# A Review of Flying Ad Hoc Networks: Key Characteristics, Applications, and Wireless Technologies
## 5 FANET Routing Protocols  
In this section, different sets of routing protocols in the network layer are defined for FANETs. One possible classification of FANET routing protocols is depicted in Figure 5, and is discussed in the following.  
* _Position-based (Figure 5, box A):_ In these protocols, the geographic information of the nodes is known from GPS, and the positions of the sender and receiver are determined in advance using _reactive_, _predictive_, _greedy_[84], and _hierarchical_[85; 86; 87] methods, as shown in Figure 5, box \(A\). The position-based hop by hop protocols usually dynamically select relay nodes. The packets are broadcast blindly by the node, and the selection of the relay is postponed until the neighbours of the node receive the packets. After the neighbours receive the packets, they calculate the dynamic forwarding delay (DFD) values according to their local position information in a distributed manner and then forward the packets to the destination greedily.The nodes closest to the destination then acquire the minimum DFD value and become the next forward [88].
* _Topology-based (Figure 5, box B):_ In topology-based hop by hop routing protocols, the senders forward packets through an optimal path using the network's topology information along with link- state information such as IP address [89]. As shown in to Figure 5, box \(B\), these can be classified as _static_[90], _proactive_[91], _reactive_[92], and _Hybrid_.
* _Delay-tolerant networks (DTNs) (Figure 5, box C):_ In DTNs, the mobile nodes are intermittently and unstably connected. As the mobile nodes experience high latency and low data rates, new routing protocols are needed to address the DTN characteristics [66]. The three main DTN-based routing protocols studied for FANETs are _deterministic_, _social network_, and _stochastic_[66], which are depicted in Figure 5, box C.
* _Heterogeneous (Figure 5, box D):_ FANETs interact with various ground networks, such as VANETs, MANETs, or fixed nodes, in which heterogeneous routing protocols are required for exchanging data between moving users. Routing protocols with heterogeneous techniques can support both mobile and fixed nodes in FANETs [93]. This technique can provide sub-network assistance coverage for both nodes on the ground and UAVs, network extension, and more [94]. The classification of heterogeneous routing protocols is shown in Figure 5, box \(D\).
* _Cluster-based (Figure 5, box E):_ In the clustering technique, nodes with similar characteristics and features are combined to form clusters. There is a cluster head in each cluster that carries out communication processing [95]. As shown in Figure 5, box \(E\), cluster-based routing protocols in FANETs can be classified into two main categories, namely, probabilistic and deterministic. A full classification of cluster-based routing protocols in FANETs is shown in Figure 5, box \(E\).  
Figure 4: FANET applications: Multi-UAV cooperation, UAV-to-Ground cooperation, and UAV-to-VANET cooperation.  
* _Swarm-based (Figure 5, box F)_: This routing technique takes advantage of the social behaviour of fish, birds, and insects to find the optimal path and topology management approach [96; 97; 98; 99].  
Selecting the best routing protocol for different UAV-based scenarios depends on a variety of characteristics, including the routing approach (dynamic, static, on-demand, hybrid), mobility models, simulation tools, and performance metrics. Although UAV routing protocols are in the early stages of development, the reactive and proactive routing approaches have thus far performed better in highly dynamic FANETs compared to other protocols. In addition, hybrid protocols appear to be better for employment in monitoring applications in large-scale FANETs [100].