# Retransmission Avoidance for Reliable Data Delivery in Underwater WSNs
## 2 Related Work
### Multi-Hopping to Avoid Void Regions  
Authors proposed in [17], the geographic and opportunistic routing protocol (GEDAR) for UWSNs. This is an anycast, geographic and opportunistic protocol which selects a potential neighbor to forward data packet using anycast method. Every sensor node greedily forward data packets to the lowest depth node in its range. When a data packet reaches void region, the current forwarder switches to recovery mode and changes depth to find a potential neighbor. GEDAR overcomes void holes and increases PDR in dense regions. However, position changing mechanism consumes more energy and increases delay.  
In [18], the authors proposed hydrocast: pressure routing for UWSNs which uses measured pressure level information to route data packets towards the destination. Every data packet is routed by anycast mechanism in only vertical direction. Every sensor node forwards each data packet to a subset of forwarders which maximizes the greedy progress towards the destination. The receiver node does not forward data packet until all the higher priority nodes in forwarding set fails. If a data packet reaches in void region, the current forwarder sends it to a higher depth node (backward transmission) and directs it to the sink through another path. The reliability increases by sending data packet to multiple forwarders and sends using different path in case of void region. However, hydrocast pays the cost of energy and delay.  
The authors of on energy hole and coverage hole avoidance in UWSNs proposed spherical hole repair technique (SHORT) in [19]. Coverage hole is un-sensed region which occurs because of uneven distribution of nodes, random deployment or by death of sensor nodes. Every sensor node embeds its residual energy information in the forwarding data packet which helps the receivers to update their neighbors information. In order to reduce packet loss and suppress retransmissions, holding time is computed for each packet before forwarding. When a sensor node is near to die, it generates control message to its neighbors so, they stop forwarding data to it. Now a sensor node in dense region moves to fill this coverage hole ensuring its movement does not create another coverage hole. The nodes which  
\begin{table}
\begin{tabular}{|c|c|c|c|} \hline
**Protocol Name** & **Features** & **Achievements** & **Limitations** \\ \hline \multirow{3}{*}{MPT [12]} & Multiple & Increases reliability & Dense regions increase \\  & path transmission & and low delay & collisions and \\  & & & energy consumption \\ \hline \multirow{2}{*}{AEDG [13]} & AUVs and & Balanced energy consumption & \multirow{2}{*}{High delay} \\  & multi-hopping & and high throughput & \\ \hline \multirow{3}{*}{PER [14]} & Forwarding data through & \multirow{3}{*}{High PDR} & High delay and \\  & multiple paths by & & energy consumption \\  & following binary tree & & \\ \hline \multirow{3}{*}{M-FEC [15]} & Data forwarding & Minimum energy & Delay increases because \\  & through multiple paths & consumption and reliability & of data packet \\ \hline \multirow{3}{*}{EBH [16]} & Direct transmission & \multirow{3}{*}{Balance energy consumption} & Void holes increase \\  & and multi-hopping & & delay and \\ \hline \end{tabular}
\end{table}
Table 2: Comparison of Multi-Path Routing Protocols.  
make cross triangles or hidden cross triangles have higher probability of movement. Coverage hole filling increases throughput and lifetime of the network at the cost of high delay. The summary of the discussed routing algorithms is provided in Table 3.