MACAW: A media access protocol for wireless LANs

This paper reads like a cookbook of modifications made to a previously proposed media access protocol, MACA, to improve both total media utilization and "fairness". While media utilization is fairly well-defined, as the authors pointed out, its not quite clear in what form "fairness" should take. Should it be bandwidth per transmitter, bandwidth per receiver, or bandwidth per stream? In this paper, it seems like that authors chose the last.

The structure of the paper seems to follow this pattern: point out a short-coming of MACA, fix it with a proposed modification, repeat. In fact, sometimes the proposed modifications could bring problems of its own, and yet another new modifcation is proposed. While the proposed modifications are well-thought out, I can't help but think (sitting from where I am instead of having to design a wireless protocol) if there could have been a more holistic approach, where one looks at the various use scenarios, and come up with a scheme that addresses all concerns. Of course, the process of putting together all the modifications could still be the same "poke-finger-into-holes-in-dikes" approach, but it could feel more satisfying. I do however like the style in the paper because its works very well as a tutorial.

Anyway, the summary (section 5) of this paper gives a good wrap-up of all the issues and solutions, but I'll just repeat them for completeness - in paper chronological order:

• The baseline, MACA, uses a RTS-CTS-DATA protocol, with a binary exponential backoff retransmission when CTS is not received.
• The backoff counter is copied and shared by all stations, so that congestion information is known to all.
• The backoff follows multiplicative increase and linear decrease to avoid big ocillations, while still backing off quickly
• Each transmitter maintains separate queues for each stream (or perhaps destination) each serviced by a separate backoff algorithm such that bandwidth can be equally shared by all streams in the cell.
• ACK is added (to get RTS-CTS-DATA-ACK) for fast recovery
• DS is added (to get RTS-CTS-DS-DATA-ACK) such that stations that cannot hear CTS will know that data is about to be transmitted (since RTS-CTS is successful) and defer transmission
• RRTS is added, such that when CTS cannot be sent due to other stations transmitting, the receiver can contend on behalf of the transmitter.
• A separate backoff counter is maintained for each end of each stream but copied in such a way that all stations trying to talk with a receiver has the same backoff value. This is done to reduce leakage of backoff counters across cells to allow high utilization in each cell.