## 2 ITS Applications and Communication Technologies, Critical Design Requirements, and Related Works  
### Review of Intelligent Transportation Systems (ITS)  
Next-generation mobility and transportation services demand the development of a smart, highly automated, and responsive transportation system. Such systems are known as Intelligent Transportation Systems (ITS) [7; 8; 9] and are based on Future Internet Connectivity. Their role is to facilitate the safe but efficient transportation of commodities and/or humans. This is accomplished through embedding location-based and communication information leveraged with the adoption of suitable transportation models [9]. Specifically, urban ITS aim to relieve the heavily congested and crowded city environments related to the rapid growth of automobile users in recent decades. The rapid increase in city transportation volumes drastically affects user behavior and driving style leading to an increased risk of traffic accidents [7; 8; 9]. A vehicular network constitutes a C-ITS-based network that supports interactions between the highly mobile and dynamic nodes-vehicles. In a Vehicle Ad Hoc Network (VANET), vehicles connect with each other through wireless short-range radio and with road infrastructures through either short-range radio or 3G, LTE-4G, and 5G [8; 13; 15]. In VANETs, by definition, vehicles act as the network nodes that are capable of transmitting (source) or receiving (destination) data, or they can even function as network routers. However, VANETs exhibit some unique features that distinguish them from other types of networks: Self-organization, high mobility (but with geographical or trajectory limitations), high transmission speed due to variable node densities, and a dynamic topology [8; 13]. The vehicular networking applications are divided into (1) active road safety applications, (2) traffic efficiency and management applications, and (3) infotainment (information and entertainment) applications [15; 16]. The first category is intended to decrease the probability of traffic accidents through the provision of various ITS services such as lane departure, forward collision warnings, and emergency vehicle warnings. The second category aims to improve vehicle traffic flow and coordination conditions (e.g., speed management operations) and the provision of local information and maps (e.g., cooperative navigation) [15; 16]. Finally, the last category includes local cooperative services (e.g., media download and voice assistance) and global internet services [15; 16]. Today, the evolution of communication technologies drives the ITS industry to connect vehicles and infrastructure, leading to Cooperative ITS (C-ITS) [8; 9]. These systems offer advanced synchronized driving modalities (e.g., cooperative cruising, cooperative awareness, cooperative positioning/navigation, speed management, and cooperative sensing capabilities) [8; 9]. In this case, system standardization dictates the use of interoperable protocols to enable communication among stations with various architectures. Additionally, the deployment of C-ITS demands cooperation among competing stakeholders [13; 16]. The available ITS technologies come with various definitions and classifications depending on the type of nodes contributing to the communication network. Moreover, they can be classified into _vehicle-to-vehicle_ (V2V), _vehicle-to-infrastructure_ (V2I), _vehicle-to-nomatic_ (V2N), _vehicle-to-pedestrian_ (V2P), _vehicle-to-everything_ (V2X), _in-vehicle network_ (IVN), _vehicular ad hoc networks_ (VANET), and other schemes [7; 8]. In Figure 1, various ITS scenarios and connected-vehicle services are illustrated. Table 1 summarizes the various ITS wireless communication standards in terms of class and type of wireless protocol [9; 17]. Advanced safety-critical ITS systems should fulfill a variety of operational and radio-communication requirements.