## 2 State of the Art
### Routing Protocols
#### 2.2.1 Topology-Based Routing Protocols
The BATMAN routing protocol is a relatively new proactive protocol that is used in MANETs. The BATMAN protocol does not discover the entire network; the protocol only learns from the nodes directly connected to them. In addition, BATMAN-Advanced (BATMAN-ADV) [23] is an improvement of the original BATMAN protocol in terms of performance due to its integration in the protocol stack. Numerous comparative studies of BATMAN-ADV performance in ad hoc networks and FANETs have been carried out [23; 24]. DOLSR was specifically designed for FANETs and is a variant of OLSR. One of the most important performance factors in OLSR is the selection of multipoint relay (MPR) nodes. Thus, in DOLSR the number of MPRs is reduced to decrease the overhead. Each node selects a set of MPRs so that it can cover two-hop neighbors. DOLSR has the advantage of minimizing end-to-end delay, which is crucial for real-time applications and offers security improvements as it is resistant to jamming. In addition to the protocols explained, there are a variety of protocols with variants that are used in FANETs, such as Predictive-OLSR (P-OLSR) [5], Mobility and Load-Aware OLSR (ML-OLSR) [25], Contention-Based OLSR (COLSR) [26], Modified-OLSR (M-OLSR) [27], Cartography-Enhanced OLSR (CE-OLSR) [28], Topology Broadcast Based on Reverse-Path Forwarding (TBRPF) [29], Fisheye State Routing (FSR) [30], and Babel [31]. Babel builds on the ideas of DSDV, AODV, and other routing protocols to derive a loop-avoiding distance vector routing protocol that is designed to be robust and efficient in both relatively stable and highly dynamic networks. Due to the large bandwidth consumption that periodically occurs in the network discovery process of proactive protocols, reactive protocols use on-demand network discovery processes, which makes reactive protocols bandwidth-efficient. These protocols have a great advantage over proactive protocols as it gives them a great dynamism, something necessary in FANETs, but it has a great disadvantage in the latency produced by the route-search process. The most important reactive routing protocols are Dynamic Source Routing (DSR) [32], Ad Hoc On-Demand Distance Vector (AODV) [33], and Time-Slotted AODV (TS-AODV) [34]. DSR is a source-routing protocol. The complete route that a packet must follow is indicated in the header and only the source node can indicate the route. AODV is the most popular reactive routing protocol and is typically used in MANETs, VANETs, and FANETs. During the route lookup process, the source node searches its routing table to see if a route to the destination node has been established in the past. If no route exists, a route request process is initiated. TS-AODV is a time slot routing protocol centered on FANETs and based on the AODV protocol. Due to the large number of UAVs that may exist in FANETs, TS-AODV uses the time-division mechanism to avoid collisions in the transmission of information, significantly reducing packet losses and increasing the available bandwidth. In addition to the reactive protocols explained above, there are other routing protocols used in the literature on ad hoc networks and FANETs, such as Multicast AODV (MAODV) [35] and AODV Security (AODVSEC) [36]. To solve the problems of bandwidth consumption of control messages and the low dynamism of proactive protocols, as well as the long delay in the route search of reactive protocols, hybrid routing protocols were introduced. These hybrid protocols are particularly suitable for large networks since they base their operation on the division of the network into sub-networks or zones. Thus, a proactive routing protocol operates within each zone and a reactive routing protocol is used for communication between zones. For hybrid solutions, the Zone Routing Protocol (ZRP) [37] is the most popular. ZRP divides the network into clusters of nodes in which their maximum separation distance is predefined. Within the clusters, a proactive routing protocol is applied, and nodes in different zones are routed to a subset that is common to both zones.