(C) All rights reserved by Professor Wen-Tsuen Chen1 Chapter 4 The Medium Access Sublayer ä Medium...

(C) All rights reserved by Professor (C) All rights reserved by Professor Wen-Tsuen ChenWen-Tsuen Chen

11

Chapter 4 The Medium Access SublayerChapter 4 The Medium Access Sublayer

Medium Access Control (MAC) sublayer in Medium Access Control (MAC) sublayer in IEEE 802 LAN standardsIEEE 802 LAN standards

Broadcast networks and their protocolsBroadcast networks and their protocols Multiple Access Protocols: Channel access Multiple Access Protocols: Channel access

schemes to shared communication channel schemes to shared communication channel

© All rights reserved. No part of these slides may be reproduced, in any© All rights reserved. No part of these slides may be reproduced, in any form or by any means, without permission in writing from form or by any means, without permission in writing from Professor Wen-Tsuen Chen (email: [email protected]).Professor Wen-Tsuen Chen (email: [email protected]).


22

Multiple Access Protocols

Contention Conflict-Free

Dynamic Resolution Static Resolution Dynamic Allocation Static Allocation

Time ofArrival

Proba-bilistic

IDTime ofArrival

Reser-vation

TokenPassing

Time andFrequency

Based

FrequencyBased

TimeBased

Classification of Multiple Access MethodsClassification of Multiple Access Methods


33

Pure ALOHAPure ALOHA

Devised in early 1970s by Norman Abramson and his colleagues at thDevised in early 1970s by Norman Abramson and his colleagues at the University of Hawaii.e University of Hawaii.

ALOHA system used ground-based radio broadcasting for communicaALOHA system used ground-based radio broadcasting for communication between terminals and the main computer.tion between terminals and the main computer.

The pure ALOHA protocol is very simple:The pure ALOHA protocol is very simple: A newly generated packet is transmitted immediately hoping for nA newly generated packet is transmitted immediately hoping for n

o interference by others.o interference by others. If the transmission is unsuccessful, every colliding user, independeIf the transmission is unsuccessful, every colliding user, independe

ntly of the others, schedules his retransmission to a random time in ntly of the others, schedules his retransmission to a random time in the future, to ensure that the same set of packets does not continue the future, to ensure that the same set of packets does not continue to collide indefinitely.to collide indefinitely.


44


55

Capacity of the pure ALOHA channelCapacity of the pure ALOHA channel

Consider a single-hop system with an infinite population generating Consider a single-hop system with an infinite population generating packets of equal length T according to a Poisson process with rate packets of equal length T according to a Poisson process with rate

The channel is error-freeThe channel is error-free At the end of every transmission, each user knows whether his At the end of every transmission, each user knows whether his

transmission was successful or a collision took place.transmission was successful or a collision took place. Let g = the rate of the scheduling points, the points in which packets, Let g = the rate of the scheduling points, the points in which packets,

new and retransmitted, are scheduled for transmission. ( g: offered new and retransmitted, are scheduled for transmission. ( g: offered load, g > load, g > ) )

Assume the scheduling points process is a Poisson process with rate Assume the scheduling points process is a Poisson process with rate g . This assumption is a good approximation as has been shown by g . This assumption is a good approximation as has been shown by simulation.simulation.


66


77

The probability that The probability that kk packets are generated during T is give packets are generated during T is given by the Poisson distribution:n by the Poisson distribution:

Transmission of a packet is successful with probability: Transmission of a packet is successful with probability:

If no other packet is scheduled for transmission in the vulnerIf no other packet is scheduled for transmission in the vulnerable period of duration 2 Table period of duration 2 T

The rate of successfully transmitted packets is .The rate of successfully transmitted packets is .

The throughput S is the fraction of time that useful informatiThe throughput S is the fraction of time that useful information is carried on the channel,on is carried on the channel,

wherewhere

, at , at

!k

egTkP

gTk

r

sucgP

GgT GegTeS 22 gTG

18.02

1max

eSCapacity

2

1G


88

Slotted ALOHASlotted ALOHA

Transmission of packets start only at next Transmission of packets start only at next slot boundary.slot boundary.

