Figure 14depicts 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 Objectcontains a connection Objectfor each address like an IP multicast address. The connection can have one or more groups with DataSetWriters. A group can publish DataSetsat the defined publishing interval.

In each publishing interval, a DataSetis collected for a PublishedDataSetwhich can be a list of sampled data items in the PublisherOPC UA Address Space. For each DataSeta DataSetMessageis created. The DataSetMessagesare sent in a NetworkMessageto the IP multicast address.

OPC UA Applicationslike HMI applications would use the values of the DataSetMessagethat they are interested in.

An OPC UA Applicationthat maps data fields from UADP DataSetMessagesto internal Variablescan be configured through the DataSetReader Objectand dispatcher in the Subscriber. The configuration of a DataSetReaderdefines how to decode the DataSetMessage to a DataSet. The SubscribedDataSetdefines which field in the DataSet is mapped to which Variablein the OPC UA Application.

With OPC UA UDP there is no guarantee of timeliness, delivery, ordering, or duplicate protection. The sequence numbers in DataSetMessagesprovide a solution for ordering and duplicate detection. The reliability is improved by the option to send the complete DataSetin every DataSetMessageor 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.