In the last two decades, underwater acoustic communications has experienced significant progress. Communication systems with increased bit-rate and reliability now enable real-time point-to-point links between underwater nodes such as ocean bottom sensors and autonomous underwater vehicles (AUVs). Current researches focused on combining various point-to-point links within a network structure to meet the emerging demand for applications such as environmental data collection, offshore exploration, pollution monitoring, and military surveillance. The traditional approach for ocean-bottom or ocean-column monitoring is to deploy oceanographic sensors, record the data, and recover the instruments. This approach has several disadvantages:
? The recorded data cannot be recovered until the end of the mission, which can be several months.
? There is no interactive communication between the underwater instrument and the onshore user. Therefore, it is not possible to reconfigure the system as interesting events occur.
? If a failure occurs before recovery, data acquisition may stop or all the data may be lost.
The ideal solution for real-time monitoring of selected ocean areas for long periods of time is to connect various instruments through wireless links within a network structure. Basic underwater acoustic networks are formed by establishing bidirectional acoustic communication between nodes such as autonomous underwater vehicles (AUVs) and fixed sensors. The network is then connected to a surface station, which can further be connected to terrestrial networks such as the Internet, through an Ri link. Onshore users can extract real-time data from multiple underwater instruments. After evaluating the obtained data, they can send control messages to individual instruments. Since data is not stored in the underwater instruments, data loss is prevented as long as isolated node failures can be circumvented by reconfiguring the network. A major constraint of underwater acoustic (UWA) networks is the limited energy supply. Whereas the batteries of a wireless modem can easily be replaced on land-based systems, the replacement of an underwater modem battery involves ship time and the retrieval of the modem from the ocean bottom which is costly and time consuming. Therefore, transmission energy is precious in underwater applications. Network protocols should conserve energy by reducing the number of retransmissions, powering down between transactions, and minimizing the energy required for transmission.
Some underwater applications require the network to be deployed quickly without substantial planning, such as in rescue and recovery missions. Therefore the network should be able to determine the node locations and configure itself automatically to provide an efficient data communication environment. Also, if the channel conditions change or some of the nodes fail during the mission, the network should be capable of reconfiguring itself dynamically to continue its operation.
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