# XCF Services

Substation ES IMS Interface connects to an XCF Group. XCF services register and announce the joining of Substation ES IMS Interface as an XCF member.

XCF, the transport layer, delivers OTMA messages between Substation ES IMS Interface and the OTMA server.

You must specify an XCF group name in the Substation ES startup parameters for connecting to the IMS server at initialization. The group name for Substation ES IMS Interface and for the targeted IMS server must be the same.

The XCF Group is created and owned by IMS. If Substation ES detects that IMS has not been started, Substation ES waits until the IMS region starts successfully and then connects. In this case, Substation ES prints messages to the log file indicating that IMS has not been started.

All members of the XCF group are clients, except IMS, which is the server. Substation ES IMS Interface, acting as a client, communicates with IMS through the XCF interface by sending OTMA messages to IMS. In turn, IMS sends the result set back to Substation ES IMS Interface.

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