Transcript Fundamentals of Computer Networks ECE 478/578 Lecture #10: Multiple Access in Wireless Networks Instructor: Loukas Lazos Dept of Electrical and Computer Engineering University of Arizona.

Fundamentals of Computer Networks
ECE 478/578
Lecture #10: Multiple Access in Wireless Networks
Instructor: Loukas Lazos
Dept of Electrical and Computer Engineering
University of Arizona
Wireless Access Protocols
Each node has a fixed range
Nodes use omni-directional antennas
Can we use CSMA?
What about reservations?
Problem: Feedback Mechanism
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The Hidden Terminal Problem
A
B
C
D
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The Exposed Terminal Problem
A
B
C
D
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Multiple Access with Collision Avoidance
A
RTS
B
C
D
CTS
Addressing the Hidden Terminal Problem
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Multiple Access with Collision Avoidance
A
CTS
RTS
B
C
RTS
Addressing the Exposed Terminal Problem
CTS
D
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Distributed Coordination Function
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
DIFS
sender
data
RTS
SIFS
receiver
other
stations
CTS SIFS
NAV (RTS)
NAV (CTS)
defer access
DIFS
new contention
data
t
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Can Collisions be useful?
Zig-zag decoding
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Implementation Issues
Finding the beginning of each packet in a collision
Use the preamble to find the start of each packet
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Implementation Issues
Obtaining 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?
ACK (1)
D
A
CTS (1)
ACK (2)
B
RTS (1, 2, 5, 7)
C
CTS (2)
D
RTS (2, 5, 7)
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Possible Solution
Force everyone to switch to a common control channel for
negotiation of channel assignment
Delay in the channel assignment, wasted bandwidth
Throughput of control channel becomes a bottleneck
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