# Survey on Routing Protocols for Vehicular Ad Hoc Networks Based on Multimetrics
## 3 Systematic Literature Review
### Study Quality Assessment  
The first search, without any exclusion-criteria, shown in Tables 1 and 2, returned a high number of documents, several of which were duplicated. After the use of the exclusion/inclusion criteria, many documents were dismissed. All separate authors of this work individually checked the activity of searching for publications to ensure the quality of the place of publication. Quality evaluation was then separately performed to verify the obtained information. After the selection of around 100 documents, only 21 papers fulfilled the requirements of this study (e.g., publication period, new protocol proposal, more than one metrics used in the selection of forwarder nodes) and were selected in this review.  
We note that the proposals were presented between 2009 and 2019 and that, between 2013 and 2018, a higher number of proposals were published (see Figure 2a. This figure also shows the simulators used by these proposals.).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Digital Libraries** & **IEEE** & **Scopus** & **ACM** & **ScienceDirect** \\ \hline _routing protocol vanet_ & 1316 & 1598 & 34,411 & 1153 \\ _metrics routing vanet_ & 223 & 273 & 18,407 & 4 \\ _metrics routing protocol vanet_ & 204 & 231 & 28,592 & 761 \\ _multi-metric routing protocol_ & 47 & 65 & 52,035 & 75 \\ _multimetric routing protocol vanet_ & 7 & 1 & 18,206 & 4 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Research results using search string.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Digital Libraries** & **IEEE** & **Scopus** & **ACM** & **ScienceDirect** \\ \hline _\"routing\" and \"protocol\" and \"VANET\"_ & 3874 & 7147 & 92 & 1153 \\ _\"multimetric\" and \"routing\" and \"protocol\" and \"VANET\"_ & 19 & 51 & 7612 & 4 \\ _\"metrics\" and \"routing\" and \"protocol\" and \"VANET\"_ & 1790 & 1365 & 28,592 & 761 \\ _\"multimetrics\" and \"routing\" and \"protocol\"_ & 42 & 328 & 17,960 & 75 \\ _\"multimetrics\" and \"routing\" and \"protocol\"_ & 1 & 50 & 18,206 & 4 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Research results using Boolean expressions.  
We can also observe in Figure 2b that proposals were often evaluated using NS-2, specifically, 71% of the analyzed proposals. NS-2 is a highly accepted network simulator in the research community. In comparison with other well-known network simulation tools, NS-2 has several advantages: (1) the code is open-source, research-community-accepted, and facilitates openness to modify the existing mechanism; (2) extensibility and stability; and (3) it can support large simulation scenarios where the number of nodes can be up to 20,000, making the simulation results more realistic [25]. NS-2 was one of the first simulators with these features. It is important to note that the two other simulators also share these characteristics, although they appeared after NS-2.  
A summary of the whole SLR process is presented in Figure 3, from the identification of the articles in the four databases until the final selection going through the different selection filters.  
Figure 3: Systematic literature-review (SLR) selection process.  
Figure 2: Multimetric routing protocols proposed for VANETs along time. (**a**) Bars account for multimetric routing proposals grouped by network simulator. All routing proposals for VANETs over the years are plotted in lines for different network simulators. (**b**) Distribution of network simulator that used multimetric routing proposals.  
The protocols selected as relevant for this study are presented and analyzed in the following sections.