## 2 UAV-Enabled ITS Framework
### Second Layer: Communication Networks for Traffic Monitoring
#### 2.2.1 Communication Networks  
Unmanned aerial vehicle communication networks (UAVCNs) are composed of multiple UAVs working together in ad hoc mode while being controlled from a ground terminal to carry out a mission, as shown in Figure 3. The UAVCNs require effective coordination and collaboration to perform the desired mission, which involves complex networking models and mechanisms. UAVCNs are a type of Flying Ad Hoc Network (FANET), which is related to Mobile Ad Hoc Networks (MANETs) and Vehicular Ad Hoc Networks (VANETs) [45]. Additionally, UAVs integrated with vehicles support vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2X) communications in ITSs. UAVCN system architectures characterize UAV networks based on several factors, including the network architecture (such as mesh topology and hierarchical structure), node roles (such as UAV nodes, ground stations, and gateway nodes), and communication protocols (such as routing protocols and data link layer protocols), as shown in Figure 4[46]. Figure 4: Categorization of UAV system architectures [46]. According to the general requirements, the UAV network could consist of one or more large or small UAVs and other connecting components. The large UAVs could be used individually in a single network, while the small UAVs could form swarms or multi-UAV systems. As shown in Figure 5, while single-UAV systems have a relatively simple communication network, multi-UAV systems introduce significant complexity due to the need for inter-UAV communication, advanced protocols, scalability concerns, and secure network management. In multi-UAV systems, multiple UAVs are interconnected with ground nodes (or stations) to form a coordinated two-way communication network. These systems rely on two main types of communication: UAV-to-infrastructure, where drones communicate with ground stations or base stations, and UAV-to-UAV, where drones exchange information directly with each other. The complexity of UAVCNs in multi-UAV systems increases with the number of UAVs and the level of intricacy of the mission. Multi-UAV systems can be categorized into two primary types as follows:  
1. Cooperative Multi-UAVs: In these systems, UAVs collaborate, sharing information and tasks to achieve common goals more effectively. This approach is beneficial for missions that involve multiple UAVs, each potentially having different communication capabilities. Examples of cooperative multi-UAV architectures include cooperative task achievement, opportunistic relaying networks, or delay-tolerant networks. 2. Multi-Layered UAV Systems: These systems are structured into different hierarchical layers, with UAVs assigned specific roles and responsibilities at each layer. This layered approach helps manage complex missions more effectively by organizing tasks and communication. In addition to interacting with each other in UAV swarms, UAVs in multi-layered systems often interact with IoT devices for enhanced data collection and processing, ground wireless sensor networks (WSNs) to gather and relay environmental data, and cloud computing for data storage, processing, and analysis, allowing for more scalable mission support. Multi-UAV systems often provide significant benefits over single-UAV systems. Although multi-UAV systems can be costly, they offer more benefits due to the increased efficiency and enhanced coverage in ITS applications. Table 2 compares single-UAV and multi-UAV systems based on scalability, speed, bandwidth, antenna, control complexity, cost, survivability, and failure. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Features** & **Single-UAV System** & **Multi-UAV System** \\ \hline Scalability & Limited & High \\ Speed & Slow & Fast \\ Bandwidth Required & High & Medium \\ Antenna & Omni-directional & Directional \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The difference in features of single-UAV and multi-UAV systems [47]. Figure 5: Complexity of communication network between single-UAV and multi-UAV systems.