MACAW: A Media Access Protocol for Wireless Lans's

The authors present their design for a media access control protocol to a shared 256kbps channel. Their overriding design consideration is fairness, and not throughput which is a decisions which appears at numerous points in the paper. They begin with a typical RTS-CTS-DATA scheme very similar to CSMA/CA and then consider a number of modifications to it to address fairness; the sender can contend for the receiver using RRTS, the data can use link-level recovery with ACKs, or the sender can acknowledge the CTS message with a DS message before sending the payload.

The design of a workable MAC is somewhat complicated, and its design will effect power consumption, throughput, latency, and reliability. Of these axis, the authors did a reasonable job evaluating throughput; however the others are not well treated (although low latency can be a function of fair sharing between senders). My main objection to their MAC is the high protocol overhead in the RTS-CTS-DS-DATA-ACK exchange, which occurs even the absence of channel contention. What we have found in our own research is that the authors are exactly right about local repair; the lossy nature of these links makes it inadvisable to just send a packet and assume it got there. However, our lesson has been that the additional packets involved in more complex coordination schemes result in a noisier channel which can be an important effect with many senders. Furthermore, if the data packets are short, the data is effectively piggybacked on the RTS; if it is received at all then no further transmissions are necessary and power consumption is reduced because of fewer sends and receives.

Several things about this paper means that our design lessons are not directly applicable. For one, they use significantly different radio technology with a "near-field" PHY and short range. Second, their workload model is significantly different (they cite a data rate at one point of 64 packets per second). Finally, all communication is to central collection points. Given these parameters, I probably would have tried to reduce contention by designing a protocol for assigning different channels to nearby receivers to reduce cross-cell interference (this should solve the RRTS problem) and evaluated weather or not the full scheme they propose is necessary in practice.

This brings us to a final criticism of the paper, which is its evaluation methodology. MAC protocols have been shown to be very sensitive to workload and environmental factors; if a protocol is designed for a specific workload and environment, it would be significantly more convincing to have implementation results for that environment then packet-level simulation. Modeling RF communication is notoriously difficult and there is still active work in conferences improving the simulation in ns-2 and TOSSIM. The implementation results, even if simplistic, are an important sanity check.