Underwater Acoustic Networking Techniques

Digital underwater communications are becoming increasingly important, with numerous applications emerging in environmental monitoring, exploration of the oceans, and military missions. Until the mid-nineties, the research was focused on hardware and on communication transmitters and receivers for the transmission of raw bits. In network terminology, this is known as the physical layer. A breakthrough was achieved in the mid-nineties by Stojanovic et al. [1], who showed that phase-coherent communication is feasible by integrating a phase-locked loop into a decision-feedback equalizer [2]. Such a receiver can be applied to a single hydrophone, although robust operation at high data rates, say >1 kbit/s, generally requires the presence of a (vertical) hydrophone array for reception. Indeed, multichannel adaptive equalizers have proven to be versatile and powerful tools. If the use of a receive array is impractical, as in multinode networks, then frequency-shift keying (FSK) is often used as a fairly robust modulation for single-receiver systems [3‐5]. However, the corresponding data rates are of the order of 100 bit/s. Although progress is still reported on the physical layer, for example on multicarrier modulations or covert communications, a basic set of modulations and receiver algorithms is now available to support research on higher levels in network architectures.

The actual need for time synchronization within an underwater acoustic network is not always present. It can be argued that given a network with an operation time of hours or a few days, any standard equipment will have a clock drift that is negligible given most applications and network protocol stacks. Given this argument, synchronization of clocks can be done on board, before deployment. This might be true for some cases, even though from a practical and logistical point of view, especially when the number of nodes gets large, it gets time consuming to access all nodes individually through their electrical interface to set their clock manually. Another option is to synchronize clocks through switching the power on simultaneously for all nodes, but this can also be impractical. Common for both approaches is that the accuracy will vary and errors might occur (human in the loop). From this point of view it would be beneficial to be able to have the nodes doing time synchronization through the actual acoustic network.

Medium access control (MAC), also known as multiple access control, is a sublayer of the data link layer and manages access to the medium. In underwater networks, MAC protocols orchestrate the access to the acoustic communication channel. Without MAC, collisions of unsolicited modem signals may greatly degrade the overall network performance. The basic MAC objective is to avoid collisions, but more generally MAC protocols deal with network throughput, latency, energy efficiency, scalability, and adaptability. Weights can be given to different MAC objectives, depending on application and requirements. MAC protocols can be subdivided in contention-free schemes and contention-based schemes.

In this chapter, we describe methods to ensure reliable information delivery to higher layers at the sink(s), while keeping overhead, retransmissions, and discarded information as low as possible. The basic mechanism to achieve this is typically ARQ (automatic repeat request), but many variants and additional mechanisms can be used.

Routing is a fundamental network primitive in any wireless network. Given typical transmit power constraints, it is very unlikely that all nodes in a network are within the transmit range of one another. For this reason, many messages may have to be relayed towards their destination through multiple hops. Other than the clear advantages this strategy brings about in terms of connectivity among far nodes, multihop routing generates two types of overhead: on one hand the messages get replicated throughout the network, as multiple nodes relay the original transmission; on the other hand, the decisions about which node should be a relay require some sort of signalling before routing actually takes place.