Overview of Intersystem Communication

ISC is a part of the IMS Transaction Manager. It is one of the ways to connect multiple subsystems. The other means of connection is Multiple Systems Coupling (MSC).

As defined under SNA, ISC is an LU 6.1 session that:
  • Connects different subsystems to communicate at the application level.
  • Provides distributed transaction processing permitting a terminal user or application in one subsystem to communicate with a terminal or application in a different subsystem and, optionally, to receive a reply. In some cases, the application is user written; in other cases, the subsystem itself acts as an application.
  • Provides distributed services. Thus, an application in one subsystem can use a service (such as a message queue or database) in a different subsystem.

The IMS implementation of ISC supports connections that are managed by TCP/IP (ISC TCP/IP) or SNA VTAM® (ISC VTAM). Both dynamically defined ISC nodes and statically defined ISC LU 6.1 terminals can use either TCP/IP or VTAM support.

ISC TCP/IP supports connections between IMS and CICS® only. ISC TCP/IP support uses a private protocol, IP interconnectivity (IPIC), that is defined by CICS. The use of IPIC is generally consistent with the protocols that are defined by SNA for ISC VTAM and is transparent to application programs that use ISC.

ISC VTAM supports communication between unlike subsystems and includes SNA-defined session control protocols, data flow control protocols, and routing parameters. The functions provided by each of these protocols and parameters are summarized below, and the IMS support for them is described in this topic and its subtopics.

Session control (SC) comprises:
  • Initiating sessions between subsystems
  • Recovering and resynchronizing sessions, maintaining the integrity of session states and recoverable resources across both session and subsystem failures
  • Terminating any or all session paths between IMS and another subsystem
Data flow control (DFC) includes:
  • Controlling send and receive protocols within a session.
  • Resolving contention for transaction initiation within a session.
  • Monitoring error recovery processing.
  • Monitoring symmetrical shutdown
  • Controlling synchronization of resources. Sync point control ensures that all resources are committed or backed out synchronously.
ISC VTAM routing comprises:
  • Using parameters in the SNA-defined function management headers to connect the process required for incoming messages and to route reply messages.

APPC/IMS only supports message switching to LU 6.2 destinations through the DFSAPPC service.