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.
Starting with the general approach mentioned in Subclause C.1.1, the logical link can be regarded as a possible asset within the area of consideration (see Figure C.1.1-1
). The conditions under which its functions are to be performed are vital for the dependability of the automation application system.
This is the link between a logical end point in a source device and the logical end point in a target device. Logical end points are elements of the reference interface, which may group several logical end points together.
The intended function of the logical link is the transmission of a sequence of messages from a logical source end point to the correct logical target end point. This is achieved by transforming each message into a form that fosters error-free transmission. The transmission process includes certain processes, for instance repetitions, in order to fulfil the intended function. After transmission, the transported package(s) is converted back into a message. The message is to be available and correct at the target within a defined time. The sequence of messages at the target is to be the same as the sequence at the source.
The functional units, which are necessary to fulfil this function are shown, in Figure C.1.2.1-1
The required function can be impaired by various influences, which can lead to communication errors. Such errors are described elsewhere in the literature  
. A summary of these errors is provided in Annex A
. The occurrence of one of these errors influences the values of the relevant dependability parameters of the logical link.
The present document addresses both OSI-layer-3 (IP) and OSI-layer-2 communication. The model in Figure C.1.2-1 can be used for describing both cases. The implementation of communication functions is split between a higher communication layer (HCL) and a lower communication layer (LCL). The partition of the layer for the two traffic options discussed in the present document is provided in Table C.1.2.2-1. This difference is of importance when discussing the implications of the service performance requirements in Clause 5
and Annex A
for the network performance (see Clause C.5
|OSI level at which the traffic occurs
||Levels comprised by the higher communication layer
||Levels comprised by the lower communication layer
|3||4 to 6||1 to 3|
|2||3 to 6 (note)||1 to 2|
In some vertical application, level 3 to 6 are not implemented.
The messages to be transmitted for the intended function of a logical link are defined by strings of characters with a certain semantic. Such a character string is handed over as user data at the reference interface for transmission. If the number of characters in a message is too great for it to be transmitted as a unit, the message is divided for transmission into several packets (fragmentation).
The communication devices—together with the physical link—determine the function and thus the dependability of the logical link. The function of the communication devices is the correct sending and correct receipt of sequences of messages. The asset "communication device" is depicted in Figure C.1.2.3-1.
The communication system as an asset represents a quantity of logical links whose message transmissions are implemented by wireless devices via one or more media. The communication system function to be provided consists in transmitting messages for all the logical links in the distributed application. This function is to be performed for a defined period, the operating time of the automation application.
In an automation application system, it is paramount that requirements pertaining to logical links are fulfilled. These requirements and the conditions can be very different from one case and implementation to the other. The functions (services and protocols) for individual logical links can therefore also be different. Despite these differences, some of the logical links share communication devices and media.