## 3 Routing Protocols for UWSN
### Protocols Featuring Network Void Hole Avoidance  
Most often UWSN has to cover some vast area and it might happen that, sender node can not find any neighboring node in its range to forward the data. This network void hole is created due to the random placement of the sensor node, dead neighboring node or water movement may take all the neighboring node out of the range of the sender. Performance of the whole network is greatly hampered due to these void holes. These routing protocols address this challenging issue to avoid network void holes. EBER\({}^{2}\), WDFAD-DBR, VSM and RSM fall into this category of the UWSN routing protocols. These schemes are suitable for a sparsely populated network where the deployment covers a vast area.  
An Energy Balanced Efficient and Reliable Routing Protocol (EBER\({}^{2}\)) has been proposed by [PERSON] et al. to achieve improved energy balancing, reliability, energy efficiency, network latency and packet delivery ratio [63]. Unlike DBR, Weighting Depth and Forwarding Area Division DBR (WDFAD-DBR) considers depth of the both current hop and expected forwarding hop thus avoid the void holes [65]. But WDFAD-DBR still faces void hole issues as it does not consider Potential Forwarding Nodes (PFNs) for the second hop. WDFAD-DBR also increases duplicate packets and packet collisions which affects the energy efficiency and lifetime of the network [63]. [PERSON] et al. in EBER\({}^{2}\) solved the problems posed by WDFAD-DBR. EBER\({}^{2}\) is formed of three types of nodes, that is, (i) sink nodes, (ii) anchored nodes, and (iii) relay nodes.  
Figure 11 shows the network architecture of EBER\({}^{2}\). Anchored nodes remain attached at the bottom of the water level and remain fixed in their positions. The sensor nodes or the relay nodes are deployed at a different layer of water levels which are movable. The relay nodes sense the surrounding data and also act as a forwarder node to transfer the data. The sinks remain floated at the water surface and are considered as the destination.  
The depths of the first two hops are considered for selecting the forwarder node which decreases the chance of the void hole issue in the network. The residual energy of the nodes is also considered. It is used to avoid duplicate packets by taking higher values of the holding time of the forwarding nodes. This allows energy optimization by avoiding duplicate packets thus increasing the packet delivery ratio. Moreover, the use of residual energy as a metric for selecting the forwarder node enables energy distribution in the network. This eventually prolongs the network  
Figure 11: Network architecture of Energy Balanced Efficient and Reliable Routing Protocol (EBER\({}^{2}\)) [63].  
issue of reliability, PFNs of the next forwarder are also taken into account which also decreases the probability void holes in the network.  
The energy consumption of the nodes nearer to the sink is always higher due to higher network traffic. If these nodes die earlier it would cause the network to be separated from the sinks. To address this problem, these nodes tune their transmission power level according to the distance to reach the nearest sink. Simulations show that, the EBER\({}^{2}\) performs better in energy efficiency, PDR, and packet loss when compared to WDFAD-DBR. But this protocol increases the end to end delay between the nodes by considering the residual energy.