## 4 Next Forwarding Node Selection in UWSNs
### Comparative Analysis and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{**Category**} & **Protocol** & **Residual Energy** & **Link Quality** & **Void-Aware** & **Reliability** & **Multi-hop** & **Shortest Path** & **Sender/Receive-Based** \\ \hline \multirow{4}{*}{**Localization** routing protocols} & **Virtual Shape** & VBF & & & & & ✓ & & Receiver-based \\  & & HH-VBF & & & & & ✓ & ✓ & Receiver-based \\  & & \multirow{2}{*}{**Energy Efficiency**} & PER & ✓ & & & & & & & \multicolumn{1}{c}{} & \multicolumn{1}{c}{} \\  & & SEANAR & ✓ & & & ✓ & & & & & \multicolumn{1}{c}{} \\  & & VBVA & & & ✓ & & ✓ & & & Receiver-based \\  & & Void-Aware & FBR & & ✓ & ✓ & ✓ & ✓ & & & Receiver-based \\  & & DFR & & ✓ & ✓ & ✓ & ✓ & & & Receiver-based \\ \hline \multirow{4}{*}{**Localization** routing protocols} & **Category** & **Protocol** & **Residual Energy** & **Link Quality** & **Void-Aware** & **Reliability** & **Multi-hop** & **Shortest Path** & **Sender/Receive-based** \\  & & H2-DAB & & & & & & ✓ & & & \multicolumn{1}{c}{} \\  & & 2H-ACK & & & & ✓ & & & & \multicolumn{1}{c}{} \\  & & APCK & ✓ & & ✓ & ✓ & ✓ & & & \multicolumn{1}{c}{} \\  & & DBR & & & & & ✓ & ✓ & ✓ & & Receiver-based \\  & & DBR & & & & & & ✓ & ✓ & & \multicolumn{1}{c}{} \\  & & DBR & & & & & & ✓ & ✓ & & Receiver-based \\  & & DBR & & ✓ & & & & ✓ & ✓ & & \multicolumn{1}{c}{} \\  & & EDER & ✓ & & & & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\  & & MPP & ✓ & & ✓ & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\  & & REFER & ✓ & & & & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\  & & R-EPER & ✓ & ✓ & & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\  & & EEF & ✓ & & & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\  & DREE & & ✓ & & & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\ \hline \multirow{4}{*}{**Non-localization** routing protocols} & **HydeCast** & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & Receiver-based \\  & VAPR & & & ✓ & ✓ & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\  & AMCD & ✓ & & ✓ & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\  & IVAR & & & ✓ & ✓ & ✓ & ✓ & \multicolumn{1}{c}{} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of the main factors in routing protocols in UWSNs. Selecting the next forwarding nodes and the best nodes to forward the data packets is a major problem in routing protocols that have a direct impact on overall routing performance such as energy consumption, packet delivery ratio, and network lifetime [14; 58; 59]. As shown in Table 1, localization routing protocols provide virtual shapes using location information from the sender node to the sink. This kind of solution is not efficient to use in UWSNs due to the cost of using localization and the lack of GPS. Moreover, most of the existing localization routing protocols select the best forwarding node based on its distance to the sink. Specifically, VBF, HH-VBF, VBVA, and FBR employ only the distance parameter. SEANAR and PER use the distance parameter along with the residual energy metric, whereas DFR utilizes the distance parameter along with link quality. On the other hand, non-localization beacon-based routing protocols select the next forwarding nodes using costly partial information (beacon messages provided by the sink). For instance, H\({}^{2}\)-DAB and 2H-ACK employ dynamic addressing based for selecting the next forwarding node. MRP and APCR employ layer ID along with residual energy. ERP\({}^{2}\)R utilizes physical distance with residual energy, whereas R-ERP\({}^{2}\)R utilizes physical distance along with residual energy and link quality for selecting the best candidate for forwarding the data packets. Finally, DREE utilizes physical distance along with link quality for selecting the best candidate for forwarding the data packets. In contrast, selecting the next forwarding nodes in non-localization pressure-based routing protocols is very simple and can be identified locally. Each node can measure its depth locally using pressure sensors and can share this information with neighbors to identify the next forwarding nodes by selecting its shallower neighbor nodes.