Abstract
The extreme conditions under which multi-hop underwater acoustic sensor networks (UASNs) operate constrain the performance of medium access control (MAC) protocols. The MAC protocol employed significantly impacts the operation of the network supported, and such impacts must be carefully considered when developing protocols for networks constrained by both bandwidth and propagation delay.
Time-based coordination, such as TDMA, have limited applicability due to the dynamic nature of the water channel used to propagate the sound signals, as well as the significant effect of relatively small changes in propagation distance on the propagation time. These effects cause inaccurate time synchronization and therefore make time-based access protocols less viable. The large propagation delays also diminish the effectiveness of carrier sense protocols as they do not predict with any certainty the status of the intended recipients at the point when the traffic would arrive. Thus, CSMA protocols do not perform well in UASNs, either.
Reservation-based protocols have seldom been successful in commercial products over the past 50 years due to many drawbacks, such as limited scalability, relatively low robustness, etc. In particular, the impact of propagation delays in UASNs and other such constrained networks obfuscate the operation of the reservation protocols and diminish, if not completely negate, the benefit of reservations. The efficacy of the well-known RTS-CTS scheme, as a reservation-based enhancement to the CSMA protocol, is also adversely impacted by long propagation delays.
An alternative to these MAC protocols is the much less complex ALOHA protocol, or one of its variants. However, the performance of such protocols within the context of multi-hop networks is not well studied. In this paper we identify the challenges of modeling contention-based MAC protocols and present models for analyzing ALOHA and p-persistent ALOHA variants for a simple string topology. As expected, an application of the model suggests that ALOHA variants are very sensitive to traffic loads. Indeed, when the traffic load is small, utilization becomes insensible to p values. A key finding, though, is the significance of the network size on the protocols' performance, in terms of successful delivery of traffic from outlying nodes, indicating that such protocols are only appropriate for very small networks, as measured by hop count.
