The set of Objectsand related information that the OPC UA Servermakes available to Clientsis referred to as its AddressSpace. The model for Objectsis defined by the OPC UA Object Model (see 4.2).

Objects and their components are represented in the AddressSpaceas a set of Nodesdescribed by Attributesand interconnected by References. Figure 3illustrates the model of a Nodeand the remainder of 4.3 discusses the details of the Node Model.

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Figure 3– AddressSpace Node Model

NodeClassesare defined in terms of the Attributesand Referencesthat shall be instantiated (given values) when a Nodeis defined in the AddressSpace. Attributesare discussed in 4.3.3and Referencesin 4.3.4.

Clause 5defines the NodeClassesfor the OPC UA AddressSpace. These NodeClassesare referred to collectively as the metadata for the AddressSpace. Each Nodein the AddressSpaceis an instance of one of these NodeClasses. No other NodeClassesshall be used to define Nodes, and as a result, Clientsand Serversare not allowed to define NodeClassesor extend the definitions of these NodeClasses.

Attributesare data elements that describe Nodes. Clientscan access Attributevalues using Read, Write, Query, and Subscription/MonitoredItem Services. These Servicesare defined in OPC 10000-4.

Attributesare elementary components of NodeClasses. Attributedefinitions are included as part of the NodeClassdefinitions in Clause 5and, therefore, are not included in the AddressSpace.

Each Attributedefinition consists of an attribute id (for attribute ids of Attributes, see OPC 10000-6), a name, a description, a data type and a mandatory/optional indicator. The set of Attributesdefined for each NodeClassshall not be extended by Clientsor Servers.

When a Nodeis instantiated in the AddressSpace, the values of the NodeClass Attributesare provided. The mandatory/optional indicator for the Attributeindicates whether the Attributehas to be instantiated.

Referencesare used to relate Nodesto each other. They can be accessed using the browsing and querying Servicesdefined in OPC 10000-4.

Like Attributes, they are defined as fundamental components of Nodes. Unlike Attributes, Referencesare defined as instances of ReferenceType Nodes. ReferenceType Nodesare visible in the AddressSpace and are defined using the ReferenceType NodeClass(see 5.3).

The Nodethat contains the Referenceis referred to as the SourceNodeand the Nodethat is referenced is referred to as the TargetNode. The combination of the SourceNode, the ReferenceTypeand the TargetNodeare used in OPC UA Servicesto uniquely identify References. Thus, each Nodecan reference another Nodewith the same ReferenceTypeonly once. Any subtypes of concrete ReferenceTypesare considered to be equal to the base concrete ReferenceTypeswhen identifying References(see 5.3for subtypes of ReferenceTypes). Figure 4illustrates this model of a Reference.

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Figure 4– Reference Model

The TargetNodeof a Referencemay be in the same AddressSpaceor in the AddressSpaceof another OPC UA Server. TargetNodeslocated in other Serversare identified in OPC UA Servicesusing a combination of the remote Servername and the identifier assigned to the Nodeby the remote Server.

OPC UA does not require that the TargetNode exists, thus Referencesmay point to a Nodethat does not exist.