

multiplicative-increase-complete-decrease (why?)Increase the waiting probability range by 2m

Decreasing the rang completely after getting through


Ethernet MAC flow in Half-Duplex Mode-Ethernet MAC flow in Half-Duplex Mode-ReceiveReceive

Receive Process

Channel SenseIdle

Busy

Start Receiving

Channel Sense

Received Frame too Small? (Jam Signal)

RecognizeAddress?

Valid Frame Check Sequence?

Extra bits?

Receive Alignment ErrorReceive Frame Check Error

SuccessfulReception

Busy

Idle

Yes

No

No

YesNo

Yes

Yes

No


Ethernet’s use of randomizationEthernet’s use of randomization

More collisionsMore collisions

Heavier Load, more nodes trying to sendHeavier Load, more nodes trying to send

Randomize retransmissions over longer time interval, to reduce collision probability

Randomize retransmissions over longer time interval, to reduce collision probability



5.1 Introduction and services5.2 Error detection and correction 5.3 Links and Access Protocols5.4 Ethernet5.5 Ethernet Model

Ethernet and RandomizationEthernet Model (throughput, response time,

efficiency)5.6 Ethernet Frame Structure 5.7 LAN addresses and ARP5.8 Ethernet Technologies5.9 Hubs, bridges, and switches5.10 Point to Point Protocol


Ethernet ModelEthernet Model

An analytical model for Ethernet is developed. The model includes

Throughput, Response Time and Efficiency (Utilization)


Model AssumptionsModel Assumptions

Large number of active nodes, with each node having a large number of frames to send.

Fixed length frames. The packets transmission probability is Poisson. Poisson model: probability of k packets transmission attempts

in t time units

infinite population model

!k

e)Gt(t]P[k,unites] time t in ontransmissi [k Prob

Gtk


Frame transmission time is unit of time (F/R).

Throughput S - number of frames successfully (without collision) transmitted per unit time.

Offered load G - number frames transmissions attempted per unit time.

Note: S <= G,

S depends on G.

Model ParametersModel Parameters


Throughput ModelThroughput Model

busy time is a random variable given by B

idle time is a random variable given by I

collision time is a random variable given by C

Unit time……

][][][ CEBEIE

frame a of ontransmissi l successfuofy ProbabilitS



Throughput vs Offered Load-Average Idle TimeThroughput vs Offered Load-Average Idle Time

Because it is a Poisson Process then:I is a variable with an exponential distribution with expected value =1/G

GIE

1][ Average (Expected) idle time =

G: Average offered load per unit time.1/G: Average time between two consecutive transmission.

Idle time:

Time units

Average Unit timeE(I)

Frame Frame Frame

1/G


Throughput vs Offered Load-Average Busy TimeThroughput vs Offered Load-Average Busy Time

1+α

αUnit time

no one transmits GekP ],0[

frame clear the channelsuccessful frame transmission

busy time length =B = (1+ α)

GeBE )1(][Average (Expected) busy time =

A successful packet transmission:

= end-to-end propagation time/unit timetime during which collisions can occur

Probability of successful transmission of a frame

α

Time units

5.0/

/

RF

vd


Throughput vs Offered Load-Average Collision TimeThroughput vs Offered Load-Average Collision Time

A collision in the channel:

transmission GekPkP 1],0[1],0[

Unit timeα

A frame enters into channel

Any transmissioncauses collision

Unit timeα

Channel cleared Collision time length = α

Unit timeα α

Collision time length = 2αChannel cleared

Collision time length = 1.5 α

Time units

)e1()5.0(]C[E G


Average TimesAverage Times

)e-(11.5Time Collision Average

)1(Tim Busy AverageTime Service Average

/1IdleTime Average

G

Gee

G

Time Units/frameTime Units/frame


Average ThroughputAverage Throughput

][][][ CEBEIE

frame a of ontransmissi l successfuofy ProbabilitS


frames/Unit timeframes/Unit time

44.51

1

21

1 then

54.0

10

2 max e

Se

Gfor

)1(5.1)1(1)1(5.1)1(1 GG

G

GG

G

eGGe

Ge

eeG

eS


Response Time ModelResponse Time Model

Average response time:

Average response time is the time during which a frame gets through.

framea of sion transmissuccessful ofy Probabilit

]C[E]B[ER

Average Response time:

)1α(e5.1α)1(e

)e1α(5.1α)e1(R αG

αG

αGαG

Time Units/frameTime Units/frame


0 5 10 15 20 25 30 35 40 45 500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

a=0.01

a=0.02

a=0.05

a=0.1

a=0.2


G = Offered Load [Frames/Unit time]

S =

Thro

ughput

[Fra

mes/

Unit

ti

me]

capacity of CSMA/CD: maximum value of S over all values of G

>

α=0.1 means “10% of frame get transmitted before every one on the channel hears (detects) it”.α=0.1 means “10% of frame get transmitted before every one on the channel hears (detects) it”.

