# IMS Transactions

This section describes the way the OTMA Interface, which is part of the IMS interface and Substation ES, handles transactions.

## Request-Reply Guaranteed

When using Substation Guaranteed services, the OTMA client (TIBCO Substation) performs these steps:

1. Specify `Synclevel=1` (`Syncpt`) in the OTMA message prefix.
2. Specify `commit mode 1`.

   This is the only option that is used.

3. The transaction is received by Substation ES, sent to OTMA, and then to IMS.
4. After an ISRT by the IMS application on IOPCB to TPIPE, the output is returned to OTMA then to Substation.
5. OTMA waits for an ACK/NACK from Substation.
6. Substation delivers the message to the ESB and waits for a response.
7. Depending on the outcome of operations, Substation sends an ACK/NACK to OTMA.
8. IMS commits or rolls back the transaction.
9. Substation ES closes its UOW.

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