MAC Protocol Design for Underwater Networks: Challenges and New Directions

In this chapter, we focus on acoustic underwater networks, which utilize acoustic communication links to communicate between the nodes. It is wellknown that RF signals do not propagate well underwater, and the optical signals [1] are useful for short distances and under clear water propagation conditions. For long distances, the Navy has utilized acoustic communication. More recently, there has been a growing emphasis in the research community on underwater ad hoc and sensor networks [2]–[6], which utilize acoustic communication over relatively short distances of about 100 meters. The advantages of such short distance communication are numerous: First, the strong multipath profile [7] that exists in the traditional applications over much longer distances is significantly reduced, thereby increasing the data rate at which the communication can take place. Second, the propagation delays are reduced. This has important consequences for the design of MAC protocols because achieving a rough synchronization between the nodes becomes possible. Third, short distances save battery energy [8], which is especially important for underwater sensor networks. For example, in scientific data collection applications [9] in which the batteries are expected to last for months under difficult conditions, the battery energy becomes the most important constraint. In contrast with terrestrial communications, underwater acoustic communications is affected by both distance-dependent path loss, as well as frequency-dependent absorp-