α=0.01α=0.02α=0.05α=0.1α=0.2>

>

>

>

>



0 5 10 15 20 25 30 35 40 45 500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

G = Offered Load [Frames/Unit time]

S =

Thro

ughput

[Fra

me/U

nit

tim

e]

α=0.01α=0.02α=0.05α=0.1α=0.2

ALOHA

S-ALOHA


Lack of collision controlLack of collision control

Collision ControlCollision Control

Offered load

Th

roughput

Controlled

Uncontrolled


0 20 40 60 80 1001

1.05

1.1

1.15

1.2

1.25

α =0 .01

α =0 .02

R-G GraphsR-G Graphs

G=Offered Load [Frame/Unit time]

R =

Resp

on

se t

ime [

Tim

e u

nit

s]

1.02

1.01

0 10 20 30 40 500

5

10

15

20

25

1. 1


R =

Resp

on

se t

ime [

Tim

e U

nit

s]

α = 0.1

0 5 10 15 20 250

10

20

30

40

50

α = 0.2


R =

Resp

on

se t

ime [

Tim

e U

nit

s]1.2

0 20 40 60 80 1000

5

10

15

α = 0.05


R =

Resp

on

se t

ime [

Tim

e U

nit

s]

1.05


Efficiency ModelEfficiency Model

)1(5.1)1(

)1(

3

)1(5.1)1(/1

)1(

2

)1(5.1)1(/1

)1( 1

2

GG

G

GG

G

GG

G

ee

e

timecontentiontimentransmisioframe

timentransmisioframeefficiency

eeG

e

timeelapse

timentransmisioframeefficiency

eeG

ethroughputtimeserviceefficiencyU


A Discussion on EfficiencyA Discussion on Efficiency

Efficiency goes to 0 as d/v=tprop goes up (long distance)

Efficiency goes to 0 as F/R=ttrans goes to 0 (high bandwidth)

People want: high bandwidth over long distances! Recommendation: Don’t use Ethernet.

44.51

12efficiency and

44.51

1

e21

1S then

54.0

10

e2G for max

R/F

v/d


Efficiency1Efficiency1

0 5 10 15 20 25 30 35 40 45 500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

a=0.01

a=0.02

a=0.05a=0.1

a=0.2

G=Offered Load [Frames/Unit time]

α=0.01α=0.02α=0.05α=0.1α=0.2

)1(5.1)1(/1

)1( 1

2

GG

G

eeG

ethroughputtimeserviceefficiencyU


Efficiency3Efficiency3

0 5 10 15 20 25 30 35 40 45 500

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

a=0.01

a=0.02

a=0.05a=0.1

a=0.2


α=0.01α=0.02α=0.05α=0.1α=0.2

)e1(5.1e)1(

e)1(

time contentiontime ntransmisio frame

time ntransmisio frame3 efficiency

GG

G


Ethernet Frame FormatEthernet Frame Format

PreamblePreamble SFDSFD DADA SASA TypeType DataData PadPad CRCCRC

7 1 6 6 2 0-46 4

1. Preamble: 10101010 …, trains clock-recovery circuits.2. Start of Frame Delimiter: 10101011, indicates start of frame.3. Destination Address: 48-bit globally unique address

assigned by manufacturer.4. Type: Indicates the higher layer protocol, mostly IP (e.g. IP =

0x0800) but others may be supported such as Novell IPX and AppleTalk.

5. Pad: Zeroes used to ensure minimum frame length6. Cyclic Redundancy Check: checked at receiver, if error is

detected, the frame is simply dropped.

Bytes: 46 to 1500 Bytes

Mini :6+6+2+46+4= 64 Bytes (512 bits)Max :6+6+2+1500+4= 1518 Bytes


Unreliable, connectionless serviceUnreliable, connectionless service

Connectionless: No handshaking between sending and receiving adapter.

