OPC UA Architecture

The communication stack is only the beginning of various innovations. The OPC UA architecture is a Service Orientated Architecture (SOA) and is based on different logical levels.

All of the Base Services defined by OPC are abstract method descriptions which are protocol independent and found the basis for the whole OPC UA functionality. The transport layer transforms these methods into a protocol, which means it serializes/de-serializes the data and transmits it over the network. Currently there are two protocols specified for this purpose. One is a binary, high performance optimized TCP protocol and the second, a Webservice-based protocol.
The OPC Information Model is not just a hierarchy based on folders, items and properties anymore, but a so-called Full Mesh Network based on Nodes instead. This network of Nodes can additionally transmit all varieties of meta information and diagnostic data. The closest image of a node would be an object, known thru object orientated programming (OOP). It can own attributes for read access (DA, HDA), methods which can be called (Commands) and triggered events which can be fired (AE, DA DataChange). The Nodes get used for process data as well as for all other types of meta data. The newly modeled OPC namespace contains now also the Type Model used for description of all possible data types.
Based on this new architecture, other organizations e.g. like EDDL, FDT or even ISA95 are specifying their own information source. Client software has the ability to verify which of the so-called Profiles a server supports. This is necessary to obtain information if a server only supports the DA functionality or additionally the AE, HDA, etc. functions. Additionally, information can be obtained whether a server supports i.e. the EDDL profile and therefore the client knows that there will be EDDL-specific device descriptions available.
Additional new and important features of OPC UA are:

• Redundancy support
• Heartbeat for connection monitoring in both directions. That means the server as well as the client recognize failures.
• Buffering of data and acknowledgements of transmitted data. Lost connections do not lead to data loss anymore. Lost data packages can be fetched again.