1. Multiple Access, Collision Avoidance for Wireless LAN’s
Preston Burden
Chris Meullion
Dwight Philpotts
John C. Jones-Walker

2. Introduction MACA – proposed by Karn and Biba
Modifies packet exchange method
Utilizes an alternative backoff (BO) algorithm

3. Motivation Xerox Coporation’s Palo Alto Research Center (PARC)
CSMA, CSMA/CA, MACA vs. MACAW

4. Background
PARC
CSMA
MACA
MACAW Design and Terminology

5. Backoff Algorithm Binary Exponential Backoff (BEB) BEB Copy
Multiplicative Increase Linear Decrease (MILD)

6. Multiple Stream Model Single packet queue per station
Fair?
Single packet queue per stream
Fair allocation of bandwidth
Throughput

7. Acknowledgement (ACK)
Link layer reliability
Effects of noise
probability of improper reception of packets
Throughput

8. Data Sending (DS) Exposed terminal problem No Carrier Sensing hardware
Unheard CTS
No Carrier Sensing hardware
Throughput

9. Does DS work in this scenario?

10. Request for Request to Send (RRTS)
Inability to reply to RTS due to deferral
Replies to initial RTS
Neighbor stations defer for 2 slot times
Sender replies with RTS
Throughput
Fair access to media

11. RRTS isn’t always the answer…

12. Multicast Multiple receivers for a packet No RTS-CTS exchange
RTS-DATA exchange
No resolution

13. BO Algorithm Revisited – Leakage

14. BO Algorithm Revisited – Interference

15. BO Algorithm Revisited – Per-destination BO Copying
Station specific congestion information shared amongst sending stations
Transmission BO value reflects congestion at destination and source

16. Preliminary Evaluation of MACAW
MACA achieves 84% channel capacity
MACAW achieves 78% channel capacity
MACAW performs better under congestion and noise?

17. 3 Cell Scenario with Non-uniform Congestion

18. 3 Cell Congestion Results
MACAW yields 37% improvement in throughput

19. 5 Cell Scenario with Noise

20. 5 Cell Results Yields 13% improvement over MACA
Channel usage is much more fair

21. Future Design Issues Better multicast design scheme
Complexity of BO algorithm
Power variations
Support for asynchronous/synchronous service
Only benefits congested networks

22. Summary Assumptions RTS-CTS-DS-DATA-ACK exchange
Congestions is at receiver not sender
Arrangement of pads and base stations is symmetric
RTS-CTS-DS-DATA-ACK exchange
Greater efficiency and fairness