# Message Size and Constructs

IMS Interface messages include OTMA messages and application messages. Note the size of these messages.

Note the following on the size of the IMS Interface messages:

- OTMA messages

  Messages that enter IMS from OTMA contain both the OTMA message prefix and other defined IMS message prefixes. Excluding the user data section, the OTMA message prefix can become very large, sometimes over 200 bytes in length. The OTMA message prefix, including the user data section, is stored on IMS message queue data sets, which increases the usage of the queue buffer pool.

- Application messages

  The current maximum size of a Substation ES message passed from and to IMS regions and applications is 3 MB.

Note: The following rules pertain to IMS applications:

- You can execute existing Fast Path application programs with OTMA. They must run as send-then-commit transactions.
- Parameters with the OTMA transaction that contradict this commit mode cause the transaction to be rejected.
- IMS translates in uppercase. To preserve lowercase characters, specify the ULC keyword while defining the transaction.

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