Comparison of ISC and MSC

Both Multiple Systems Coupling (MSC) and Intersystem Communication (ISC) can be used to couple multiple IMS subsystems. Both MSC and ISC enable you to route transactions, distribute transaction processing, and expand beyond the capacity of one IMS system.

MSC is an IMS protocol that enables coupling of IMS systems to other IMS systems only. ISC, however, allows you to connect IMS subsystems with any other subsystem that supports the ISC protocol. This other system can be another IMS, CICS®, or a user-written system.

MSC supports four types of links between IMS systems: channel-to-channel (CTC), memory-to-memory (MTM), TCP/IP, and VTAM® LU 6.1. ISC uses VTAM LU 6.1 or TCP/IP; however ISC TCP/IP is supported only for connections between IMS and IBM® CICS Transaction Server for z/OS®.

The following figure compares MSC to ISC.

Figure 1. IMS Multiple Systems Coupling and Intersystem Communication
Begin figure description: This figure merely illustrates what is discussed in the surrounding text. End description.

ISC, MSC VTAM, and MSC TCP/IP all provide parallel session support. Some key differences exist, however. The following table highlights the major functions of MSC and ISC, and shows the differences in support.

Table 1. Comparing MSC and ISC functions
MSC functions ISC functions
MSC connects multiple IMS systems only to each other. These IMS systems can all reside in one processor, or they can reside in different processors. ISC can connect either like or unlike subsystems, as long as the connected subsystems both implement ISC. ISC can couple an IMS subsystem to:
  • Another IMS subsystem
  • A CICS subsystem (using VTAM or TCP/IP support)
  • A user-written subsystem
Communication in the MSC environment is subsystem-to-subsystem. Communication is between application programs in the two subsystems. The subsystems themselves are session partners, supporting logical flows between the applications.
Processing is transparent to the user. That is, to the user, MSC processing appears as if it is occurring in a single system. Because ISC supports coupling of unlike subsystems, message routing requires involvement by the terminal user or the application to determine the message destination. Specified routing parameters for ISC VTAM connections can be overridden, modified, or deleted by the Message Format Service (MFS).
Unless MSC-directed routing is used, routing is automatic based on system definition parameters. The terminal operator or application program does not need to know routing information. ISC provides a unique message-switching capability that permits message routing to occur without involvement of a user application.
MSC supports the steps of a conversation to be distributed over multiple IMS subsystems, transparent to both the source terminal operator and to each conversational step (application). ISC connections that are supported by VTAM support MFS in IMS subsystems to assist in the routing and formatting of messages between subsystems. ISC TCP/IP connections do not support MFS.
MSC does not support the use of the Fast Path Expedited Message Handler (EMH). When VTAM connections are used, ISC supports the use of Fast Path Expedited Message Handler (EMH) between IMS subsystems.