Optimization of a data link protocol for an underwater acoustic channel

Acoustic modems typically operate in half-duplex, which limits the choice of a data link control protocol to the Stop and Wait (S&W) type. Unfortunately, on channels with poor quality and long propagation delay-such as the majority of acoustic channels-S&W protocol has low throughput efficiency. The basic S&W can be improved by using a modification in which packets are transmitted in groups and acknowledged selectively. Throughput efficiency can now be maximized by selecting the optimal packet size, which is a function of range, rate, and error probability. Quantitative analysis for typical acoustic links shows that modified S&W protocols offer good performance, provided that packet size is chosen close to optimal. In addition, as the group size increases, sensitivity to packet size selection is reduced. To ensure best ARQ performance in mobile acoustic systems where link conditions vary with time, future generation of acoustic modems must focus on adaptive selection of protocol parameters.