For our discussion of communication in automation we apply a definition of the area of consideration for industrial radio communication that is found elsewhere in the literature 
. This definition is illustrated in Figure C.1.1-1.
Here, a distributed automation application system is depicted. This system includes a distributed automation application, which is the aggregation of several automation functions. These can be functions in sensors, measurement devices, drives, switches, I/O devices, encoders etc. All of these functions contribute toward the control of physical objects. Field bus systems, industrial Ethernet systems, or wireless communication systems can be used for connecting the distributed functions. The essential function of these communication systems is the distribution of messages among the distributed automation functions. For cyber-physical control applications, the dependability of the entire communication system and/or of its devices or its links is essential. Communication functions are realised by the respective hardware and software implementation.
In order for the automation application system to operate, messages need to be exchanged between spatially distributed application functions. For that process, messages are exchanged at an interface between the automation application system and the communication system. This interface is termed the reference interface. Required and guaranteed values for characteristic parameters, which describe the behavioural properties of the radio communication system, as well as some influence quantities refer to that interface.
The conditions that influence the behaviour of wireless communication are framed by the communication requirements of the application (e.g., end-to-end latency), the characteristics of the communication system (e.g., output power of a transmitter), and the transmission conditions of the media (e.g., signal fluctuations caused by multipath propagation).
General requirements from the application point of view for the time and failure behaviour of a communication system are mostly related to an end-to-end link. It is assumed in the present document that the behaviour of the link is representative of the communication system as a whole and of the entire scope of the application.