# Synchronous Callout (ICAL) or Request/Reply Support

The Substation ES OTMA interface supports the use of synchronous callouts (ICAL), also known as request/replies.

To use synchronous callouts from an MPR, BMP, IMS Fast Path, or Java processing facilities, use the ICAL call to send a request to a destination defined in DFSYDTx with a TPIPE that matches the `TRIG-SYNC` parameter in the Substation ES IMS interface definition. Substation ES receives the request, and passes it on to TIBCO Enterprise Message Service or Rendezvous. When the reply is received, Substation ES posts the response to the ICAL issuer.

A sample program written in COBOL, `SXICIRR1`, is provided in the COB library.

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