# Cooperative Communication Based Protocols for Underwater Wireless Sensors Networks: A Review
## 4 Network Layer Protocols
This latter features, combined with spatial diversity achieved with nodes cooperation at physical layer, allows not only energy saving, but also performance enhancements in terms packet delivery ratio. Specifically related to a vertical underwater network scenario, the authors in [54] investigate a routing strategy driven by the knowledge about depth of the sensor nodes. By characterizing each node with a location value that is function of depth, routing is performed by selecting those nodes with the lowest value, so to guarantee that the information flows towards the water surface where the destination is supposed to be located. A vertical underwater network routing scenario is also considered in [55], where the authors assume the presence of mobile sink nodes exploitable to realize incremental cooperative routing, with the goal to minimize the nodes energy consumption. Interestingly, many works in the literature regard opportunistic routing. In opportunistic routing, the source identifies first a set of potential relay nodes, ordered following specific priority rules. So, the relay characterized by the highest priority forwards the packet received by the source to the next-hop node. The other nodes instead pause their other transmission for a certain period, and perform packet retransmission whether the highest priority node transmission fails. The authors in [56] propose three opportunistic pressure based routing mechanisms based on a greedy algorithm to achieve energy saving and minimize the number of transmission hops. Furthermore, relay cooperation is considered to realize spatial diversity and apply MRC at the destination node, in order to improve the communication reliability as well. In [57], opportunistic routing is applied in a vertical underwater network, where path selection is performed based on a depth fitness factor characterizing each node. Such factor accounts for different features like node energy, link distance and packet delivery probability. A fuzzy logic-based relay selection is considered in [58] to realize opportunistic routing, with the best relay node being identified by evaluating the nodes energy consumption and packet delivery success. As outlined at the beginning of this section, routing in UWSNs is mainly oriented to energy efficiency and/or throughput optimization, without any specific focus on traffic management that typically characterizes other terrestrial networks. Differently, the work in [59] introduces a particular approach where both energy efficiency and traffic optimization are jointly addressed. In fact, the authors investigate a novel routing mechanism based on ant colony algorithm and cooperative relaying, where path next-hop is selected based on both nodes residual energy and transmitted data priority. Merging such features allowsrouting performance to be improved in terms of network lifetime and load. The studies about cooperative UWSN clustering and routing discussed above are summarized with their key features in Table 3. It is worth recalling that, differently from what happens in terrestrial RF systems, UWSNs management at the network layer is strictly influenced by the challenges posed by underwater signal propagation. An efficient routing and energy saving depend also on what happens below the network layer. Therefore, the major challenge is UWSNs probably regards the formulation of a cross-layer protocol aimed to harmonize the nodes cooperation at the different levels, so as to maximize the overall network performance in terms of energy and traffic management.