A GDS is an OPC UA Serverwhich allows Clientsto search for Serverswithin the administrative domain of the GDS. It provides Methodsthat allow applications to search for other applications (See 6). To access the GDS, the Clientuses the Callservice to invoke the QueryApplications Method (see 6.6.11) to retrieve a list of Serversthat meet the filter criteria provided. The QueryApplications Method is similar to the FindServersservice except that it provides more advanced search and filter criteria. The discovery process is illustrated in Figure 5.

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Figure 5– The Global Discovery Process

The GDS may be coupled with any of the previous network architectures. For each MulticastSubnet, one or more LDSs may be registered with a GDS.

The Clientcan also be configured with the URL of the GDS using an out of band mechanism.

The complete discovery process is shown in Figure 6.