Vulnerable period is reduced to a single slot Vulnerable period is reduced to a single slot time T.time T. S gTe GegT G

Capacity Se

G max . ,1

0 36 1


99


1010

Instability of the ALOHA protocolsInstability of the ALOHA protocols

stable point instable point


1111

Splitting AlgorithmsSplitting Algorithms

From From Data NetworksData Networks, by D. Bertsekas and , by D. Bertsekas and R. Gallager, 2nd Ed. Prentice Hall, 1992R. Gallager, 2nd Ed. Prentice Hall, 1992

Tree algorithmsTree algorithms FCFS splitting algorithmsFCFS splitting algorithms Maximum stable throughput = 0.4878Maximum stable throughput = 0.4878


1212

Tree AlgorithmTree Algorithm


1313

Improvements to the Tree AlgorithmImprovements to the Tree Algorithm


1414


1515

FCFS Splitting AlgorithmFCFS Splitting Algorithm


1616


1717

FCFS splitting algorithmFCFS splitting algorithm

If feedback = If feedback = e e ,then,then

T(T(kk)=T()=T(kk-1)-1)

If feedback =1 and If feedback =1 and then then

T(T(kk)=T()=T(kk-1)+-1)+

Lk

kk

)(2

)1()(

Lk )1()1( k

Rk

kk

)(

)1()(


1818

FCFS splitting algorithm(cont.)FCFS splitting algorithm(cont.)

If feedback = 0 and If feedback = 0 and = L then,= L then,

T(T(kk) = T() = T(kk-1) +-1) +

If feedback = 0 or 1 and If feedback = 0 or 1 and = R then, = R then,

T(T(kk) = T() = T(kk-1) +-1) +

)1( k

)1( k

Lk

kk

)(2

)1()(

)1( k

)1( k

Rk

kTkk

)(

)](min[)(0


1919

Improvements in the FCFS splitting algorithmImprovements in the FCFS splitting algorithm

When each interval is split into the When each interval is split into the optimally sized subintervals, the maximum optimally sized subintervals, the maximum stable throughput increases to 0.4878.stable throughput increases to 0.4878.

Considerable research has been devoted to Considerable research has been devoted to finding upper bounds to throughput,and the finding upper bounds to throughput,and the tightest such bound is 0.587.tightest such bound is 0.587.


2020

Carrier Sense Multiple Access ProtocolsCarrier Sense Multiple Access Protocols

When a user generates a new packet, the channel is sensed When a user generates a new packet, the channel is sensed and if found idle, the packet is transmitted.and if found idle, the packet is transmitted.

When a collision takes place, every transmitting user reschWhen a collision takes place, every transmitting user reschedules a retransmission of the collided packet to some otheedules a retransmission of the collided packet to some other random time in the future, at which time the same operatir random time in the future, at which time the same operation is repeated.on is repeated.

nonpersistent CSMAnonpersistent CSMA If found busy, the user behaves as if its packet collides i.e. If found busy, the user behaves as if its packet collides i.e.

schedules retransmission to some random time in the futurschedules retransmission to some random time in the future.e.

For slotted SystemFor slotted SystemS

aGea e

aaG

aG T

1,

GG1

as a 0


2121

Carrier Sense Multiple Access ProtocolsCarrier Sense Multiple Access Protocols

1-persistent CSMA1-persistent CSMA If found busy, persists to wait until the channel become idlIf found busy, persists to wait until the channel become idl

ee If found idle, transmit with probability 1If found idle, transmit with probability 1

p-persistent CSMAp-persistent CSMA the same as 1-persistent CSMA except if found idle, transthe same as 1-persistent CSMA except if found idle, trans

mit with probability P.mit with probability P. For slotted system, 1-persistentFor slotted system, 1-persistent

SGe a e

a e ae

a e aG

aG a G

aG

1 1

1

11 1

( )

( )

( )( )( )

SGe GG ea

G

G

0

1( )


2222


2323

Throughput Analysis of Slotted NonpThroughput Analysis of Slotted Nonpersistent CSMAersistent CSMA

S = S = where U = expected # of successful slots where U = expected # of successful slots

B = expected # of busy slots B = expected # of busy slots

I = expected # of idle slotsI = expected # of idle slots

IB

U


2424

Throughput of Slotted Nonpersistent CSMAThroughput of Slotted Nonpersistent CSMA

G

G

ea

TeT

eTg

eeT

eeg

eT

IB

PT

BT

IB

US

e

eg

Someob

ob

arrivalssomeontransmissithe

successfulobP

PT

BTUEU

e

TBsokeeTkBob

eIsokeekIP

B

aG

g

g

gg

g

g

gsuc

g

g

suc

suc

ggkg

ggkg

~

1S small, very is aWhen ,

1

aGe=S

gT=G and =a Using1

1

1slot-mini ain arrivals Pr

slot-mini ain arrival singlePr

beforeslot -minilast in arrival singleProb=

period]ion transmiss[Pr

][

, ...2,1,)1()]([Pr

1, ....2,1),1()()(

cycle a within timeofamount the:U period, idle :I period,busy :

