Medium Access for Underwater Acoustic Sensor Networks

The underwater acoustic channel has many fundamentally different properties to the conventional radio-based wireless channel. Communication is changed drastically by acoustic propagation speeds that are five orders of magnitude slower than radio. This, coupled with bandwidth limitations, high transmit energy cost, complex multi-path effects, and high bit-error rates make the issue of medium access control (MAC) in an acoustic medium a challenging problem. Recent work has begun to explore how multi-hop communication over short distances (500m or less) can reduce many of the channel complexities and improve energy efficiency (for example, see [1]), but multi-hop communication requires channel coordination. Our current work is therefore focused on understanding the challenges and potential solutions in MAC design for such short-range acoustic underwater sensor networks. Prior work on MAC for underwater acoustic networks has developed code distribution techniques with CDMA [6]. Recent work has extended radio-based sensor network MAC approaches to underwater for a specific class of applications [3]. More recent work has adapted CSMA techniques for underwater networks [2]. However we believe that the opportunities in underwater MACs have not been fully explored, particularly in low-cost, short-range networks. In this abstract, we present a reservation based MAC protocol, called Tone Lohi (T-Lohi). (Lohi aptly means “slow” in Hawaiian.) The T-Lohi MAC introduces two novel ideas. The first is to detect and count the number of contenders during the reservation and use this information in building a traffic adaptive back-off algorithm. The slow propagation enables nodes to detect and count contenders, as long as the contention packets occupy the channel with a duration much less than the propagation delay. The above ability is a major paradigm shift from the collision avoidance (CA) mechanism in terrestrial wireless net-