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7/30/2019 12-MultipleAccess (1)

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Multiple Access

01204325: Data Communication

and Computer Networks

Asst. Prof. Chaiporn Jaikaeo, [email protected]

http://www.cpe.ku.ac.th/~cpj

Computer Engineering DepartmentKasetsart University, Bangkok, ThailandAdapted from lecture slides by Behrouz A. Forouzan The McGraw-Hill Companies, Inc. All rights reserved
mailto:[email protected]:[email protected]


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Outline

Multiple access mechanisms Random access

Controlled access

Channelization


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Multiple Access Mechanisms


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Random Access


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Random Access

Also called contention-based access No station is assigned to control another


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ALOHA Network


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Frames in Pure ALOHA


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ALOHA Protocol


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ALOHA: Vulnerable Time


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ALOHA: Throughput

Assume number of stations trying totransmit follow Poisson Distribution

The throughput for pure ALOHA is

S = G e2Gwhere G is the average number of framesrequested per frame-time

The maximum throughput Smax = 0.184 when G= 1/2


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Example

A pure ALOHA network transmits 200-bitframes on a shared channel of 200 kbps.What is the throughput if the system (allstations together) produces

1000 frames per second




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Slotted ALOHA


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Slotted ALOHA: Vulnerable Time


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Slotted ALOHA: Throughput

The throughput for Slotted ALOHA is

S = G eG

where G is the average number of framesrequested per frame-time

The maximum throughput Smax = 0.368 when G= 1


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Example

A Slotted ALOHA network transmits 200-bit frames on a shared channel of 200kbps. What is the throughput if the system(all stations together) produces





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CSMA

Carrier Sense MultipleAccess "Listen before talk"

Reduce the possibility of collision

But cannot completely eliminate it


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Collision in CSMA


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Persistence Methods

What a station does when channel is idle or busy


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Persistence Methods


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CSMA/CD

Carrier Sense MultipleAccess withCollision Detection

Station monitors channel when sending a

frame


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Energy Levels


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CSMA/CD: Minimum Frame Size

Each frame must be large enough for a senderto detect a collision

Worst case scenario:

"A" is transmitting

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

A B C D

Long enough to

hear colliding signalfrom D


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Example

A CSMA/CD network has a bandwidth of10 Mbps. If the maximum propagationtime is 25.6 s, what is the minimum sizeof the frame?


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CSMA/CA

Carrier Sense MultipleAccess withCollisionAvoidance

Used in a network where collision cannot

be detected E.g., wireless LAN

IFS Interframe Space


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Control Access

A station must be authorized by someone(e.g., other stations) before transmitting

Three common methods:

Reservation Polling

Token passing


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Reservation Method


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Polling Method


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Channelization


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FDMA


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TDMA


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CDMA

One channel carries all transmissions atthe same time

Each channel is separated by code


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CDMA: Chip Sequences

Each station is assigned a unique chip sequence

Chip sequences are orthogonal vectors

Inner product of any pair must be zero

With N stations, sequences must have the

following properties: They are of length N

Their self inner product is always N


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CDMA: Bit Representation


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Transmission in CDMA


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Signal Created by CDMA


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Sequence Generation

Common method: Walsh Table Number of sequences is always a power of two


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