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OPC UA 57-7<br />

and the TargetNode are used in OPC UA services to uniquely identify References. Thus, each Node can<br />

reference another Node with the same ReferenceType only once.<br />

The TargetNode of a reference may be in the same AddressSpace or in the AddressSpace of another<br />

OPC UA server. TargetNodes located in other servers are identified in OPC UA Services using a combination<br />

of the remote server name and the identifier assigned to the Node by the remote server. OPC UA<br />

does not require that the TargetNode exists, thus References may point to a Node that does not exist!<br />

57.4 Overview of UA Services<br />

This section contains a summary of the description of UA services presented in Part 1 of the UA<br />

specifications. For a detailed description of UA Services, it is advised to read Part 4 of the UA specifications.<br />

Table 57.1 lists different parts of the UA specifications.<br />

57.4.1 General Services<br />

Request/response services are services invoked by the UA client application through the OPC UA service<br />

interface to perform a specific task on one or more Nodes in the AddressSpace and to return a response.<br />

Publisher Services are services invoked through the OPC UA service interface for the purpose of<br />

periodically sending Notifications to UA clients. Notifications include Events, Alarms, Data Changes,<br />

and Program Outputs.<br />

57.4.2 Discovery Service Set<br />

This service set defines services used to discover OPC UA servers that are available in a system. It also<br />

provides a manner in which clients can read the security configuration required for connection to the<br />

Server. The Discovery services are implemented by individual UA servers and by dedicated Discovery<br />

servers. Well-known dedicated Discovery servers provide a way for clients to discover all registered OPC<br />

UA servers.<br />

57.4.3 SecureChannel Service Set<br />

This service set defines services used to open a <strong>communication</strong> channel that ensures the confidentiality<br />

and integrity of all messages exchanged with the UA server. More details about the base concepts for UA<br />

security are defined in Part 2 of the UA specifications.<br />

TABLE 57.1<br />

List of the UA Specifications<br />

OPC UA Part N° Description Status<br />

Part 1 Overview and concepts 1.01 specification Released<br />

Part 2 Security model 1.01 specification Released<br />

Part 3 Address space model 1.01 specification Released<br />

Part 4 Services 1.01 specification Released<br />

Part 5 Information model 1.01 specification Released<br />

Part 6 Mappings 1.00 specification Released<br />

Part 7 Profiles 1.00 specification Released<br />

Part 8 Data access 1.01 specification Released<br />

Part 9 Alarms draft 0.62 specification Draft<br />

Part 10 Programs 1.00 specification Released<br />

Part 11 Historical access 1.00 specification Released<br />

© <strong>2011</strong> by Taylor and Francis Group, LLC

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