## 4 Evaluation of the Routing Protocols
EBER\({}^{2}\), CoDBR, RE-PBR, VTP give best performance in terms of delivery ratio whereas EBER\({}^{2}\) and SEEC are most energy efficient. The delay efficiency of RE-PBR and VTP is better than any other considered routing protocols. On the other hand, EBER\({}^{2}\), CoDBR and RE-PBR offer highest reliability in data transmission in compared to others. Considering all the metrics and factors, the overall performances of EBER\({}^{2}\) and RE-PBR are better than any other considered routing protocols for UWSN. Even though UWSN has improved a lot due to massive efforts from researchers, it still possesses several challenges which include time synchronization, localization accuracy, wave speed variation and security issues. Most of these challenges are not addressed in the above-discussed routing protocols and these are open research issues. Time synchronization is one of the important factors for localization [42]. The sink nodes floating at the water surface can synchronize their time from GPS data whereas the underwater nodes face problem in time-synchronization due to clock skew and offsets [73]. Most of the routing protocols described here do not consider the speed variation of the acoustic waves due salinity, water pressure and temperature. This might cause discrepancies in distance measurement which affects the the localization and routing decisions [73]. Security in UWSN is one of the very important issues that has to be considered for practical deployment [74]. Countermeasures against security attacks needs to be considered while designing the routing protocols and this challenge is still open for research. \begin{table}
\begin{tabular}{l l l l l l l l l} \hline \hline \multirow{2}{*}{**Sl.**} & \multirow{2}{*}{**Protocol No.**} & \multirow{2}{*}{**End-to-End/**} & **Delivery** & **Energy** & **Delay** & **Localization** & **Reliability** & **Performance** \\  & & & **Multi-Hop/** & **Ratio** & **Efficiency** & **Efficiency** & **Requirement** & & \\  & & **Clustering** & & & & & & & \\ \hline
1 & VBF [38] & End-to-End & Low & Medium & Low & Not Needed & Low & Low \\ \hline
2 & HH-VBF [51] & Multi-Hop & High & Low & Medium & Not Needed & High & Medium \\ \hline
3 & DBR [52] & Multi-Hop & High & Medium & High & Not Needed & High & High \\ \hline
4 & VTP [53] & Multi-Hop & Very High & Low & Very High & Not Needed & High & High \\  & & and & Clustering & & & & & \\ \hline
5 & CoDBR [54] & Multi-Hop & Very High & Low & Medium & Not Needed & Very High & High \\ \hline
6 & EEDBR [55] & Multi-Hop & High & High & High & Not Needed & High & High \\ \hline
7 & RE-PBR [56] & Multi-Hop & Very High & High & Very High & Not Needed & Very High & Very High \\ \hline
8 & DFR [57] & Multi-Hop & Medium & Medium & Medium & Needed & High & Medium \\ \hline
9 & LARP [58] & Multi-Hop & High & Medium & Low & Needed & High & High \\ \hline
10 & FBR [59] & Multi-Hop & Medium & Medium & High & Partially & Medium & High \\  & & & & & Needed & & & \\ \hline
11 & DUCS [60] & Multi-Hop & Medium & High & Low & Not Needed & Low & Low \\ \hline
12 & SEEC [61] & Clustering & Medium & Very High & High & Not Needed & High & High \\ \hline
13 & EBER\({}^{2}\)[63] & Multi-Hop & Very High & Very High & Medium & Not Needed & Very High & Very High \\ \hline
14 & RSM and & Multi-Hop & High & Medium & High & Not Needed & High & High \\  & & and & Clustering & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Evaluation of the Routing protocols.