Management of the Device Topologyis a configuration task, i.e., the elements in the topology (Devices, Networks, and Connection Points) are usually configured “offline” and – at a later time – will be validated against their physical representative in a real network.
To support explicit access to either the online or the offline information, each element may be represented by two instances that are schematically identical, i.e., there exist component Objects, FunctionalGroups, and so on. A Referenceconnects online and offline representations and allows to navigate between them.
This is illustrated in Figure 27.
If Online/Offline is supported, the main (leading) instance represents the offline information. Its HasTypeDefinition Referencepoints to the concrete configured or identified ObjectType. All Parametersof this instance represent offline data points and reading or writing them will typically result in configuration database access. Propertieswill also represent offline information.
A Devicecan be engineered through the offline instance without online access.
The online data for a topology element are kept in an associated Objectwith the BrowseName Onlineas illustrated in Figure 27. The Online Objectis referenced via an IsOnline Reference. It is always of the same ObjectTypeas the offline instance.
The online Parameter Nodesreflect values in a physical element (typically a Device), i.e., reading or writing to a Parametervalue will then result in a communication request to this element. When elements are not connected, reading or writing to the online Parameter will return a proper status code (Bad_NotConnected).
The transfer of information (Parameters) between offline nodes and the physical device in correct order is supported through TransferToDevice, TransferFromDevicetogether with FetchTransferResultData. These Methodsare exposed by means of an AddIninstance of TransferServicesTypedescribed in 6.4.2.
Both offline and online are created and driven by the same ObjectType. According to their usability, certain components (Parameters, Methods, and FunctionalGroups) may exist only in either the online or the offline element.
A Parameterin the offline ParameterSetand its corresponding counterpart in the online ParameterSetshall have the same BrowseName. Their NodeIdsneed to be different, though, since this is the identifier passed by the Clientin read/write requests.
The Identification FunctionalGrouporganizes Parametersthat help identify a topology element. Clientscan compare the values of these Parametersin the online and the offline instance to detect mismatches between the configuration data and the currently connected element.