# Void Avoidance Opportunistic Routing Protocol for Underwater Wireless Sensor Networks
## 2 Related Works  
A number of protocols have been proposed that use OR techniques to deal with the void communication area problem. In this section, we first give a brief classification of routing protocols found in the literature that address the void problem as well as other comparative protocols. We then present a review and discussion of these protocols and their benefits and drawbacks. The existing OR protocols in UWSNs can be classified into two main categories based on their positioning information: Geography-based routing protocols and Pressure-based routing protocols. This classification can be seen in Figure 1.  
In the Geography-based category such as [16; 17; 18; 19], selecting the forwarding set candidates and making the forwarding packet decisions in OR requires information about the geographic location of sensor nodes, while in the Pressure-based category, as in [15; 20; 21; 22; 23; 24; 25], the depth information of nodes is needed to select the next forwarding set candidates and make forwarding packet decisions. Void-Aware Pressure Routing (VAPR) [16] is an anycast soft-state routing protocol that was proposed to address the void node issue in UWSNs. VAPR consists of two stages: an enhanced beaconing stage and an opportunistic data forwarding stage. Instead of encountering a void area and then implementing a recovery mode, VAPR takes advantage of geographic routing and employs the regular beaconing messages method, which includessome useful local information about the node to be used in the forwarding set selection stage. VAPR suffers from high-energy consumption since it measures the distance to the neighboring nodes and uses enhanced beaconing to exchange node information between the neighbors periodically. In [17], Geographic and opportunistic routing with Depth Adjustment-based topology control for communication Recovery (GEDAR) utilizes the greedy forwarding technique by knowing the position information of each current forwarding node, its neighbors, and the known sink. GEDAR is a sender-side OR technique, where the forwarding set of candidates is determined in each hop by the sender node. GEDAR uses network topology control techniques to increase the connectivity of the network and reduce the number of packet retransmissions. Moreover, the topology control method is also utilized to address the void zone problem. On the other hand, the depth adjustment technique used by void nodes to move to a new depth that allows them to communicate with other node(s) consumes a significant amount of energy. Another drawback of GEDAR is that sensor node energy is not considered when forwarder nodes are selected, which may result in poor forwarder node selection. Void handling using Geo-Opportunistic Routing in underwater wireless sensor networks (VHGOR) [18] adopts Geography-based Opportunistic Routing (GOR) to forward data packets to reach the destination over multi-hops. It is a heuristic protocol implemented with two methods to form optimal forwarder selection. The first metric is Opportunistic Routing based Expected Packet Progress (OREPP), which is calculated based on the difference between the geographic distance between the source and destination, and the geographic distance between any node and the destination, residual energy and packet  
Figure 1: Classification of Opportunistic Routing (OR) protocols based on positioning information.