# OTMA Messages

The communication data passed between the IMS Interface and the OTMA Server is within an OTMA message.

The following figure shows the layout of Substation ES IMS Interface OTMA messages:

An OTMA message contains the following segments:

- Prefix

  The prefix contains one or more of the following components:

  - A Message Control information segment is provided for every message.
  - The State Data segment contains transaction-related information and has different formats.
  - The Security Data segment is mandatory for each transaction or command.
  - The User Data segment is of variable length and is managed by the IMS OTMA Interface.
- Application data

  The application data can be an IMS command or a transaction. The data in this segment is unchanged by the receiver (OTMA Server) and is transmitted directly to an application that can obtain or send the data in the I/O area with DL/I calls.

Note: For details on OTMA message segments, see IMS Communications and Connections.
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