# An Overview of Vehicular Communications
## 2 Vehicle-To-Vehicle  
V2V technology consists of wireless data transmissions between motor vehicles. The primary purpose of this communication is to prevent possible accidents, allowing vehicles in transit to transfer data on their position and their speed within an ad-hoc mesh network [31]. The latter uses a decentralized connection system, which may provide either a fully connected mesh topology or a partially connected mesh topology (Figure 1). In the first case, each node is connected directly to others in the network. In the second case, some nodes can be connected to all the others, while the remaining ones are attached only to those with which they frequently exchange most of the data. By exploiting this network topology, the nodes of a mesh network can exchange messages and information with neighboring nodes to which they are directly connected (only one hop, in the case of totally connected network), or they can choose one of the different paths available to reach the destination (multi-hop, in the case of a partially connected network). This topology also increases the robustness of the network structure. In fact, in case of collapse or temporary malfunction of a node, the routes are recalculated within the forwarding tables to reach all destinations. In the past, when mesh networks were only wired, this topology was very expensive and difficult to realize, because each node had to be physically connected to the others. Today, by employing the advantages introduced by wireless communications and with the appearance of the Wireless Personal Area Networks (WPANs), these limitations have been overcome. In these networks, each autonomous and mobile node, connected to the others, forms a graph of arbitrary size (partially connected mesh network). Instead of relying on a base station that coordinates the flow of messages to each entity in the network, the individual nodes forward the packets to each other. These nodes can move randomly and organize themselves arbitrarily, although wireless topology varies rapidly and unpredictably. Furthermore, these networks can operate alone or be connected to the Internet to provide additional services. Depending on how the technology is developed, the driver of a vehicle can receive a warning in the event of an accident risk or the vehicle itself can independently take preventive actions, such as emergency braking, if it is designed to carry out safety interventions [32]. It is expected that V2V communications will be far more effective than current embedded systems designed by the Original Equipment Manufacturer (OEM) since, to date, the vehicle safety is entirely dependent on the functionality of onboard sensors, cameras, and radars [33]. The system reacts to any dangerous situations based on specific parameters detected by these devices placed on the vehicle. Typically, the main examined parameters are the travel speed, the distance from an obstacle or the presence of a vehicle in the blind spot. However, although the used technologies are increasingly reliable, the calculation errors should not be underestimated. On the contrary, V2V communication protocols will improve the performance in the security field, since, by making all the vehicles close to interact each other, they will help the car in danger (for instance, driver's sleep, a component malfunction, obstacle in the lane, and so on) to undertake a more effective choice to solve the emerging problem. Therefore, the primary purpose of each node forming part of the mesh network will be the data collection to guarantee good security to itself and its neighbors. This system is called cooperative awareness [34]. Anyhow, it is useful to adopt proper coding methods for real-time data access, as suggested in [35; 36]. An overview of the current autonomous OEM security systems (used in the latest generation vehicles) will provide complete knowledge of the technologies currently available on the market.