# Void Avoiding Opportunistic Routing Protocols for Underwater Wireless Sensor Networks: A Survey
## 3 Review on Opportunistic Routing Void Avoidance Protocols for UWSNs
In \(\underline{\text{EBER}^{2}}\), the network architecture consists of three types of sensor nodes (sink nodes, anchored nodes, and relay nodes), as demonstrated in Figure 6:0  
The authors of \(\underline{\text{EBER}^{2}}\) take into account three factors as primary parameters for choosing the next forwarder in order to address the WDFAD-DBR weaknesses. The first parameter is the weighting depth difference of two hops; by choosing the next forwarder node based on the depths of the first two hops, the likelihood of a network void area problem is reduced. The second factor is the number of PFNs, which are nodes that are within the source node's upper hemisphere of its transmission range. A void node is one that has no PFNs; as a result, it is excluded from the upcoming forwarding set, which improves network stability. The residual energy is the third parameter, and it is used to provide PFNs with the same depth but varying holding durations in order to prevent duplicate packets. In addition to forming the next forwarder set and preventing void nodes from being chosen as candidates for the next forwarder, these three parameters also support energy efficiency, boost packet delivery ratio, and lengthen network lifetime by preventing duplicate packets and the ensuing collisions. Figure 6:1 Layering structure in TORA at the node localization phase [53]. To further assist these nodes in communicating with the embedded sinks and delivering data packets to them rather than travelling through a long path to reach the sinks on the surface, the EBER\({}^{2}\) protocol deploys two additional embedded sinks in the underwater area of interest that have high traffic density, as can be seen from Figure 6:2 In general, since the nodes placed in these dense and high traffic areas transfer the received packet to the closest embedded sink rather than transmitting further to the surface, this strategy enhances network packet delivery ratio while consuming less energy. Instead, the cost of communication rises because of the high-speed optical fiber links used to connect embedded sinks and on-surface sinks. Additionally, EBER\({}^{2}\) employs a transmission energy adaptation mechanism that enables nodes that are closer to sinks to reduce their transmission power level in accordance with their distance from that sink. This minimizes the void area created by the death of these nodes by preventing the nodes close to the sinks from rapidly exhausting their energy due to being involved in the majority of forwarding procedures. **EDORQ**  
The authors of [55] proposed a new receiver side-based routing protocol for UWSNs named Energy-efficient Depth-based Opportunistic Routing with Q-Learning (EDORQ). The EDORQ contains two phases: (1) the candidate set selection phase to choose a subset of neighbor nodes to carry on forwarding data packets until delivered to the destination; (2) the candidate set coordination phase, where the candidate nodes collaborate according to their priorities by applying the timer-based mechanism to suppress redundant forwarding. Moreover, the authors adopted the Q-learning technique to design the holding time of the candidate nodes. By defining a holding time for each candidate, the candidate node with the larger Q-value has a higher priority, a lower holding time, and will transmit the packet first. Figure 6:3 EBER\({}^{2}\) Network Topology. EDORQ starts the forwarding process using greedy mode, where the current forwarding node broadcasts the data packet to its neighbors. Each candidate neighbor extracted the depth (_d_) and void-flag information of the current node from the packet header after receiving the data packet and then compared \(d\) with its own depth. In order to ensure that the data packets are quickly sent in the sink's direction, the greedy mode helps locate a collection of candidate nodes closer to the water's surface.