Hydro-acoustic communications and networking in contemporary underwater robotics: instruments and case studies

This chapter describes the results of an easy-to-achieve implementation of an underwater acoustic (UWA) sensor network capable of operating in different modes for the exchange of UWA data among mobile and/or stationary network nodes that may include autonomous surface vehicles, autonomous underwater vehicles (AUVs), remotely operated vehicles, and benthic stations. The data exchanged are crucial in a number of applications that include cooperative multiple vehicle navigation and control, mission status assessment, and environmental sensing. A key novelty of the implementation described is the use of UWA modems incorporating the EviNS dedicated Networking Software Framework. The latter is a compact open-source, open-architecture software that is undemanding in terms of computer resources and can be installed directly on a UWA modem platform as part of its standard software. The practical usefulness and advantage of this solution stems from the fact that it significantly reduces the costs involved in setting up a UWA network while retaining small dimensions, small weight, and high energy efficiency of each hydro-acoustic network node (e.g. modems performing the functions of network communication devices). Several case studies with experimental networks based on UWA modems incorporating the EviNS Framework, with pre-installed but freely changeable mediaaccess and routing protocols, are presented in this chapter. Experimental results aimed at assessing the performance of the UWA network herein described are presented in relation to the combination of simple medium-access control protocols (providing uncoordinated access), wide-spread flooding-based routing protocols (based on so-called sequence number control), and polling protocols (implementing sequential and broadcast communications in networks with a centralized topology). Apart from providing details on the application of the EviNS Framework, the chapter offers a detailed analysis of the performance of an ad-hoc underwater sensor network operating in a shallow water area, i.e. in a hydro-acoustic environment of large practical interest. The chapter provides also a thorough analysis of the communication performance achievable with multiple nodes in a network with a centralized topology and affords the reader details of the practical estimation of the data rates achievable in communications involving multiple underwater modems performing simultaneous and asynchronous communications as elements of a mobile network. Even though the experiments reported were limited to specific cases, the number of network nodes used, the network geometries, and the fact that the underwater networks involved operated in complicated hydro-acoustic environments capture frequently occurring combinations of circumstances that occur in real off-shore practice.