Fundamentals of Computer NetworksECE 478/578

Lecture #10: Multiple Access in Wireless NetworksInstructor: Loukas Lazos

Dept of Electrical and Computer EngineeringUniversity of Arizona

Wireless Access ProtocolsEach node has a fixed rangeNodes use omni-directional antennas

Can we use CSMA?What about reservations?Problem: Feedback Mechanism

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The Hidden Terminal Problem

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A B C D

The Exposed Terminal Problem

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A B C D

Multiple Access with Collision Avoidance

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A B C DRTS

CTS

Addressing the Hidden Terminal Problem

Multiple Access with Collision Avoidance

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A B C DCTS

RTS

RTS

CTS

Addressing the Exposed Terminal Problem

Distributed Coordination Function

7t

DIFS

data

defer access

otherstations

receiver

senderdata

DIFS

new contention

RTS

CTSSIFS SIFS

NAV (RTS)NAV (CTS)

Senses channel for DIFS (Distributed Interframe Space)

If busy backoff random time, else

Sender sends RTS with NAV (Network Allocation Vector)

Receiver acknowledges via CTS after SIFS (Short interframe space)CTS reserves channel for sender, notifying possibly hidden stations;

any station hearing CTS should be silent for NAV

Sender can now send data at once

Can Collisions be useful?Zig-zag decoding

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Implementation IssuesFinding the beginning of each packet in a collision

Use the preamble to find the start of each packet

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Implementation IssuesObtaining matching collisions

Use the correlation to find the start of Pb, then use the header bits to ensure that Pb’ = Pb

Resolving more than two collisions

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What if Multiple Channels are Available?

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A B C DCTS (1)

RTS (1, 2, 5, 7)

ACK (1)

D CTS (2)

RTS (2, 5, 7)

ACK (2)

Possible SolutionForce everyone to switch to a common control channel for negotiation of channel assignmentDelay in the channel assignment, wasted bandwidthThroughput of control channel becomes a bottleneck

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