Underwater acoustic networks are often used in mission-critical scenarios, such as military underwater networks and assets deployed for tsunami prevention, hence an attack performed against these types of networks can easily lead to disastrous consequences. Nevertheless, countermeasures to possible network attacks have not been widely investigated so far. A reputation system, where a node gains trust each time it exhibits a good behavior, and loses trust each time it behaves suspiciously, is an effective way to identify possible attackers in the network. The main challenge when applying a reputation system in an underwater network is to understand whether the network performance degrades because a node is acting maliciously intentionally, or because of changed channel conditions causing a large packet drop. For instance, when a ship travels close to an underwater network deployment, it causes an increased packet loss, and so does the change of environmental conditions, such as a drop of temperature, the presence of rain or the increase of the wind speed. This behavior of the acoustic channel can be characterized with a Hidden Markov Model, whose parameters are obtained observing the time evolution of the acoustic channel in a sea experiment. This paper presents a trust model based on the knowledge of the channel state, inferred from the perceived noise and received signal strength, in which misbehavior and correct behavior are differently considered according to the actual channel state. We evaluate the model both analytically and through simulations, implementing the trust mechanism in the DESERT Underwater Network framework.

This paper reports on the results achieved in the European Defence Agency (EDA) project Smart Adaptive Long-and Short-range underwater Acoustic network (SALSA), that was running from November 2018 to November 2022. SALSA was a European defence cooperation between The Netherlands (lead nation), Germany, Norway, Sweden and Finland. The objective was the development of a protocol stack for self-reconfigurable underwater acoustic networks that autonomously adapt to changing environmental conditions and operational needs.

The recent years increasing interest for research on underwater communication and networking has sparked the interest for a software defined modem (SDM). This concept has been realized on a sophisticated hardware platform, which is provided in form of the commercially available develogic Hydro-Acoustic Modem HAM.Base. This is a small sized, energy efficient, and deep water approved underwater communication modem. It provides widespread flexibility especially in the physical layer, but also broad accessibility to higher layers. Such an SDM is especially suitable for scientific research in underwater communication networks, where the implemented software architecture may be a result and not a prerequisite. This paper is about the tools, on how, and to what extent reprogramming and reconfiguration of this SDM are performable. As an example it is shown how this SDM is programmed to be used in the framework of TUWACS (Transient Underwater Acoustic Communication System) used in the robust acoustic communications in underwater networks (RACUN) EDA project.

Wide-sense stationary and uncorrelated scattering (WSSUS) assumptions are often applied for the statistical description of wireless communication channels. However, in the case of underwater acoustic channels the WSSUS model is of limited value. The degree of similarity of in-phase and quadrature components of the channel impulse response, measured with the use of bandpass modulated signals, can be used as an indicator of WSSUS assumption fulfillment. The paper describes an experimental method that uses quadrature Pseudo-Random Binary Sequence to evaluate the validity of the WSSUS assumption. The technique was developed by analyzing the shallow water experiment in the Bornholm Basin of the Baltic Sea.

Security in underwater networks is a crucial aspect to maintain correct network operations, but has only partially been investigated so far. Most of the applications of an underwater network are related to military or public safety scenarios, which should exhibit a high robustness to attacks and failures in order to avoid disastrous consequences even with simple Denial-of-Service attacks. The defense mechanisms and countermeasures are usually tailored to specific types of attacks. Albeit this procedure allows to obtain very effective defense mechanisms, it requires the development of different countermeasures for each possible attack. Another possible solution is to use reputation systems to identify the attackers; hence, in this paper we design a trust model able to tackle a wide range of attacks. The idea of the trust model is to observe the behavior of the nodes overhearing the neighbors’ transmissions and use this information in a subjective logic framework to assign a trust metric to each of the neighbors. In addition, in order to take into account the unique characteristics of the underwater acoustic channel, which alternates periods of high packet loss to periods with low errors, we include the channel state in the trust model to avoid to erroneously mark as not trustworthy nodes that correctly forwarded the packets, but could not be overheard due to an unreliable channel.