OPC UA Client/Server
The SOA based communication concept in OPC UA, based on the Client/Server principle, where a client sends a "Request" to the server and the server sends back a "Response" within a peer-to-peer connection context, is well established in industrial applications. Reason is that it has a reliable, acknowledged data transfer with reduced bandwidth and it has several techniques to avoid data loss in harsh environments and it comes with ability to monitor the live time of the communication partner. In addition the security mechanisms are well defined and allow authenticated, authorized and confidential access to information. Therefore mandatory OPC UA Client/Server communication is the preferred approach in nearly all application scenarios and is supported by all OPC UA Servers and Clients on the market as of today. You can realize controller-to-controller and controller-to-HMI type of application scenarios as well as MES/ERP-to-automation and telemetry-data-to-Cloud type of applications. For diagnostic and configuration type of applications this communication schema is used. The communication is based on TCP/IP and it is firewall friendly, reliable and secure.
OPC UA Pub/Sub
In addition to the above there exist some exceptional use cases where one application wants to share the same data with multiple receivers. Instead of being "asked" for data by separate clients individually, a server can cyclically send a predefined set of data values to a group of data consumers who are interested in all or parts of such data stream. This broadcast type of preconfigured data stream is actively send by the publisher in a preconfigured cyclic rate. So called one-to-many and many-to-one communication can be realized easily and with extreme low resource consumption on the sender side. The communication is based on UDP and it is simple, cyclic and fast.
1) OPC UA to Cloud
Besides the aready existing UA Client appliactions collecting data from the plant flor and converting them into cloud messages and sending (publish) them to the Broker, so called cloud gateways, which traditionally run in subnet (on premise), there may be the demand for direct cloud connectivity of an individual device in certain scenarios. In typical Cloud applications the messages are send form the publisher directly to the Relay/Broker. By this the firewall of the plant remains closed, the publisher initiates the communication and calls the Broker that resides in the global network or on the edge. Based on the pre-configured set of data to be published, the message can directly be transported over common Cloud protocol like AMQP or MQTT. An advanced OPC UA Server can create the message either UA-Binary endoced or JSON encoded and hence transmit via the prefered cloud protocol. With this optional technology enhancement OPC UA delivers another important building block for directly connected IT application scenarios in Industrie 4.0 and the IIoT.
2) OPC UA over TSN
In fast, local networks and having the appropriate hardware support, the Publisher/Subscriber communication mechanism can fulfill requirements of deterministic communications. Therefore messages must have fixed content and constant length and transmission on the network (layer 2) must be run with exactly identical duration through all network nodes. Such "planned" layer 2 transfer is enabled by Time Sensitive Network (TSN). All participants and all switches within the deterministic TSN-network must be time synchronized and must be configured to be able to transmit the data to the final receiver (scheduling). TSN is a set of several Ethernet specifications in IEEE 802.1 and 801.3. OPC UA needs at least two of these Ethernet enhancements i) 802.1 AS-Rev for time synchronization and ii) 802.1Qbv for Scheduling. The OPC Foundation is pushing forward specification and implementation work with a dedicated OPC UA TSN working group consisting of 85+ members. As soon as TSN enabled Ethernet switches and devices are considerably available and can consistently be configured, OPC UA (Pub/Sub) over TSN will enable deterministic data transfer between controllers. For example robot-body-controller and robot-hand-controller could communicate in real time. The OPC Foundation plans to enhance their certification efforts for vendor-independent, deterministic Controller-to-Controller communication. With this optional technology enhancement OPC UA delivers another important building block for special, deterministic application scenarios in Industrie 4.0 and the IIoT.