CS 313 Introduction to Computer Networking & Telecommunication

Chi-Cheng Lin, Winona State University

CS 313 Introduction to Computer Networking &

Telecommunication

Medium Access Control Sublayer

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Topics Introduction

Channel Allocation Problem

Multiple Access Protocols

CDMA

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Introduction Broadcast networks

Key issue: who gets to use the channel when there is competition

Referred to asMultiaccess channelsRandom access channels

MAC (Medium Access Control) sublayerLANsWireless networksSatellite networks

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Channel Allocation Problem Channel Allocation

StaticDynamic

Performance factorsMedium access delay

Time between a frame is ready and the frame can be transmitted

Throughput#frames can be transmitted in unit time

interval

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Static Channel Allocation FDM

Bandwidth divided into N equal sized portions for N users

Problems#senders large#senders continuously variesbursty traffic

Discussion: #users > N ? < N ? = N ?N times worse than all frames queued in one big

queue

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Static Channel Allocation TDM

Each user is statically allocated every Nth time slot

Same problems as FDM

Under what circumstances are static channel allocation schemes efficient?

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Dynamic Channel Allocation Key assumptions

1. Station modelIndependentWork is generated constantlyOne program per stationStation is blocked once a frame has been

generated until the frame has been successfully transmitted

2. Single channel assumption

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Dynamic Channel Allocation Key assumptions

3. Collision AssumptionCollision:

Two frames are transmitted simultaneously, overlapped in time and resulting signal garbled

Can be detected by all stationsNo other errors

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Dynamic Channel Allocation Key assumptions

4. Time: either continuous or discrete (slotted)Continuous

Frame transmission can begin at any instant No "master clock" needed

Slotted Time divided into discrete intervals (slots) Frame transmissions begin at the start of a slot #frames contained in a slot: 0 ? 1 ? >1 ?

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Dynamic Channel Allocation Carrier sense ("carrier" refers to

electrical signal): either Y or NYes

A station can check channel before transmission

If busy, station idleWired LANs

No“Just do it"Can tell if transmission successful laterWireless networks, cable modems

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Multiple Access Protocols ALOHA Carrier sense multiple access

protocols (CSMA) CSMA w/ collision detection

(CSMA/CD) Collision-free protocols Limited-contention protocols

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ALOHA Applicable to any contention

systemSystem in which uncoordinated users

are competing for the use of a single shared channel

Two versionsPure ALOHASlotted ALOHA

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Pure ALOHA Let users transmit whenever they have data to

be sent Colliding frames are destroyed Sender can always find out destroyed or not

Feedback (property of broadcasting) or ACKLANs: immediatelySatellites: propagation delay (e.g., 270msec)

By listening to the channelIf frame is destroyed wait a random amount of time and retransmit (why "random"?)

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Pure ALOHA

Where are the collisions?

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Slotted ALOHA Discrete time Agreed slot boundaries Synchronization needed Performance

Which ALOHA has a shorter medium access delay?

Which ALOHA has a higher throughput?

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Performance of ALOHA Slotted ALOHA can double the

throughput of pure ALOHA

Throughput versus offered traffic for ALOHA systems.

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Carrier Sense Multiple Access (CSMA) Protocols

Stations can listen to the channel (i.e., sense a carrier in the channel)

Types1-persistent CSMANonpersistent CSMAp-persistent CSMA

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Comparison of the channel utilization versus load for various random access protocols.

Performance of MAC Protocols

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CSMA w/ Collision Detection (CSMA/CD)

Can listen to the channel and detect collisionStop transmitting as soon as collision

detected Widely used on LANs (e.g.,

Ethernet) Collision detection

Analog processSpecial encoding is used

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CSMA w/ Collision Detection (CSMA/CD)

Conceptual model3 states

ContentionTransmissionIdle

Minimum time to detect collision determines time slotDepends on propagation delay of

medium

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CSMA/CD Model

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CSMA/CD Algorithm

Source: http://www.10gea.org/gigabit-ethernet/

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Collision-Free Protocols Model

N Stations: 0,1, ..., (N-1) Question

Which station gets the channel after a successful transmission?

ProtocolsBit-map (i.e., reservation) protocolToken passing protocol

Example: Token ring

Collision-Free Protocol

Token ring

Station

Direction oftransmission

Token

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Performance of Contention and Collision-Free Protocols

ContentionLow load => low medium access delay

:)High load => low channel efficiency :(

Collision-FreeLow load => high medium access

delay :(High load => high channel

efficiency :)

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Summary of Channel Allocation Methods/Systems

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* | Token Passing | Contention-free protocol |