1. 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

2. Wireless Access Protocols
Each node has a fixed range Nodes use omni-directional antennas Can we use CSMA? What about reservations? Problem: Feedback Mechanism

3. The Hidden Terminal ProblemC D

4. The Exposed Terminal ProblemC D

5. Multiple Access with Collision Avoidance
RTS A B C D
CTS
Addressing the Hidden Terminal Problem

6. Multiple Access with Collision Avoidance
RTS
CTS A B C D
CTS
RTS
Addressing the Exposed Terminal Problem

7. 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
RTS
data
sender
SIFS
CTS
SIFS
receiver
NAV (RTS)
DIFS
data
other
stations
NAV (CTS) t
defer access
new contention

8. Can Collisions be useful?
Zig-zag decoding

9. Implementation Issues
Finding the beginning of each packet in a collision
Use the preamble to find the start of each packet

10. 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

11. What if Multiple Channels are Available?
ACK (1)
ACK (2) D A B C D
CTS (1)
CTS (2)
RTS (1, 2, 5, 7)
RTS (2, 5, 7)

12. 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