Multiple AccessMultiple Access

01204325: Data 01204325: Data Communication and Communication and Computer NetworksComputer Networks

Asst. Prof. Chaiporn Jaikaeo, Ph.D.Asst. Prof. Chaiporn Jaikaeo, Ph.D.chaiporn.j@ku.ac.th

http://www.cpe.ku.ac.th/~cpjhttp://www.cpe.ku.ac.th/~cpjComputer Engineering DepartmentComputer Engineering Department

Kasetsart University, Bangkok, ThailandKasetsart University, Bangkok, ThailandAdapted from lecture slides by Behrouz A. Forouzan© The McGraw-Hill Companies, Inc. All rights reserved

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OutlineOutline Multiple access mechanismsMultiple access mechanisms Random accessRandom access Controlled accessControlled access ChannelizationChannelization

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Sublayers of Data Link Sublayers of Data Link LayerLayer

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Multiple Access Multiple Access MechanismsMechanisms

Random AccessRandom Access

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Random AccessRandom Access Also called Also called contention-basedcontention-based access access No station is assigned to control No station is assigned to control

anotheranother

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ALOHA NetworkALOHA Network

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Frames in Pure ALOHAFrames in Pure ALOHA

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ALOHA ProtocolALOHA Protocol

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ExampleExample Calculate possible values of Calculate possible values of TTBB when when

stations on an ALOHA network are a stations on an ALOHA network are a maximum of 600 km apartmaximum of 600 km apart

TTpp = (600 × 10 = (600 × 1033) / (3 × 10) / (3 × 1088) = 2 ms) = 2 ms

When When KK=1, =1, TTBB {0ms,2ms} {0ms,2ms} When When KK=2, =2, TTBB {0ms,2ms,4ms,6ms} {0ms,2ms,4ms,6ms} ::

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ALOHA: Vulnerable TimeALOHA: Vulnerable Time

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ALOHA: ThroughputALOHA: Throughput Assume number of stations trying to Assume number of stations trying to

transmit follow Poisson Distributiontransmit follow Poisson Distribution The throughput for pure ALOHA isThe throughput for pure ALOHA is

S = G × eS = G × e−2G−2G

where G is the average number of where G is the average number of frames requested per frame-timeframes requested per frame-time

The maximum throughputThe maximum throughput SSmaxmax = 0.184 when G= 1/2 = 0.184 when G= 1/2

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ExampleExample A pure ALOHA network transmits A pure ALOHA network transmits

200-bit frames on a shared channel 200-bit frames on a shared channel of 200 kbps. What is the throughput of 200 kbps. What is the throughput if the system (all stations together) if the system (all stations together) producesproduces 1000 frames per second1000 frames per second 500 frames per second500 frames per second 250 frames per second250 frames per second

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Slotted ALOHASlotted ALOHA

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Slotted ALOHA: Vulnerable Slotted ALOHA: Vulnerable TimeTime

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Slotted ALOHA: Slotted ALOHA: ThroughputThroughput The throughput for Slotted ALOHA isThe throughput for Slotted ALOHA is

S = G × eS = G × e−G−G

where G is the average number of where G is the average number of frames requested per frame-timeframes requested per frame-time

The maximum throughputThe maximum throughput SSmaxmax = 0.368 when G= 1 = 0.368 when G= 1

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ExampleExample A Slotted ALOHA network transmits A Slotted ALOHA network transmits

200-bit frames on a shared channel 200-bit frames on a shared channel of 200 kbps. What is the throughput of 200 kbps. What is the throughput if the system (all stations together) if the system (all stations together) producesproduces 1000 frames per second1000 frames per second 500 frames per second500 frames per second 250 frames per second250 frames per second

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CSMACSMA CCarrier arrier SSense ense MMultiple ultiple AAccessccess

"Listen before talk""Listen before talk" Reduce the possibility of collisionReduce the possibility of collision

But cannot completely eliminate itBut cannot completely eliminate it

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Collision in CSMACollision in CSMA