Unreliable: receiving adapter doesn’t send ACKs or NACKs to sending adapter stream of datagrams passed to network layer can have

gaps gaps will be filled if application is using TCP otherwise, application will see the gaps


LAN Addresses and ARPLAN Addresses and ARP

32-bit IP address: network-layer address used to get datagram to destination IP network (recall IP

network definition)

LAN (or MAC or physical or Ethernet) address: used to get datagram from one interface to another

physically-connected interface (same network) 48 bit MAC address (for most LANs)

burned in the adapter ROM


LAN AddressLAN Address

MAC address allocation administered by IEEE Manufacturer buys portion of MAC address

space (to assure uniqueness)

MAC flat address —› portability can move LAN card from one LAN to another

IP hierarchical address NOT portable depends on IP network to which node is attached


Ethernet Address FormatEthernet Address Format

Every vendor (e.g., 3COM) is assigned a vendor block code.

Therefore, every globally administered address is globally unique.


LAN Addresses and ARP-1LAN Addresses and ARP-1

1A-23-F9-CD-06-9B

8B-B2-2F-54-1A-0F

49-BD-D2-C7-56-2A

5C-66-AB-90-75-B161-BC-85-50-C1-7B

B1-C6-A1-0B-B9-80

LANLAN

240.108.12.01

240.108.12.02

240.108.12.03

240.108.12.04

240.108.12.05

240.108.12.06

Each Adapter on LAN has unique LAN address

Network Interface Card (Network Interface Card (AdaptorAdaptor))


Recall earlier routing discussionRecall earlier routing discussion

Starting at A, given IP datagram addressed to B:

look up net. address of B, find B on same net. as A

link layer send datagram to B inside link-layer frame

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

datagram

B’s MACaddr

A’s MACaddr

A’s IPaddr

B’s IPaddr

IP payload

frame source,dest address

datagram source,dest address

CRC

framedatagram


ARP: Address Resolution ProtocolARP: Address Resolution Protocol

Each IP node (Host, Router) on LAN has ARP table

ARP Table: IP/MAC address mappings for some LAN nodes

< IP address; MAC address; TTL> TTL (Time To Live): time

after which address mapping will be forgotten (typically 20 min)

Question: how to determineMAC address of Bknowing B’s IP address?

Question: how to determineMAC address of Bknowing B’s IP address?

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B


Operation of ARPOperation of ARP

FTP

TCP

IP

Ethernet driver

ARP

TCP

resolver

IPARP

Ethernet driver

Ethernet driver

ARP

hostnamehostname

IP addr Establish connection with IP address

Send IP datagram to IP address

ARP request (Ethernet broadcast)

(4)(5)

(6)

(3)

(1)

(2)

(7)

(8) (9)

A

B

LAN


Routing to inside a LANRouting to inside a LAN

A wants to send datagram to B, and A knows B’s IP address.

Suppose B’s MAC address is not in A’s ARP table.

A broadcasts ARP query packet, containing B's IP address all machines on LAN

receive ARP query B receives ARP packet,

replies to A with its (B's) MAC address frame sent to A’s 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 “plug-and-play”: nodes create their ARP

tables without intervention from network administrator


Routing to another LANRouting to another LAN

LAN1LAN1

1A-23-F9-CD-06-9B240.108.12.01 49-BD-D2-C7-56-2A

240.108.12.03

61-BC-85-50-C1-7B

240.108.12.02 LAN2LAN2

B1-C6-A1-0B-B9-80

40.211.7.200

40.211.7.20033-5A-18-0E-CC-12

AB

walkthrough: send datagram from A to B via R (assume A knows B’s IP address)

walkthrough: send datagram from A to B via R (assume A knows B’s IP address)

Two ARP tables in router, one for each IP network (LAN) In ARP table at source, find R’s MAC address 49-BD-D2-C7-56-

2A

Two ARP tables in router, one for each IP network (LAN) In ARP table at source, find R’s MAC address 49-BD-D2-C7-56-

2A


A creates datagram with source A, destination B A uses ARP to get R’s MAC address for

