Comparison and Evaluation of the T-Lohi MAC for Underwater Acoustic Sensor Networks

This paper introduces T-Lohi, a new class of distributed and energy-efficient media-access protocols (MAC) for underwater acoustic sensor networks (UWSN). MAC design for UWSN faces significant challenges. For example, acoustic communication suffers from latencies five orders-of-magnitude larger than radio communication, so a naive CSMA MAC would require very long listen time resulting in low throughput and poor energy efficiency. In this paper, we first identify unique characteristics in underwater networking that may affect all MACs, such as space-time uncertainty and deafness conditions. We then develop T-Lohi employing a novel tone-based contention resolution mechanism that exploits space-time uncertainty and high latency to detect collisions and count contenders, achieving good throughput across all offered loads. T-Lohi exploits a low-power wake-up receiver to significantly reduce energy consumption. We evaluate design choices and protocol performance through extensive simulation. Finally, we compare T-Lohi against a few canonical MAC protocols. The results show that the energy cost of packet transmission is within 3-9% of optimal, and that Lohi achieves good channel utilization, within 30% utilization of the theoretical maximum. We also show that Lohi is stable and fair under both low and very high offered loads. Finally, we compare Lohi with other alternatives, including TDMA, CSMA, and ALOHA. Except for TDMA under heavy load, Lohi provides the best utilization in all cases, and it is always the most energy efficient.