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 [4]. 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.

Table C.2.2-1 and Table C.2.3-1 summarise candidate interface parameters for the description of the communication service performance. The lists are grouped according to whether the parameter stands for automation characteristic parameters (Table C.2.2-1) or influence quantities (Table C.2.3-1). The meaning of the columns and rows is explained after each table.

  Parameter description Communication service availability

• This parameter indicates if the communication system works as contracted ("available"/"unavailable" state). The communication system is in the "available" state as long as the availability criteria for transmitted packets are met. The service is unavailable if the packets received at the target are impaired and/or untimely (e.g. update time > stipulated maximum). If the survival time (see Table C.2.3-1) is larger than zero, consecutive impairments and/or delays are ignored until the respective time has expired.

End-to-end latency

• This parameter indicates the time allotted to the communication system for transmitting a message and the permitted timeliness.

Communication service reliability

• Mean time between failures is one of the typical indicators for communication service reliability. This parameter states the mean value of how long the communication service is available before it becomes unavailable. For instance, a mean time between failures of one month indicates that a communication service runs error-free for one month on average before an error/errors make the communication service unavailable. Usually, an exponential distribution is assumed. This means, there will be several failures where the time between two subsequent errors is below the mean value (1 month in the example).
• Communication service availability and communication service reliability (mean time between failures) give an indication on the time between failures and the length of the failures.

Service bit rate

1. deterministic communication
   The target value indicates committed data rate in bit/s sought from the communication service. This is the minimum data rate the communication system guarantees to provide at any time, i.e. in this case target value = user experienced data rate.
2. non-deterministic communication
   The target value indicates the target data rate in bit/s. This is the information rate the communication system aims at providing on average during a given (moving) time window (unit: s). The user experienced data rate the lower data rate threshold for any of the time windows.

Update time

Applicable only to periodic communication, the update time indicates the time interval between any two consecutive messages delivered from the egress (of the communication system) to the application.

Traffic classes

• In practice, vertical communication networks serve applications exhibiting a wide range of communication requirements. In order to facilitate efficient modelling of the communication network during engineering, and for reducing the complexity of network optimisation, disjoint QoS sets have been identified. These sets are referred to as traffic classes [6]. Typically, only three traffic classes are needed in industrial environments [6], i.e.
  • deterministic periodic communication;
  • deterministic aperiodic communication; and
  • non-deterministic communication.
• Deterministic periodic communication stands for periodic communication with stringent requirements on timeliness of the transmission.
• Deterministic aperiodic communication stands for communication without a pre-set sending time. Typical activity patterns for which this kind of communication is suitable are event-driven actions.
• Non-deterministic communication subsumes all other types of traffic. Periodic non-real time and aperiodic non-real time traffic are subsumed by the non-deterministic traffic class, since periodicity is irrelevant in case the communication is not time-critical.

Usage of the parameters in Table C.2.2-1

  The transmission of, for instance, program code and configuration data may be handed to the 3GPP system as data burst. In this case, the ingress data rate exceeds the capacity of the network, which implies that some of the data has to be stored within the ingress node of the communication system before it can be transmitted to the egress interface(s). However, the application consuming the communication service requires that the data of such a burst needs to be transmitted completely. This is in contrast to periodic data transmission, where new messages overwrite old ones. The user data length indicates the (maximum) size of the user data packet delivered from the application to the ingress of the communication system and from the egress of the communication system to the application. For periodic communication this parameter can be used for calculating the requested user-experienced data rate. If this parameter is not provided, the default is the maximum value supported by the PDU type (e.g. Ethernet PDU: maximum frame length is 1,522 octets, IP PDU: maximum packet length is 65,535 octets). Describes the start and end time of a communication service. Note that there are other ways to describe the service time interval numerically, for instance as the tuple [start time, service duration]. The maximum survival time indicates the time period the communication service may not meet the application's requirement before the communication service is deemed to be in an unavailable state. Applicable only to periodic communication, the transfer interval indicates the time elapsed between any two consecutive messages delivered by the automation application to the ingress of the communication system.