The established technologies – C-ITS and DSRC
C-ITS and DSRC both use variants of the familiar IEEE 802.11 Wi-Fi standards as the physical and medium access layers of their protocol stack. For DSRC this protocol layer is defined by the IEEE 802.11p standard, while C-ITS uses the similar ETSI-defined ITS-G5 standard, which is basically a version of 802.11p adapted to suit European regulations. While there are significant differences in the higher-level functioning of C-ITS and DSRC, both can run on broadly-compatible hardware and reside in almost the same frequency range. Both these V2X systems can communicate effectively between fast-moving vehicles over a distance of at least 300m and at data rates ranging from approximately 3-27 Mbps.
In 1999, the US FCC allocated the 5.850-5.925 GHz band for high-priority road safety and traffic management applications, and the European Commission has dedicated 5.875-5.905 GHz for similar purposes. IEEE 802.11p and ITS-G5 are designed to work in these respective ranges. To make them better optimised for the cluttered and fast-moving automotive environment, the standards feature modifications to typical IEEE 802.11 Wi-Fi frequency usage which reduce the impact of Doppler shifts and multipath fading.
One of the key differences between these automotive protocols and other members of the 802.11 family is that the initial handshake and association period can be reduced to a bare minimum. The protocol can send data almost immediately, and can also defer authentication, encryption and full identification to higher level protocols. Therefore, vehicles and infrastructure can begin exchanging essential data on speed and position within tens of milliseconds of detecting each other. This is obviously ideal for applications such as collision avoidance, and a stark difference from the seconds-long handshaking negotiation that are common in other Wi-Fi variants.
While C-ITS and DSRC are very similar at the MAC/PHY access level, above that, they diverge significantly at the networking and transport level. Both support TCP/UDP over IPv6, but for time-critical features they each have their own specialised low-overhead messaging standard. DSRC uses the wave short message protocol (WSMP), which is defined as part of IEEE 1609. However, C-ITS employs a more ambitious multi-hop routing system, the basic transport protocol (BTP) over the GeoNetworking service, as defined by the ETSI EN 302 636 series of standards. GeoNetworking is a geographically aware routing technique, which establishes an ad-hoc network that is efficiently arranged according to the physical locations of nodes.