0a

aG-

T

~

1~

1

~~~


2525

Throughput of Slotted 1-persistent CSMAThroughput of Slotted 1-persistent CSMA

G

G

Gag

gTg

sucsuc

Tg

Tg

suc

g

g

suc

TgTgkTg

eG

GGe

eeT

TTgTe

IB

US

PT

TBPTU

e

eTg

ob

obPe

egob

successfulobP

e

TBsokeeTkBob

]1[S small, very is aWhen

]1)[(

][

)(1

)(

]arrival oneleast at periodion transmissain arrival single[Pr

]periodbusy ain periodfirst -non in theion transmisssuccessful[Pr1

]arrival oneleast at slot-mini ain arrival single[Pr=

periodbusy a of periodon transmissi

first in theion transmissPr

, ...2,1,)1()]([Pr

0a

)1(

)(

21

)(

)(

2

1

)()(1)(

~


2626


2727


2828

CSMA with Collision DetectionCSMA with Collision Detection If a collision is detected, the station aborts its transmission, If a collision is detected, the station aborts its transmission,

waits a random period of time, and then tries again.waits a random period of time, and then tries again.

Reservationperiod


2929

CSMA with Collision Detection (Cont.)CSMA with Collision Detection (Cont.)

The notable example is Ethernet.The notable example is Ethernet. Only a small amount of bandwidth, instead Only a small amount of bandwidth, instead

of the whole collided frame transmission of the whole collided frame transmission time, is wasted.time, is wasted.

The contention period should be longer than The contention period should be longer than the time to detect collision.the time to detect collision.


3030

Collision-Free ProtocolsCollision-Free Protocols


3131

The binary countdown protocolThe binary countdown protocol


3232

IEEE Standard 802 for LANs and MANsIEEE Standard 802 for LANs and MANs

802.3802.3 CSMA/CD or EthernetCSMA/CD or Ethernet 802.4802.4 Token BusToken Bus 802.5802.5 Token RingToken Ring 802.6802.6 Distributed Queue Dual BusDistributed Queue Dual Bus 802.2802.2 Logical Link ControlLogical Link Control


3333

IEEE 802.3 CSMA/CDIEEE 802.3 CSMA/CD

Ethernet by R.M. Metcalfe and D.R. Boggs Ethernet by R.M. Metcalfe and D.R. Boggs at Xerox PARC in 1976. Data rate is 2.94 at Xerox PARC in 1976. Data rate is 2.94 MbpsMbps

10 Mbps Ethernet standards by Xerox, 10 Mbps Ethernet standards by Xerox, DEC, and Intel in 1980DEC, and Intel in 1980


3434

802.3 Protocol802.3 Protocol

In IEEE 802.3, In IEEE 802.3, 1-persistent CSMA is employed.1-persistent CSMA is employed. use truncated binary exponential backoff algorithm to deuse truncated binary exponential backoff algorithm to de

cide the random period of time, i.e. After cide the random period of time, i.e. After ii collisions, a collisions, a random numberrandom number RR between 0 and 2 between 0 and 2ii-1 is chosen, and the -1 is chosen, and the number of number of RR slots is skipped.However, if slots is skipped.However, if ii>10 the interv>10 the interval[0, 2al[0, 21010-1] is used.-1] is used.

Last Come First Serve EffectLast Come First Serve Effect


3535

contention period contention period 2 2where where is the propagation time between two farthest is the propagation time between two farthest stations.stations.In Ethernet, the contention period is 51.2In Ethernet, the contention period is 51.2uus, the time to transmits, the time to transmit 64 bytes64 bytes


3636


3737

Fast EthernetFast Ethernet

802.3u : approved by IEEE in June 1995802.3u : approved by IEEE in June 1995 Backward compatible with existing LANsBackward compatible with existing LANs Keep all the old packet format, interfaces, aKeep all the old packet format, interfaces, a

nd procedure rules, but reduce the bit time fnd procedure rules, but reduce the bit time from 100 rom 100 nsns to 10 to 10 ns.ns.


3838

100 Base - T4100 Base - T4 25 MHz25 MHz Four wires: to the hub. from the hub, two switchable to the current Four wires: to the hub. from the hub, two switchable to the current

transmission direction.transmission direction. Ternary signals 8B6T-NRZ on each of three wires. Ternary signals 8B6T-NRZ on each of three wires. 2 2

7 possible symbols => 4 bits per clock time7 possible symbols => 4 bits per clock time

100 Base - TX100 Base - TX 125 MHz125 MHz 2 wires2 wires 4B5B-NRZI coding4B5B-NRZI coding


3939


4040


4141


4242

