## 4 Underwater Sensor Networks
By exploiting controlled mobility of AUVs and the fact that AUVs can move closer to other underwater acoustic sensor nodes, a short-range high data rate underwater channel for large amount of data transmissions let GWs forward the aggregated data to an AUV. AUVs act as relay nodes, which collect sensed data from GWs and then forward them using a short-range high rate acoustic channel to the sink, which will in turn forward the data to the surface unit. AUVs have another acoustic interface for a long distance and low data rate communications to receive control signal and to send urgent data from/to the mothership via a surface unit. This procedure lets the system improve the network performance in terms of data delivery ratio and energy consumption. Authors simulated AURP performance using NS2 and they compared it with another Directed Diffusion based routing protocol. Their simulations used a realistic underwater acoustic communication channel model to evaluate the performance of the proposed scheme. Their results indicate AURP has higher delivery ratio and low energy consumption than the other. Moreover, they have observed that it could be used for applications that require mass data transmissions, such as multimedia applications. In [28] [PERSON] _et al._ propose both a security suite, which is designed with the goal of reducing the communication overhead introduced by security in terms of number and size of messages between autonomous mobile underwater sensors, and a cooperative algorithm in which the mobile underwater sensors (installed on Autonomous Underwater Vehicles) respond to simple local rules based on the available information. The cooperative algorithm relies on the fact that the available information from the other sensors, though not necessarily complete, is trustworthy. It considers that when there is a loss of communication among the vehicles, the coverage performance is degraded but not lost. The proposed security suite is aimed at communication integrity and confidentiality, which jointly with the robustness of the cooperative algorithm, which is intrinsically reactive against DoS due to the emergent behaviour approach, makes it ideal to perform a mission and maintain the communication link with the network in the underwater scenario. Authors also explain the implementation details of the integration between both and provide statistics of the system performance collected from a real project. Finally, they report some results from an Underwater Acoustic Network project to characterize its application level performance including the cooperative strategy and the security suite proposed in the paper. They demonstrate that the security solution is efficient in terms of number of messages and message size, reducing the time and the energy of the transmission. [PERSON] _et al._ propose in [29] a series of improvements in the time-slot routing algorithm to achieve a complete routing protocol called time-slot based balancing network coding (TSBNC) routing algorithm for underwater wireless sensor networks. They designed a probability balanced mechanism and applied it to the time-slot based routing algorithm in order to increase energy consumption efficiency and extend network lifetime. Finally, they evaluated the performance of TSBNC and compared it with other classical underwater routing protocols. The simulation results show that the proposed protocol can reduce the probability of node conflicts, shorten the process of routing construction, balance energy consumption of each node and effectively prolong the network lifetime. In [30], [PERSON] _et al._ examine the hydrological and biogeochemical variables obtained from the Guadalquivir estuary during three years through a real-time remote monitoring network. This network was developed with the aim of providing accurate data in order to study the influence of hydrodynamical and hydrological features within the estuary on the functioning of the pelagic ecosystem and know the trophic status of the aquatic environment.