# Void Avoiding Opportunistic Routing Protocols for Underwater Wireless Sensor Networks: A Survey
## 3 Review on Opportunistic Routing Void Avoidance Protocols for UWSNs  
Only a few protocols have been proposed to deal with the void communication area problem in UWSNs using the opportunistic routing technique. In this section, we will give a quick review of all the existing protocols. **HydroCast**  
Authors in [43] presented a hydraulic pressure routing for underwater sensor networks protocol (HydroCast). HydroCast forms a cluster of nodes by using only the local knowledge of the topology, excluding hidden terminals among them, while also maximiing the expected packet advance (EPA) of this cluster. When adopting the time of arrival technique, which is frequently used in UWSNs, the current forwarder node in HydroCast can define the pairwise distances and two-hop connections for the nearby nodes in order to determine its forwarding set. Additionally, the forwarding set candidates are prioritised using a distance-based timer approach. To help organise the transmission and reduce collisions, when nodes in the forwarding set receive a data packet from a recent forwarder node, they set their timers in order, starting with the node with the longest distance. HydroCast also proposes a _Local Lower-Depth-First Recovery_ approach and _2-D Void Floor Surface Flooding for Recovery Path Search_ for a recovery mode. Where each void node (i.e., local minimum node as used in the paper) seeks out its neighbors to find a node with a lesser depth than itself, this lesser depth node could be another void node with a new recovery path or a sensor node in a position that helps to resume the greedy forwarding techniques. Figure 5 shows the recovery path in the HydroCast protocol. In the 3D network topology, nodes experiencing a void area employ a costly flooding technique to learn which nodes are best suited to resume greedy forwarding or identify alternative routes to better forwarding channels. However, it is difficult to estimate the limited 3D flooding probability value because the flooding could involve every sensor node and affect the entire network topology. They suggest 2D flooding on the surface of the void floor to get around this restriction and increase the effectiveness of the procedure. This flood will include the best possible collection of nodes. As a result, nodes on the surface will monitor their void floor surface status using their local connectivity information and forward packets accordingly, whereas nodes that are not on the surface but are controlled by surface neighbors will not forward packets. HydroCast addresses the void area issue using an OR approach, which also successfully enables increasing the packet delivery ratio with small end-to-end delays since a  
Figure 5: HydroCast void handling technique. subset of the neighboring nodes simultaneously receive the data packet appropriately. However, at the same time, as a result of using opportunistic routing, the HydroCast protocol suffers from redundant packet transmission, where a data packet may be delivered to the sink multiple times, causing the depletion of network resources. In addition, in terms of energy efficiency, implementing the recovery mode results in additional energy costs. Moreover, there is no evidence provided about the energy consumed by the pressure sensor in order to find its depth. **VAPR**  
Void-Aware Pressure Routing (VAPR) [44] is an anycast soft-state routing protocol. It was proposed in order to address the void node issue in UWSNs. VAPR is built up of two stages: the enhanced beaconing stage and the opportunistic data forwarding stage. Instead of falling into a void area and then implementing a recovery mode, VAPR takes advantage of the geographic routing and employs the regular beaconing messages method, which includes some useful local information about the node, in the forwarding set selection stage. In VAPR, any node that receives a beaconing message from a neighbor updates its neighboring table and examines its depth with the received depth information.