IEEE 802.4: Token BusIEEE 802.4: Token Bus

Data point ARCnet, 2.5 MbpsData point ARCnet, 2.5 Mbps Promoted by General Motors for factory autPromoted by General Motors for factory aut

omationomation Use the 75-ohm broadband coaxial cable foUse the 75-ohm broadband coaxial cable fo

r Cable TVr Cable TV


4343


4444

Token BusToken Bus

Logical ring on a physical bus.Logical ring on a physical bus. Nodes can join or quit the logic ring dynamically.Nodes can join or quit the logic ring dynamically. Nodes can response to a query from the token holder even Nodes can response to a query from the token holder even

without being part of the logical ring.without being part of the logical ring. Provides deterministic worst-case bounds on access delay fProvides deterministic worst-case bounds on access delay f

or any loading configurationor any loading configuration Permits multiple classes of services.Permits multiple classes of services. Imposes no additional reguirements on the medium and the Imposes no additional reguirements on the medium and the

modem capabilities over those necessory for transmission modem capabilities over those necessory for transmission and acquisition of multi-bit, multi-frame sequences.and acquisition of multi-bit, multi-frame sequences.


4545

Start delimiter and end delimiter contain analog encoding Start delimiter and end delimiter contain analog encoding of symbols other than 0of symbols other than 0ss and 1 and 1ss..

Frame control is used to distinguish data frames from Frame control is used to distinguish data frames from control frames. For data frames, it carries the frame’s control frames. For data frames, it carries the frame’s priority. For control frames, it specifies the frame types.priority. For control frames, it specifies the frame types.


4646


4747

Token Bus MAC ProtocolsToken Bus MAC Protocols

AA CC

BB DD

P=CP=CS=DS=D

P=BP=BS=AS=A

P=DP=DS=CS=C

P=AP=AS=BS=B

TokenToken


4848

Addition of a node:Addition of a node: Each node in the ring has the responsibility of periodically Each node in the ring has the responsibility of periodically

granting an opportunity for new nodes to enter the ring.granting an opportunity for new nodes to enter the ring. While holding the token, the node issues a While holding the token, the node issues a solicit-successorsolicit-successor frame frame

inviting nodes with an address between itself and the next node in inviting nodes with an address between itself and the next node in logical sequence to demand entrancelogical sequence to demand entrance

B:B:DADA SASA

CC BB Case 1: No response:Case 1: No response:

The token holder transfers the token to its successor.The token holder transfers the token to its successor.


4949

Case 2: One responseCase 2: One response One node issuses a One node issuses a set-successorset-successor frame. The token holder sets its succ frame. The token holder sets its succ

essor node to be the requesting node and transmits the token to it.essor node to be the requesting node and transmits the token to it.

KK

DADA SASA

KK

AA CC

DD

P=CP=CS=DS=D

P=KP=KS=AS=A

P=AP=AS=BS=B

KKP=BP=BS=CS=C

BBP=DP=DS=KS=K


5050

Case 3: Multiple responseCase 3: Multiple response The token holder transmits a The token holder transmits a resouce-contentionresouce-contention frame and waits four frame and waits four

demand windows.demand windows.

Each demander can respond in one of these windows based on the firEach demander can respond in one of these windows based on the first two bits of its address.st two bits of its address.

If the token holder receives garbled respone, it tries again, and only thIf the token holder receives garbled respone, it tries again, and only those nodes that responded the first time are allowed to respond, based ose nodes that responded the first time are allowed to respond, based on the second pair of bits in their address.on the second pair of bits in their address.

1111 1010 0101 0000


5151

Deletion of a node:Deletion of a node: It waits until it receives the token, then sends a set-successor frame It waits until it receives the token, then sends a set-successor frame

