# Void Avoiding Opportunistic Routing Protocols for Underwater Wireless Sensor Networks: A Survey
## 4 Comparison Study of OR Protocols for UWSNs  
In the previous section, the literature review of the state-of-the-art of the OR protocols that are proposed for UWSNs to address the void area problem is presented. The main challenge in the protocols was to handle the void area problem by using different approaches. The occurrence of the void area in the routing path can significantly reduce network performance. In this section, the general comparison of these reviewed protocols based on their characteristics and features is summarized below in Table 1.  
In Table 1, we can see that the existing void avoiding OR protocols for UWSNs are classified into two main classifications: geographic-based and pressure-based. In the first category, geographic-based, which includes [44, 45, 57, 58, 54], selecting the forwarding set candidates and making the forwarding packet decisions in OR requires information about the geographic position of sensor nodes. While in the pressure-based category, which includes [43, 46, 47, 49, 50, 51, 52, 54, 59], the depth information of nodes is needed to select forwarding set candidates and make forwarding packet decisions. The reviewed protocols are divided into sender-side and receiver-side categories based on which node will decide if the candidate node can be added to the next hop forwarder set or not. A higher communication overhead is needed on the sender-side because the sensors frequently need to exchange node information in order to update their neighbors' tables. As a result, the limited resources of the node (i.e., battery and memory) are used up. On the receiver-side, the sender is unaware of its neighbors and is unaware of its forwarding set. This may result in a significant number of redundant broadcasts and raise the possibility of transmission collisions requiring reiterate transmissions. The entire network stability period may be shortened as a result of packet loss and sensor node energy consumption. A number of these protocols take advantage of the multi-sink architecture and consider the data packet as delivered if it reaches one of the deployed sinks on the water surface. This improves the network reliability. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Protocol** & **Category** & **Forwarding Set Selection Category** & **Sink(s)** & **Requirements** & **Knowledge Required/Maintained** & **Advantage** & **Disadvantage** \\ \hline HydroCast [43] & Pressure-based routing & \multirow{2}{*}{Sender-side} & \multirow{2}{*}{Multi-sink} & Nodes with special & 2-hop connectivity & * & Reduce end-to-end delay. & * & High energy consumption \\  & &  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Protocol** & **Category** & **Forwarding Set Selection Category** & **Sink(s)** & **Requirements** & **Knowledge Required/Matinned** & **Advantage** & **Disadvantage** \\ \hline \multirow{4}{*}{IVAR [46]} & Pressure-based routing & \multirow{2}{*}{Receiver-side} & \multirow{2}{*}{Receiver-side} & \multirow{2}{*}{Receiver-side} & \multirow{2}{*}{Risky nodes and anchored nodes} & Own depth, 1-hop neighbors and sink location & \multirow{2}{*}{ \begin{tabular}{} \end{tabular}  } & Eliminates all the routes leading to a void area & \multirow{2}{*}{ \begin{tabular}{} \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{} \end{tabular}  } \\  & & & & & & & \\ \cline{1-1}  & Pressure-based routing & \multirow{2}{*}{Receiver-side} & \multirow{2}{*}{Receiver-side} & \multirow{2}{*}{Receiver-side} & \multirow{2}{*}{Received-side} & \multirow{2}{*}{ \begin{tabular}{} \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{} \end{tabular}  } & \multirow{2}{*}{\ \begin{tabular}{} \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{} \end{tabular}