# Architecture

With Substation ES IMS Interface, external applications can communicate (publish, request, or reply) with transactions that are resident in the IMS Message Processing Program (MPP) regions.

Additionally, IMS applications, such as MPPs, Batch Message Programs (BMPs), and 3270, can also publish or redirect (trigger) output to external applications through Substation ES IMS Interface.

The Transaction Interface for IMS is part of Substation ES and couples together Substation ES and host IMS systems. With IMS Interface, multiple users can simultaneously connect and communicate with IMS Transaction Manager (TM). To understand the role and position of IMS Interface within z/OS, see Substation ES Architecture.

Substation ES IMS Interface uses IBM cross-system coupling facility (XCF) and IMS Open Transaction Manager Access (OTMA) to communicate with IMS Transaction Manager. OTMA is a transaction-based, connectionless client/server protocol.

Upon receiving a request published by TIBCO messaging clients, Substation ES determines the service level required for the message and forwards it to Substation ES IMS Interface. The interface interacts with IMS according to the desired service level. Therefore, messages sent to IMS transactions can be transactional or nonconversational.

A transaction level of service depends on the parameters, which you can configure in Substation ES, and on the type of TIBCO messaging client that initiates the request.

Substation ES can also receive messages initiated from IMS and will route these to the destination defined on the specified service. Also, Substation ES fully supports synchronous callout requests initiated from IMS.

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