240.108.12.03 A creates link-layer frame with R's MAC address

as destination, frame contains A-to-B IP datagram

A’s data link layer sends frame R’s data link layer receives frame R removes IP datagram from Ethernet frame,

sees its destined to B R uses ARP to get B’s physical layer address R creates frame containing A-to-B IP datagram

sends to B

A creates datagram with source A, destination B A uses ARP to get R’s MAC address for

240.108.12.03 A creates link-layer frame with R's MAC address

as destination, frame contains A-to-B IP datagram

A’s data link layer sends frame R’s data link layer receives frame R removes IP datagram from Ethernet frame,

sees its destined to B R uses ARP to get B’s physical layer address R creates frame containing A-to-B IP datagram

sends to B

Routing to another LAN’Routing to another LAN’


10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology

repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! has become a legacy technology

Old Ethernet Technologies: 10Base2Old Ethernet Technologies: 10Base2


Hubs are essentially physical-layer repeaters: bits coming in one link go out all other links no frame buffering no CSMA/CD at hub: adapters detect

collisions provides network management functionality

HubsHubshub

node1

node2

node3

node4

node5

node6

To higher level hubs/switches


10Base510Base5


Used in 10BaseT, 10Base2 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!

This is physical-layer!

Used in 10BaseT, 10Base2 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!

This is physical-layer!

Manchester EncodingManchester Encoding


IEEE 802.3 10Mbps MediumsIEEE 802.3 10Mbps Mediums

Coaxial Cable(50

Ohm)

Coaxial Cable (50

Ohm)

Unshielded twisted pair

850-nm optical fiber

pair

Baseband (Manchester)

Baseband (Manchester)

Baseband (Manchester

)

Manchester/On-Off

Bus Bus Star Star

500 185 100 500

100 30 ---- 33

10 5 0.4 to 0.662.5/12.5

µm

10Base5 10Base2 10Base-T 10base-FP

Transmissionmedium

Signalingtechnology

Topology

Max segmentlength [m]

Nodes persegment

Cable diameter[mm]


802.3 Ethernet Standards: Link & Physical 802.3 Ethernet Standards: Link & Physical LayersLayers

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 (twister pair) physical layer


2 pair, STP2 pair,

Category 5 UTP

2 optical fibers

4 pair, cat.3, 4 or

5 UTP

MLT-3 MLT-3 4B5B, NRZI 8B6T, NRZ

100 Mbps 100 Mbps 100 Mbps 100 Mbps

100 100 100 100

200 200 400 200

100Base-TX 100Base-FX 100Base-T4

Transmissionmedium

Signalingtechnology

Data rate

Max segmentlength [m]

Networkspan [m]

IEEE 802.3 100Mbps MediumsIEEE 802.3 100Mbps Mediums

STP: Shielded twisted pair; UTP: Unshielded twisted pairNRZ: NonReturn to Zero; NRZI: NRZ Invertwd


Gbit EthernetGbit Ethernet

use standard Ethernet frame format allows for point-to-point links and shared

broadcast channels in shared mode, CSMA/CD is used; short

distances between nodes to be efficient uses hubs, called here “Buffered Distributors” Full-Duplex at 1 Gbps for point-to-point links 10 or 40 Gbps now !


Interconnecting LAN segmentsInterconnecting LAN segments

Hubs Bridges Switches

Remark: switches are essentially multi-port bridges. What we say about bridges also holds for switches!


Bridges: Traffic IsolationBridges: Traffic Isolation

Bridge installation breaks LAN into LAN segments Bridges filter packets:

Same-LAN-segment frames not usually forwarded onto other LAN segments

Segments become separate collision domains

bridge collision domain

collision domain

= hub

= host

LAN (IP network)

LAN segment LAN segment


ForwardingForwarding

How do determine to which LAN segment to forward frame?• Looks like a routing problem...


Self learningSelf learning

A bridge has a bridge table Entry in bridge table:

(Node LAN Address, Bridge Interface, Time Stamp) Stale entries in table dropped (TTL can be 60 min)

Bridges learn which hosts can be reached through which interfaces When frame received, bridge “learns” location of

sender: incoming LAN segment Records sender/location pair in bridge table


Filtering/ForwardingFiltering/Forwarding

When bridge receives a frame:

if entry found for destinationthen{

if destination 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


Bridge ExampleBridge Example

Suppose C sends frame to D and D replies back with frame to C.

