Abstract
Underwater sensor networks have become increasingly interesting in the
past four decades. They can be used in a multitude of scenarios,
commercial and military alike. Underwater networks can communicate in
several ways, but when nodes are far apart, underwater acoustic
communication is the only feasible way. The complex underwater acoustic
channel puts high demands on the network protocols. The physical layer
needs to contend with short coherence times, high intersymbol
interference and significant Doppler spread. The routing protocol needs
to handle intermittent connectivity and mobile network topologies, such
as autonomous underwater vehicle networks. The medium access control
protocol needs to manage medium access with high latency and potentially
high packet loss ratios without congesting the network. The available
acoustic modems are still rather expensive, which limits the size of a
sensor network. Voices have also been raised from the academia for a
paradigm shift, from hardware-defined, proprietary modems to
software-defined, open-architecture modems, in order to accelerate
research in the field and enable interoperability. This paper reviews
the recent advancements in designing and implementing underwater
networks on several levels and discusses some interesting approaches to
underwater ad-hoc networking. The focus lies on acoustic
communication.