A Location Service Protocol for mobile Underwater Sensor Networks

The Underwater Sensor Network (UWSN) architecture has been proposed to explore the ocean to support solutions for time-critical aquatic applications such as submarine tracking and harbor monitoring. Assume that a large number of underwater sensor nodes are air-dropped to the venue of interest to create a SEA Swarm (Sensor Equipped Aquatic Swarm). Each node is equipped with a low bandwidth acoustic modem and with various sensors. It can dynamically control its depth through a fish-like bladder apparatus and a pressure gauge. The purpose is to implement a device that provides an efficient, scalable, and robust "location service" for the SEA Swarm. In general, a location service protocol maintains a set of location servers for a given node. These servers are updated and consulted for location updates and queries. In a SEA Swarm, however, it is non-trivial to adopt Hierarchical schemes such as HIGH-GRADE and Quorum-based scheme such as XYLS, because the entire swarm "moves" along water current. Since high-frequency "radio" signals are quickly absorbed by water, underwater networking must rely on an underwater acoustic channel that has low bandwidth and large propagation latency. An acoustic data transmission consumes much more energy than terrestrial microwave data communications. Moreover, such drastic reduction in communication bandwidth coupled with high latency makes the whole network vulnerable to congestion due to packet collisions. Consequently, minimizing the number of packet transmissions is a very important criterion for protocol design. The results of the proposed design show that "Trail" protocol yields better performance than the existing approaches.