Content for  TS 22.104  Word version:  19.1.0

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4  Overviewp. 10

4.1  Introductionp. 10

For the purpose of this document, a vertical domain is a particular industry or group of enterprises in which similar products or services are developed, produced, and provided. Automation refers to the control of processes, devices, or systems in vertical domains by automatic means. The main control functions of automated control systems include taking measurements, comparing results, computing any detected or anticipated errors, and correcting the process to avoid future errors. These functions are performed by sensors, transmitters, controllers, and actuators.
In the context of this document, cyber-physical systems are referred to as systems that include engineered, interacting networks of physical and computational components. Cyber-physical control applications are to be understood as applications that control physical processes. Cyber-physical control applications in automation follow certain activity patterns, which are open-loop control, closed-loop control, sequence control, and batch control (see Clause 4.2).
Communication services supporting cyber-physical control applications need to be ultra-reliable, dependable with a high communication service availability, and often require low or (in some cases) very low end-to-end latency.
Communication in automation in vertical domains follows certain communication patterns. The most well-known is periodic deterministic communication, others are aperiodic deterministic communication and non-deterministic communication (see Clause 4.3).
Communication for cyber-physical control applications supports operation in various vertical domains, for instance industrial automation and energy automation. This document addresses service requirements for cyber-physical control applications and supporting communication services from the vertical domains of factories of the future (smart manufacturing), electric power distribution, and central power generation. Service requirements for cyber-physical control applications and supporting communication services for rail-bound mass transit are addressed in TS 22.289.

4.2  Activity patterns in automationp. 11

Open-loop control:
The salient aspect of open-loop control is the lack of feedback from the output to the control; when providing commands to an actuator, it is assumed that the output of the influenced process is predetermined and within an acceptable range. This kind of control loop works if the influences of the environment on process and actuator are negligible. Also, this kind of control is applied in case unwanted output can be tolerated [8].
Closed-loop control:
Closed-loop control enables the manipulation of processes even if the environment influences the process or the performance of the actuator changes over time. This type of control is realised by sensing the process output and by feeding these measurements back into a controller [8].
Sequence control:
Sequence control may either step through a fixed sequence or employ logic that performs different actions based on various system states and system input [8]. Sequence control can be seen as an extension of both open-loop and closed-loop control, but instead of achieving only one output instance, an entire sequence of output instances can be produced [9].
Batch control:
Batch processes lead to the production of finite quantities of material (batches) by subjecting input materials to a defined order of processing actions by use of one or more pieces of equipment [10].

4.3  Communication attributesp. 11

Communication in automation can be characterised by two main attributes: periodicity and determinism.
Periodicity means that a transmission interval is repeated. For example, a transmission occurs every 15 ms. Reasons for a periodical transmission can be the periodic update of a position or the repeated monitoring of a characteristic parameter. Most periodic intervals in communication for automation are rather short. The transmission is started once and continuous unless a stop command is provided.
An aperiodic transmission is, for example, a transmission which is triggered instantaneously by an event, i.e., events are the trigger of the transmission. Events are defined by the control system or by the user. Example events are:
  • Process events: events that come from the process when thresholds are exceeded or fallen below, e.g., temperature, pressure, level, etc.
  • Diagnostic events: events that indicate malfunctions of an automation device or module, e.g., power supply defective; short circuit; too high temperature; etc.
  • Maintenance events: events based on information that indicates necessary maintenance work to prevent the failure of an automation device.
Most events, and especially alarms, are confirmed. In this context, alarms are messages that inform a controller or operator that an event has occurred, e.g., an equipment malfunction, process deviation, or other abnormal condition requiring a response. The receipt of the alarm is acknowledged usually within a short time period by the application that received the alarm. If no acknowledgment is received from the target application after a preset time, the so-called monitoring time, the alarm is sent again after a preset time or some failure response action is started.
Determinism refers to whether the delay between transmission of a message and receipt of the message at the destination address is stable (within bounds). Usually, communication is called deterministic if it is bounded by a given threshold for the latency/transmission time. In case of a periodic transmission, the variation of the interval is bounded.

4.4  Control systems and related communication patternsp. 12

There are preferences in the mapping between the type of control and the communication pattern. Open-loop control is characterised by one or many messages sent to an actuator. These can be sent in a periodic or an aperiodic pattern. However, the communication means used need to be deterministic since typically an activity response from the receiver and/or the receiving application is expected.
Closed-loop control produces both periodic and aperiodic communication patterns. Closed-loop control is often used for the control of continuous processes with tight time-control limits, e.g., the control of a printing press. In this case, one typically relies on periodic communication patterns. Note that in both the aperiodic and periodic case, the communication needs to be deterministic.
Logging of device states, measurements, etc. for maintenance purposes and such typically entails aperiodic communication patterns. In case the transmitted logging information can be time-stamped by the respective function, determinism is often not mandatory.
In practice, vertical communication networks serve a large number of 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, traffic classes or communication patterns have been identified [6]. There are three typical traffic classes or communication patterns in industrial environments [6], i.e.,
  • deterministic periodic communication: periodic communication with stringent requirements on timeliness of the transmission.
  • deterministic aperiodic communication: communication without a preset 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, including periodic non-real time and aperiodic non-real time traffic. Periodicity is irrelevant in case the communication is not time-critical.
Some communication services exhibit traffic patterns that cannot be assigned to one of the above communication patterns exclusively (mixed traffic).

4.5  Implications for 5G systemsp. 12

In order to be suitable for automation in vertical domains, 5G systems need to be dependable and flexible to meet specific KPIs to serve specific applications and use cases. They need to come with the system properties of reliability, availability, maintainability, safety, and integrity. What particular requirements each property needs to meet depends on the particularities of the domain and the use case. Annex F discusses the difference between reliability and communication service availability. The requirements in this document provide various sets of performance criteria that need to be met to satisfactorily support different use cases of cyber-physical control applications used by various vertical markets.

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