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Multiple AccessMultiple Access
01204325: Data 01204325: Data Communication and Communication and Computer NetworksComputer Networks
Asst. Prof. Chaiporn Jaikaeo, Ph.D.Asst. Prof. Chaiporn Jaikaeo, [email protected]
http://www.cpe.ku.ac.th/~cpjhttp://www.cpe.ku.ac.th/~cpjComputer Engineering DepartmentComputer Engineering Department
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
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
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
"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
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?