# Reinforcement Learning-Based Routing Protocols in Flying Ad Hoc Networks (FANET): A Review
## 5 Proposed Classification
### Classification of RL-Based Routing Protocols Based on the Data Dissemination Process  
In this section, the routing methods are divided into four classes based on the data dissemination process: unicast, multicast, broadcast, and geocast. Table 7 compares these methods with each other. In the following section, we describe these four categories in summary. * **Unicast-based routing:** This routing technique uses point-to-point communication, meaning there is only one source and one destination. In unicast-based routing, UAVs need to know the precise location of themselves and the destination. Thus, they must use a localization system such as a global positioning system (GPS). Figure 22 displays this routing technique. Note that the data traffic in FANETs, similar to other wireless networks, has a broadcast nature. Therefore, a unicast-based routing technique is not compatible with the nature of these networks. These routing methods suffer from problems such as high communication overhead compared to the multicast technique, high delay in the route discovery process, and high bandwidth consumption, and show poor performance in dynamic topology networks [56; 57; 58]. * **Multicast-based routing:** Multicast means that data packets are disseminated for a group of UAVs on the network. This method is suitable for applications with limited energy and bandwidth. Multicast-based routing must define multiple multicast groups. A multicast group includes a set of UAVs. Therefore, if a UAV wants to receive a multicast message, it must become a member of a multicast group because when the source UAV sends a multicast message to the multicast group, all group members receive this message. This process is shown in Figure 23. Multicast-based routing protocols use tree-like or mesh-like structures to transfer multicast data from the source to a group of destination nodes. The main weakness of this routing technique is that it must constantly reconstruct the routing tree when changing the network topology [59; 60]. This is very challenging in FANETs. Figure 22: Unicast-based routing. * **Broadcast-based routing:** In this technique, UAVs flood routing messages in the whole network. This process is represented in Figure 24. The flooding strategy can increase the reception probability of the routing message by the destination, but it consumes a lot of bandwidth. This strategy does not need the spatial information of UAVs in the network and is implemented easily. This method is useful for low-density networks. However, it has a weak performance in dense networks and can cause communication overhead, network congestion, and the broadcast storm problem. The most important disadvantage of this technique is high energy and bandwidth consumption. In addition, this process has a lot of redundancy [61; 62; 63]. Figure 23: Multicast-based routing. Figure 24: Broadcast-based routing. \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline \hline
**Routing Scheme** & **Location** & **Routing** & **Delay in the** & **Implementation** & **Network Density** & **Bandwidth** & **Energy** & **Broadcast Storm** \\  & **Information** & **Overhead** & **Rout Discovery** & **Implementation** & **Network Density** & **Consumption** & **Consumption** & **Broadcast Storm** \\ \hline
**Unicast-based** & Yes & High & High & Complex & Low or high & High & High & No \\ \hline
**Multicast-based** & Yes & Low & Low & Complex & Low or high & Low & Low & No \\ \hline
**Broadcast-based** & No & High & Low & Simple & Low & Very high & Very high & Yes \\ \hline
**Geostat-based** & Yes & Low & Low & Complex & Low or high & Low & Low & No \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Comparison of various data dissemination methods. * **Geocast-based routing:** It is a type of multicast technique. The purpose of this routing technique is to send data packets from the source to all UAVs in a particular geographic area. This process is shown in Figure 25. In this method, the geographic area is inserted into each geocast packet.