# Security Threats in Intelligent Transportation Systems and Their Risk Levels
## 2 ETSI ITS-S Communication Architecture  
ITS communications are based on ITS-S architecture of communication described in ETSI EN 302 665 (ETSI 2010). ITS-S standards are based on a common communication architecture (ITS-S reference architecture ISO 21217). This is essential to ensure the interoperability of systems deployed in vehicles, road infrastructure, urban infrastructure and control centers available through the internet infrastructure. The communication architecture of ETSI ITS-S is structured in layers and is based on the OSI or TCP/IP communication models. ETSI ITS-S architecture supports wired and wireless communication technologies, versus to the IEEE WAVE communication architecture (IEEE 2010) (based on IEEE 802.11p). Separation into layers allows applications to be developed that operate independently of the underlying technologies, thus enabling portability across distinct hardware and software platforms, and allowing features or technologies to be replaced or added in the lower layers without impacting the higher layers. This is the model that made internet communication successful because it allows end-to-end exchanges between two remote peers that use separate means of communication (e.g., a vehicle connected only to the cellular network can communicate with another vehicle connected to the internet via WiFi). The \"ETSI ITS-S\" architecture includes (cf. Figure 1):  
* _A lower \"access technologies\" layer (ITS-S access technologies layer)_ allowing the integration of all existing and future access technologies, provided that each new access technology respects the rules of integration to the architecture (specified in ISO 21218). Today, vehicular WiFi (IEEE 802.11p, with ISO M5, ETSI ITS G5, IEEE P1609 variants), urban WiFi (IEEE 802.11n/b/g/ac), cellular (2G, 3G, 4G, etc.), satellite and 6 LoWPAN sensor networks (IEEE 802.14) are already supported.
* _An ITS-S networking & transport layer_ allowing both localized communications (ISO FNTP, ETSI GeoNetworking, IPv6) for direct exchanges between vehicles and infrastructure (V2V, V2R) and remote communications (IPv6) with distant peers. GeoNetworking is designed to use vehicular WiFi only; however IPv6 allows transmission over any locally available technology (IPv6 transmission can also be done over an existing IPv4 access network when IPv6 is not deployed).
* _A \"services\" layer (ITS-S facilities layer)_ allowing applications to benefit from shared services, in particular standardized messaging, a database (LDM), datagram tagging services (time-stamping, geo-stamping), reliable positioning (fusion of data from several sources: GPS, roadside beacons, CAN bus, etc.). This layer also has the ability to direct datagrams to the most suitable communication stack according to the communication profile and the current capabilities of the ITS station.  
An \"applications\" layer_ where all applications can benefit from the shared or communication services of the lower layers without being constrained. To benefit from these services, the applications must make their communication needs known by providing the management entity with the characteristics of each of the data flows likely to be transmitted by the application.  
Two cross-layers: (1) A vertical (cross-layer) management entity, allowing management of the internal functionalities of the ITS-S (in particular the functionalities available in each layer) to determine which access technologies are available in a given place and at a given time, and to manage the data flows (ISO 24102-6) as best as possible. (2) A vertical entity (cross-layer) of security, allowing all layers to benefit from the mechanisms necessary to secure communications (encryption, authentication, etc.).