MACAW: A Media Access Protocol for Wireless LANs

This article proposes a media access protocol targetted at a particular contended wireless network. In the network environment considered by the authors, connectivity is symmetric (if A can receive from B, B can from A) and relatively short range with a sharp strength drop off. Thus, the primary efficiency concerns with implementing wireless in the authors environment are collisions at the receiver, including from signals out of range of the transmitter.

The hidden terminal problem makes carrier sense inappropriate. Instead, the authors explicit Request-to-Send and Clear-to-Send messages to manage the channel. Additionally the authors add an explicit acknowledgment (to guard against giving the false congestion signals to TCP-like protocols) and an explicit Data-Sending message (so other nodes within range of the transmitter can avoid interfering with its reception of the CTS and ACK).

Nodes control their use of the network based on these messages. A CTS or DS message reserves the channel for long enough to send the corresponding data and its acknowledgment. Based on past apparent collisions and their (apparent) locations, nodes backoff the rate at which they send RTSs, separately estimating congestion at their location and at the receiver.

The authors evaluated the MAC protocol with simulations of a variety of network topologies, verifying that reasonably fairness between data streams was provided by the MAC protocol and good link utilization was achieved (even in the presence of hidden terminals, unresponsive nodes, etc.) Most of the simulations used loss-insensitive flows and assumed the reception success was dependent entirely on distance from the receiver and competing transmitters.

It is very nice that the authors evaluated many network topologies and these clearly evaluations where the author's improvements produce huge differences. However, the paper seems to lack evaluation of more realistic scenarios. Even though the authors ACK scheme is motivated by transport protocol performance, almost none of the authors simulations consider performance of a TCP-like transport protocol or even how intermittent noise affects their link-layer protocol alone. The authors also do not consider any metric besides stream throughput, such as how much jitter and latency this MAC protocol introduces into the network.