to its predecessor and successor.to its predecessor and successor.

DADA SASA

CC BB

BB

AA CC

BB DD

P=CP=CS=DS=D

P=DP=DS=AS=A

P=AP=AS=CS=C

A=A=B=B=


5252

Fault Management:Fault Management: After sending a token, the token issuer will listen for one slot time After sending a token, the token issuer will listen for one slot time

to make sure that its successor is active.to make sure that its successor is active. If hear a valid frame, the successor node is active. If not, re-issues If hear a valid frame, the successor node is active. If not, re-issues

the token one more time.the token one more time. After two failures, assume successor failed and issue a After two failures, assume successor failed and issue a who-followwho-follow

ss frame. The issuer should get back a set-successor frame from the frame. The issuer should get back a set-successor frame from the second node down the line. If so, adjusts its linkage and issues a tosecond node down the line. If so, adjusts its linkage and issues a token.ken.

If no response, issue If no response, issue who-followswho-follows again. again. If If who-followswho-follows tactic fails, issue a tactic fails, issue a solicit-successorsolicit-successor frame with the frame with the

full address range.full address range.

If it works, a two-node ring is establishedIf it works, a two-node ring is established If no, tries again. If it fails, assume a catastrophe has occured.If no, tries again. If it fails, assume a catastrophe has occured.

DADA SASA

1111111111111111


5353

Ring initialization:Ring initialization: When one station dectects a lack of bus activity of duration longer When one station dectects a lack of bus activity of duration longer

than a time-out value, the token is assumed to be lost.than a time-out value, the token is assumed to be lost. The node issues a The node issues a claim-tokenclaim-token frame. frame. Contenting claimants are resolved in a manner similar to the respoContenting claimants are resolved in a manner similar to the respo

nse-window process.nse-window process.


5454

Classes of ServiceClasses of Service Synchronous (6)Synchronous (6) Asynchronous urgent (4)Asynchronous urgent (4) Asynchronous normal (2)Asynchronous normal (2) Asynchronous time-available (0)Asynchronous time-available (0) THT: Token holding timeTHT: Token holding time

The maximum time that a station can hold the token to transmit The maximum time that a station can hold the token to transmit class 6 data.class 6 data.

TRT4: Token rotation time for class4:TRT4: Token rotation time for class4: The maximum time that a token can take to circulate and still The maximum time that a token can take to circulate and still

permit class 4 transmission.permit class 4 transmission.

TRT2: TRT for class 2TRT2: TRT for class 2 TRT0: TRT for class 0TRT0: TRT for class 0


5555


5656

IEEE 802.4 Physical-Layer Medium AlternativeIEEE 802.4 Physical-Layer Medium Alternative

ParameterParameter

Data rate (Mbps)Data rate (Mbps)

BandwidthBandwidth

Center frequencyCenter frequency

ModulationModulation

TopologyTopology

TransmissionTransmissionmediummedium

ScramblingScrambling

Phase-Phase-ContinuousContinuousCarrierbandCarrierband

11

N.A.N.A.

5 MHz5 MHz

Manchester/phase-Manchester/phase-continuous FSKcontinuous FSKOmnidirectionalOmnidirectionalbusbusCoaxial cableCoaxial cable(75 ohm)(75 ohm)

NoNo

Phase-Phase-CoherentCoherentCarrierbandCarrierband

5 105 10

N.A. N.A.N.A. N.A.

7.5 MHz 15 MHz7.5 MHz 15 MHz

phase-continuousphase-continuousFSKFSKOmnidirectionalOmnidirectionalbusbusCoaxial cableCoaxial cable(75 ohm)(75 ohm)

NoNo

BroadbandBroadband

1 5 101 5 10

1.5MHz 6MHz 12MHz 1.5MHz 6MHz 12MHz

─ ─ ─ ─ ─ ─

Multilevel duobinary AM/PSKMultilevel duobinary AM/PSK

Directional bus (tree)Directional bus (tree)

Coaxial cable (75 ohm)Coaxial cable (75 ohm)

YesYes

Optical FiberOptical Fiber

1, 10, 201, 10, 20

270 nm270 nm

800-900 nm800-900 nm

On-offOn-off

Active or Active or passive starpassive starOptical fiberOptical fiber

NoNo

(C) All rights reserved by Professor Wen-Tsuen Chen1 Chapter 4 The Medium Access Sublayer ä Medium...

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