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

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

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

Background PARC CSMA MACA MACAW Design and Terminology

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

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

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

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

Does DS work in this scenario?

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

RRTS isn’t always the answer…

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

BO Algorithm Revisited – Leakage

BO Algorithm Revisited – Interference

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

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

3 Cell Scenario with Non-uniform Congestion

3 Cell Congestion Results MACAW yields 37% improvement in throughput

5 Cell Scenario with Noise

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

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

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