**2.5 Reference protocol stack**

Each one of the above components contains an ITS station which in turn comprises a number of ITS specific functions and a set of devices implementing these functions. From a communication's point of view an ITS station is based on the reference protocol stack depicted in Figure 3. This protocol stack follows the ISO/OSI reference model and consists of four horizontal layers and two vertical ones that flank the horizontal stack. Access, networking and transport, facilities and applications layers are the horizontal ones, whereas management and security are the vertical ones.

Fig. 3. Reference protocol stack of an ITS station.
