OPC UA Applications may demand Quality of Service (QoS) for the transport of NetworkMessages. These QoS requirements have to be fulfilled by the broker-less Message Oriented Middleware and therefore need to be mapped to concrete network technologies like TSN, Deterministic Networking (DetNet) or differentiated services (DiffServ). This mapping should be hidden to the application engineer from a PubSub perspective but may be monitored or configurable via the information model of OPC 10000-22.
Figure 17 – Message Oriented Middleware providing QoS
QoS requirements of an OPC UA Applications shall be configurable with OPC UA means and without dependencies to the underlying network technology. Hiding network details from the application simplifies to migrate OPC UA Applications from one network technology to another or to interconnect OPC UA Applications over different network technologies.
QoS requirements can be fulfilled by different network mechanisms and may require different QoS control mechanisms in the network depending on the requested level of QoS.
Figure 18 – Mapping of priority-based QoS
As shown in Figure 17 and Figure 18 the PriorityLabel from a WriterGroup, DataSetReader or PubSubConnection TransportSettings will be translated into actual values to be used on the wire. Together with the optional QosCategory the PriorityLabel will be present in the mapping table for the network interface used to transmit the data. If the combination of QosCategory and PriorityLabel is not present in the mapping table, the communication cannot be established. The reference from the network interface to the mapping table is defined in OPC 10000-22.
Standard values for QosCategory with the according required structures in the DatagramQos array are defined in Table 102. This list can be extended by specifications built on top of OPC UA PubSub. Each QosCategory will be described in detail by a list of measurable QoS KPIs like assured bandwidth or maximum latency in the parameter DatagramQos.