In recent years, whether autonomous monitoring of ocean environment, deep seabed resources surveying, underwater sensor networks, or military underwater detection networks, etc, there are widespread needs of underwater acoustic networks (UANs). This paper provides a brief overview of the characteristics and advances of UANs, and the main research contents and key technologies of UANs are analyzed, including physical layer technologies, media access control (MAC) protocols and routing protocols. Underwater acoustic communication (UWAC) in the physical layer is the basis for UANs, and its research covers multi-frequency shift keying (MFSK), multi-phase shift keying (MPSK), direct-sequence spread spectrum (DSSS), and orthogonal frequency division multiplexing (OFDM). Common MAC protocols include Time Division Multiple Access (TDMA), ALOHA and MACAW (MACA for Wireless), and are all used in UANs. Routing protocols involve static routing, hybrid routing, and self-organizing routing. The experimental research on underwater acoustic network in the Sanya area is introduced in detail. The network was composed of 15 nodes developed by three institutes. The modems of the nodes deployed used UWAC technology, such as MFSK, MPSK and OFDM. Each network node installed TD, CTD, acoustic Doppler current profiler (ADCP) or other ocean monitoring equipment, gateway was connected with the shore station via radio, and the server of the shore station was connected to the Internet. The network was running for 43 days, and online monitoring of the ocean environment was realized. This paper gives analysis on packet loss ratio, transmission delay and network energy efficiency of the nodes developed by the Hangzhou Applied Acoustics Research Institute. The node packet loss ratio was 2.9%, the average delay was 0.947 minute per hop, when the packet size was 404 bits, and the energy efficiency was 0.7831 bit/J.
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