## 9 Comparison and Roadmap of Routing Protocols  
As we have seen in the previous sections, UAVs are varied from very small to large. They are either adapted to be deployed with a traditional or a decentralized communication architecture. To this, we must add the interoperability between UAVs, which is mainly based on routing protocols. Thus, each protocol has its benefits and drawbacks. Table 4 summarizes the key performance of the FANET routing protocols reviewed in this paper. We note that topology-based routing protocols are only suitable for centralized and single-group architectures. They do not provide routing for highly mobile networks. It should also be noted that only hybrid and BATMAN protocols are suitable for sending messages without a risk of collision or packet loss. OLSR and DSDV protocols are much more expensive in terms of bandwidth, even if they are fast and avoid redundancy during message transmission. As for geographic position-based protocols, they operate on centralized, single-group, and multi-group architectures. These types of protocols promote interoperability and ensure good message deliverability while reducing bandwidth throughput. On the other hand, they do not guarantee fault tolerance in case of the failure or inaccessibility (wrong GPS coordinates) of a UAV. Only the GPSR and GLSR protocols are suitable for communication in a large-scale network. Finally, only the hierarchical-based routing protocols are operational on the four types of architectures. They are suitable for collaborative missions that involve frequent data exchange over several hierarchical levels of the network. Although they are efficient, some limitations should be noted. (1) The MPC protocol requires more time to send a message, which largely impacts the actions of other UAVs if they depend on it. (2) In case of message loss, the CAN protocol does not ensure its recovery, which can lead to the paralysis of the mission and cause a network bottleneck. Figure 18: Hierarchical routing paradigm. The choice of routing protocol depends on the type of communication architecture, the mission requirements, and the relevant features seen in Table 3. It is very important to analyze this step thoroughly in order to avoid problems of incompatibility between the protocols and the communication architecture. Figure 19 shows how to choose the right protocol depending on the infrastructure and the communication architecture. For a centralized architecture, it is better to opt for topology-based protocols. If the deliverability of messages is an important factor, then it is better to use static protocols. In case interactions with as few collisions as possible are required, it is more advantageous to use hybrid protocols. For decentralized architectures involving highly mobile UAVs in a large-scale environment, it is better to use geo-based protocols. In contrast, if it is a collaborative mission, hierarchical-based routing protocols are ideal. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multicolumn{1}{c}{\multirow{2}{*}{**Protocols**}} & \multicolumn{6}{c}{**Architectures**} & \multicolumn{6}{c}{**Performance Metrics**} \\ \cline{3-11}  & & & & & & & & & & & & \\ \multicolumn{1}{c}{} & \multirow{2}{*}{**Protocols**} & & & & & & & & & & & \\ \hline \multirow{10}{*}{Topology} & LCAD & & & & & & & & & & & & \\  & DCR & & & & & & & & & & & & \\  & MLHR & & & & & & & & & & & & \\  & OLSR & & & & & & & & & & & & \\  & DSDV & & & & & & & & & & & & \\  & BATMAN & & & & & & & & & & & & \\  & DSR & & & & & & & & & & & & \\  & AODV & & & & & & & & & & & & \\  & TSDD & & & & & & & & & & & & & \\  & ZRP & & & & & & & & & & & & \\  & TORA & & & & & & & & & & & & \\ \hline \multirow{10}{*}{Geographic position} & GPSR & & & & & & & & & & & & & & \\  & GLSR & & & & & & & & & & & & & \\  & GPMOR & & & & & & & & & & & & & \\  & RGR & & & & & & & & & & & & & \\ \hline \multirow{2}{*}{Hierarchical} & CAN & & & & & & & & & & & & & & \\  & MPC & & & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}