B

C

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CSMA: Vulnerable TimeCSMA: Vulnerable Time

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Persistence MethodsPersistence Methods What a station does when channel is idle or What a station does when channel is idle or

busybusy

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Persistence MethodsPersistence Methods

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CSMA/CDCSMA/CD CCarrier arrier SSense ense MMultiple ultiple AAccess with ccess with

CCollision ollision DDetectionetection Station monitors channel while Station monitors channel while

sending a framesending a frame

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Energy LevelsEnergy Levels

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CSMA/CD: Minimum Frame CSMA/CD: Minimum Frame SizeSize Each frame must be large enough for a Each frame must be large enough for a

sender to detect a collisionsender to detect a collision Worst case scenario:Worst case scenario:

"A" is transmitting"A" is transmitting "D" starts transmitting just before A's signal "D" starts transmitting just before A's signal

arrivesarrivesA B C D

Long enough tohear colliding signalfrom D

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ExampleExample A CSMA/CD network has a bandwidth A CSMA/CD network has a bandwidth

of 10 Mbps. If the maximum of 10 Mbps. If the maximum propagation time is 25.6 μs, what is propagation time is 25.6 μs, what is the minimum size of the frame?the minimum size of the frame?

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CSMA/CD: Flow DiagramCSMA/CD: Flow Diagram

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CSMA/CACSMA/CA CCarrier arrier SSense ense MMultiple ultiple AAccess with ccess with

CCollision ollision AAvoidancevoidance Used in a network where collision Used in a network where collision

cannot be detectedcannot be detected E.g., wireless LANE.g., wireless LAN

IFS – Interframe Space

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CSMA/CA: Flow DiagramCSMA/CA: Flow Diagram

contention window size is 2K-1

After each slot:- If idle, continue counting- If busy, stop counting

Controlled AccessControlled Access

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Control AccessControl Access A station must be authorized by A station must be authorized by

someone (e.g., other stations) before someone (e.g., other stations) before transmittingtransmitting

Three common methods:Three common methods: ReservationReservation PollingPolling Token passingToken passing

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Reservation MethodReservation Method

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Polling MethodPolling Method

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Token PassingToken Passing

ChannelizationChannelization

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ChannelizationChannelization Similar to Similar to multiplexingmultiplexing Three schemesThree schemes

Frequency-Division Multiple Access Frequency-Division Multiple Access (FDMA)(FDMA)

Time-Division Multiple Access (TDMA)Time-Division Multiple Access (TDMA) Code-Division Multiple Access (CDMA)Code-Division Multiple Access (CDMA)

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FDMAFDMA

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TDMATDMA

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CDMACDMA One channel carries all transmissions One channel carries all transmissions

at the same timeat the same time Each channel is separated by Each channel is separated by codecode

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CDMA: Chip SequencesCDMA: Chip Sequences Each station is assigned a unique Each station is assigned a unique chip chip

sequencesequence

Chip sequences are Chip sequences are orthogonal vectorsorthogonal vectors Inner productInner product of any pair must be zero of any pair must be zero

With N stations, sequences must have the With N stations, sequences must have the following properties:following properties: They are of length NThey are of length N Their self inner product is always NTheir self inner product is always N

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CDMA: Bit CDMA: Bit RepresentationRepresentation

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Transmission in CDMATransmission in CDMA

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CDMA EncodingCDMA Encoding

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Signal Created by CDMASignal Created by CDMA

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CDMA DecodingCDMA Decoding

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Sequence GenerationSequence Generation Common method: Common method: Walsh TableWalsh Table

Number of sequences is always a power Number of sequences is always a power of twoof two

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Example: Walsh TableExample: Walsh Table Find chip sequences for eight Find chip sequences for eight

stationsstations

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Example: Walsh TableExample: Walsh Table There are 80 stations in a CDMA There are 80 stations in a CDMA

network. What is the length of the network. What is the length of the sequences generated by Walsh sequences generated by Walsh Table?Table?