# Void Avoidance Opportunistic Routing Protocol for Underwater Wireless Sensor Networks
## 2 Related Works
Void-Aware Pressure Routing (VAPR), Geographic and opportunistic routing with Depth Adjustment-based topology control for communication Recovery (GEDAR), Void handling using Geo-Opportunistic Routing in underwater wireless sensor networks (VHGOR), a novel power controlled opportunistic routing protocol for internet of underwater things (PCR), Depth-Based Routing (DBR), Pressure Routing for Underwater Sensor Networks (HydroCast), Inherently Void Avoidance Routing Protocol for Underwater Sensor Networks (IVAR), weighting depth and forwarding area division DBR routing protocol (WDFAD-DBR), Energy-efficient and Void Avoidance Depth Based Routing (EVA-DBR), Energy and Depth variance-based Opportunistic Void avoidance (EDOVE), Energy Efficient Depth-Based Opportunistic Routing protocol (EEDOR). delivery probability. The second metric is Node Closer to the Destination (NCD); NCD can be defined as the best node with maximum OREPP to forward the current packet. VHGOR uses a greedy forwarding approach to advance the packet towards the destination, and if a packet gets into a void node, the protocol switches to the void mode. The limitations of VH-GOR include consuming restricted resources such as node memory, through maintaining a neighboring table, and the node energy through node beacons. Recently, a novel power control-based opportunistic (PCR) routing protocol for internet of underwater things (IoUTs) was proposed in [19]. To achieve energy-efficient data delivery in IoUTs, the authors designed an opportunistic routing protocol that includes transmission power control. In PCR, each node considers more than one transmission power level to choose its candidate set for each next-hop, and then the energy waste for each candidate set is calculated to determine the appropriate transmission power level and the next-hop forwarding set. The packet delivery ratio is increased by modifying the transmission power level at each hop in order to choose the appropriate candidate node set from the sender neighbors to continue forwarding the data packets to reach the sink(s) on the water surface. PCR also reduces the transmission power level when the number of deployed nodes is increased to reduce the number of retransmissions, which decreases the energy consumption in some cases. However, as we can see from the results presented, the energy consumption is greater than that in the compared related works, which will affect the network lifetime. The Depth-Based Routing (DBR) presented in [20] was the first OR protocol proposed for UWSNs using sensor node depth. In DBR, the current forwarding node uses the flooding technique to send the packet to its neighbors. The receiving neighbors then decide to be a forwarding candidate by comparing their depth with the depth in the received packet. The depth threshold is also implemented during the forwarding set formation to select the nodes that are far from the current forwarder to continue the forwarding process. The holding time used to manage the coordination phase between the forwarding nodes is determined based on node depth. Using only the depth of the sensor nodes as a metric for forwarding set selection reduces the protocol's performance because the nodes with smaller depths are involved in the forwarding process most of the time. Therefore, such nodes drain their energy before the other nodes in the network, which generates void zones. Moreover, the DBR flooding technique increases the probability of packet delivery to the surface but also increases redundant packets and packet retransmissions. Consequently, an extreme amount of energy will be consumed. In DBR, redundant packets and packet retransmissions happen because of the node's holding time. That is, nodes may have the same depth, and using only the depth in calculating the hold time will give a number of nodes the same transmission times. In [21], the Pressure Routing for Underwater Sensor Networks (HydroCast) protocol is presented.