OPC UA Publish-Subscribe

OPC UA stands out from technologies like Ethernet-based Time-Sensitive Networking (TSN) since it focuses on information rather than data. While TSN helps facilitate Industry 4.0 by setting out rules for operation technology (OT) and information technology (IT) nodes to communicate effectively, OPC UA structures and transforms data into information. Significant OPC UA functions include detecting servers, accessing server data, notification of events and execution of methods. This technology is comprehensive and configurable, offering a framework for defining data structures without prescribing any specific set of data structures. Additionally, OPC UA offers a security system for authentication, authorization, and encryption of data.

The publish-subscribe (Pub/Sub) model was implemented as an alternative to the traditional client-server model with its request-response approach. Pub/Sub is supported by KPA Automation softPLC. In this model, publishers can send data either to a broker for subsequent distribution to all the subscribed clients, or just send a unicast/multicast package, while subscribers listen at their local address (unicast) or at a predefined multicast address. The data itself can be packed in the binary format, as well as in JSON, that eases the application in the web-based solutions.

As opposed to the client-server model, this approach decouples the data sources and consumers, thus transforming the system into an event-driven structure. The Pub/Sub system simplifies communication in comparison to client-server, eliminating the need for redundant polling and automatically transmitting data as soon as it is changed. Multicast further enhances this system by enabling Pub/Sub without the addition of a discrete broker to the network. Additionally, the nested layering of data sets increases network efficiency.

Multicast allows simultaneous network packet transmissions to multiple hosts and can be operated both via IP and Ethernet. OPC UA can be used to map messages to IP by utilizing UDP or bypass IP and map them directly to Ethernet. The advantage of multicast over the classic Pub/Sub model is that it gets rid of the need for a broker. Everything is handled through the multicast group, and whatever data is sent by the source is automatically sent to every consumer subscribed to that group.

Furthermore, the OPC UA format for Pub/Sub messages has nested layers. An OPC UA network message includes numerous datasets, each of which is composed of a variety of fields. This enables, for instance, an industrial PC that is in charge of multiple PLCs to group datasets for each PLC into a single network message, decreasing network traffic.

The following are a few potential applications for Pub/Sub:

  1. Configurable peer-to-peer communication between controllers as well as controllers and HMIs without the need for either party to be aware of the other’s existence. This data exchange is often on a set timeline and could be a one-way connection or a broadcast to multiple receivers.
  2. Asynchronous workflows where, for example, an order processing program can place an order on a message queue or an enterprise service bus, and it can then be managed by one or various workers.
  3. Logging to multiple systems is also possible, for example, sensors or actuators can send logs to a monitoring system, HMI, an archive database for future queries, and so on.
  4. OPC UA Servers that represent services or devices can also stream data to cloud-hosted applications. This could include backend servers, big data analytics for optimization of the system and predictive maintenance.

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