# Void Avoidance Opportunistic Routing Protocol for Underwater Wireless Sensor Networks
## 2 Related Works
HydroCast applies only the local information of the topology to form a cluster with nodes excluding hidden-terminal among them, and at the same time maximizing the Expected Packet Advance (EPA) of this cluster. In HydroCast, the current forwarder node needs to know the two-hop connectivity and the pairwise distances for the neighboring nodes. Nodes in the forwarding set are prioritized using a distance-based timer approach that results in the most distant node from the source having the shortest timer, and so on, to help in scheduling the transmissions and suppressing collisions. HydroCast addresses the void area issue using an OR approach, which also enhances the packet delivery ratio with small end-to-end delays since a subset of the neighboring nodes simultaneously receive the data packet properly. However, as a result of using OR, HydroCast suffers from redundant packet transmissions where a data packet may be delivered to the sink multiple times, causing the depletion of network resources. In addition, implementing the recovery mode increases the energy cost, and the simulation results presented in the paper do not show any details about the energy consumed by the pressure sensor in order to find its depth. Inherently Void Avoidance Routing Protocol for Underwater Sensor Networks (IVAR) [22] is a receiver-based forwarding protocol where the forwarding node does not need to store its neighbor's information. In IVAR, a hop-by-hop forwarding set selection technique is used to forward the data packets from the sensed node to the sink. Each packet holder uses local information of hop distance and packet advancement to determine its own forwarding set. The nodes in these forwarding sets are given a priority depending on two metrics to forward packets: their hop count as a first metric and their depths as a second one. IVAR suffers from redundant packet transmission due to a hidden node problem. Consequently, redundant packet transmissions lead to an increase in energy consumption. In [23], another pressure-based routing protocol is described, namely weighting depth and forwarding area division DBR routing protocol (WDFAD-DBR). To increase the reliability of the packet transmission and decrease the probability of the void area problem, WDFAD-DBR uses the weighting depth difference of two-hop nodes to construct its routing decision. Broadcast control packets and acknowledgment messages (ACKs) periodically consume the node's battery and memory. Duplicated packets are handled by dividing the forwarding area and using a neighbor node prediction mechanism that helps to reduce the energy consumption. Void holes are avoided by using the depth of the expected next hop. However, retransmission is required if the best forwarding node fails to transmit the packet. Additionally, routing flexibility might be affected due to choosing a fixed primary forwarding area to form the forwarding set so that a void area is not dealt with completely since trapped nodes are not eliminated from the forwarding set. The Energy-efficient and Void Avoidance Depth Based Routing (EVA-DBR) protocol was proposed in [24]. The EVA-DBR routing protocol consists of two phases: an updating phase and a routing phase. The protocol depends on the information broadcasted periodically in the updating phase from the neighbor nodes that are one-hop away from the source node for void detection and bypassing in the routing phase. Using a forward area resizing technique helps in addressing the hidden node problem and, in some cases, can also detect the void and trapped nodes before the data packet gets caught in a void node. However, in both phases of the protocol, periodically broadcasting neighbor information consumes node resources and many duplicate packet transmissions seem to happen in sparse networks. In addition, the hidden node problem may be present if the forwarding range is chosen to be more than half of the transmission range.