Bridge receives frame from from C notes in bridge table that C is on interface 1 because D is not in table, bridge sends frame into

interfaces 2 and 3 Frame received by D


Bridge Learning: Example’Bridge Learning: Example’

D generates frame for C, sends Bridge receives frame

Notes in bridge table that D is on interface 2 Bridge knows C is on interface 1, so selectively forwards

frame to interface 1 Bridge adds D on interface 2 of the table.


Interconnection Without BackboneInterconnection Without Backbone

Not recommended for two reasons:- Single point of failure at Computer Science hub- All traffic between EE and SE must path over CS segment


Backbone ConfigurationBackbone Configuration

Recommended !


Bridges Spanning TreeBridges Spanning Tree

For increased reliability, desirable to have redundant, alternative paths from source to destination.

With multiple paths, cycles result - bridges may multiply and forward frame forever

Solution: organize bridges in a spanning tree by disabling subset of interfaces

Disabled


Some Bridge FeaturesSome Bridge Features

Isolates collision domains resulting in higher total max throughput

Limitless number of nodes and geographical coverage

Can connect different Ethernet types Transparent (“plug-and-play”): no

configuration necessary


Bridges vs. RoutersBridges vs. Routers

Both store-and-forward devices routers: network layer devices (examine network layer headers) bridges are link layer devices

Routers maintain routing tables, implement routing algorithms Bridges maintain bridge tables, implement filtering, learning

and spanning tree algorithms


(a) Hub

(b) Switch

High-Speed Backplane or Interconnection fabric

Single collision domain

Ethernet SwitchesEthernet Switches


Ethernet Switches.Ethernet Switches.

Essentially a multi-interface bridge

Layer 2 (frame) forwarding, filtering using LAN addresses

Switching: A-to-A’ and B-to-B’ simultaneously, no collisions

Large number of interfaces Often: individual hosts, star-

connected into switch Ethernet, but no collisions!

AA’

B

B’

Ethernet Switch


Ethernet Switches’Ethernet Switches’

Cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame slight reduction in latency

Combinations of shared/dedicated, 10/100/1000 Mbps interfaces


Switching: Cut-ThroughSwitching: Cut-Through

A packet starts being forwarded (sent) as soon as its header is received

Sender ReceiverR1 = 10 Mbps R2 = 10 Mbps

time

Header

What happens if R2 > R1 ?


Typical LAN (IP network)Typical LAN (IP network)

Repeater hub1

Client

Printer

Server

Client

Switching hub

Client

Remote Access ServerModem pools

TelephoneLines

Router

External Link

Serversmodem

modem

Client

Repeater hubn


hubs

bridges routers switches

Traffic isolation no yes yes yes

plug & play yes yes no yes

Optimal routing

no no yes no

Cut through yes no no yes

Summary comparison-1Summary comparison-1


Summary comparison-2Summary comparison-2

restricted,no loops

arbitrary no loops arbitrary

alwaysbroadcast &

unknown

broadcast (w/ default

route)never

1 2 2 3Works at Layer...

Yes Yes Yes NoTransparent?

worst ok high high delayPerformance

low medium high way-highComplexity

Topology

Packet Flooding

catastrophic catastrophic catastrophic TTL kills itLooping packet

floodflood or

opt. discarddefault

default ordiscard

Unknown address

instant store & fwd cut thru (typ) store & fwdForwarding

none STP opt. STP L3 protocolTopology learning

Repeating Bridging Switching Routing


Point to Point Link Layer ControlPoint to Point Link Layer Control

One sender, one receiver, one link: easier than broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, DHL connection, ISDN line

Popular point-to-point LLC protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to

be considered “high layer” in protocol stack!)


PPP Design Requirements [RFC 1557]PPP Design Requirements [RFC 1557]

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 de-multiplex upwards.

Bit transparency: must carry any bit pattern in the data field.

Error detection (no correction). Connection live-ness: detect, signal link failure to

network layer. Network layer address negotiation: endpoint can

learn/configure each other’s network address.


PPP non-requirementsPPP non-requirements

no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling)

Error recovery, flow control, data re-ordering all relegated to higher layers!


PPP Data FramePPP 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, PPP-LCP, IP, IPCP, etc)


PPP Data FramePPP Data Frame

info: upper layer data being carried check: cyclic redundancy check for error detection


Byte StuffingByte 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

5-1 link & physical layers (2-89-90) link & physical layers computer networks

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