## 2 ITS Architecture and Applications
### ITS Characteristics and Challenges  
Even though ITS will have great potential in the near future, there are many open research challenges and issues that need to be addressed in order to deploy effective and safe ITS applications. In particular, the design of ITS applications requires special attention and is characterized by the following main features:  
* Powerful capacity: ITS stations (_i.e._, RSUs, OBUs) are powerful in terms of energy, localization, computation, storage and data rate capabilities;
* High mobility: ITS involves a huge number of nodes that move from one location to another, with different speeds and directions, thus making the prediction of node position very difficult and necessitating node protection [25];
* Dynamic network topology: depending on their locations and speeds, ITS stations can join and/or leave the network very quickly; the resulting network topology is thus highly dynamic;
* Time sensitivity: safety information must be delivered to nodes in a short period of time (e.g., 100-ms delay for safety-related messages), thus making latency one of the most important quality of service (QoS) limitations for these kinds of networks;
* Sufficient energy: unlike wireless sensor networks (WSN), where nodes have limited resources and a small battery lifetime, ITS entities are considered as resource-rich devices (_i.e._, energy, storage and computation); this allows the implementation of complex algorithms to achieve a higher throughput in vehicular environments [26];
* Good physical protection: inside each ITS entity, physical protection can be ensured, giving the network immunity against physical attacks [26];
* Unbounded network size: ITS can be implemented in a small geographical area, or in cities, or even across several countries; this means that the size of the ITS network is not limited to a specific geographical area [27];
* Wireless communications: ITS entities communicate with each other and exchange information via a wireless connection; hence, some security measures and protocols must be used to ensure safe and protected communication;
* Heterogeneous V2X communication technologies: vehicles exploit different types of communication modes, such as multi-hop V2V, point-to-point V2I, short/long range V2I, _etc._; in addition, connected vehicles support a wide range of communication technologies, such as IEEE 802.11p, Wi-Fi, Bluetooth, 3G/LTE, _etc._;
* Heterogeneous environments: vehicles operate in various environments, such as indoor, outdoor, low and high network density, _etc._;
* Security and privacy: security and privacy are of paramount importance in ITS; in most scenarios, attackers aim to affect the authentication, integrity and even the availability of the network; in this context, security protocols must be implemented with low communication overhead due to time constraint and low computation complexity to exchange quick and safe information.  
A careful analysis of the above characteristics reveals that the ITS design challenges are sometimes contradictory. On the one hand, vehicular communications should be efficient and provide real-time performance, but on the other hand, extra processing and message overheads are required to ensure the security and privacy of these communications. Hence, it is necessary to achieve optimal trade-offs between these two issues in order to meet the QoS requirements in terms of the safety and practicality of ITS applications.