With this option, OPC UA PubSub relies on the network infrastructure to deliver NetworkMessages to one or more receivers. Network devices – like network routers, switches, or bridges – are typically used for this purpose.
One example is a switched network and the use of UDP with unicast or multicast messages shown in Figure 13.
Figure 13 – PubSub using network infrastructure
Advantages of this model include:
- Only requires standard network equipment and no additional software components like a Broker.
- Message delivery is assumed to be direct without software intermediaries and therefore provides reduced latency and overhead.
- UDP protocol supports multiple subscribers using multicast addressing.
Figure 14 depicts the applications, entities and messages involved in peer-to-peer communication using UDP as a protocol that does not require a Broker.
Figure 14 – UDP Multicast overview
The PublishSubscribe Object contains a connection Object for each address like an IP multicast address. The connection can have one or more groups with DataSetWriters. A group can publish DataSets at the defined publishing interval.
In each publishing interval, a DataSet is collected for a PublishedDataSet which can be a list of sampled data items in the Publisher OPC UA Address Space. For each DataSet a DataSetMessage is created. The DataSetMessages are sent in a NetworkMessage to the IP multicast address.
OPC UA Applications like HMI applications would use the values of the DataSetMessage that they are interested in.
An OPC UA Application that maps data fields from UADP DataSetMessages to internal Variables can be configured through the DataSetReader Object and dispatcher in the Subscriber. The configuration of a DataSetReader defines how to decode the DataSetMessage to a DataSet. The SubscribedDataSet defines which field in the DataSet is mapped to which Variable in the OPC UA Application.
With OPC UA UDP there is no guarantee of timeliness, delivery, ordering, or duplicate protection. The sequence numbers in DataSetMessages provide a solution for ordering and duplicate detection. The reliability is improved by the option to send the complete DataSet in every DataSetMessage or with the option to repeat NetworkMessages.
Other transport protocol mappings used with the broker-less model could provide guarantee of timeliness, delivery, ordering, or duplicate protection.