MSC logical link paths

Messages are routed in an MSC network by using logical link paths. Logical link paths identify the remote IMS system to which a message must be delivered and the local IMS system that is delivering the message.

You must define at least two corresponding logical link paths to submit a transaction at a terminal in an input IMS system, process the transaction in destination IMS system, and return a response to the terminal in the input IMS system: One logical link path in the input IMS system and another in the destination IMS system.

In the logical link path that is defined in the input IMS system, the input IMS system is defined as local and the destination IMS system is defined as remote.

In the logical link path that is defined in the destination IMS system, the destination IMS system is defined as local and the input IMS system is defined as remote.

In the definition of a logical link path, each IMS system is identified by one of its assigned system identifiers (SYSIDs), a numeric value from 1 to 2036.

IMS associates the SYSID of the input IMS system with the input transaction message and its corresponding response, so that the response can be returned to the input terminal.

Before it can be used, a logical link path must be assigned to a logical link. You can assign multiple logical link paths to a single logical link.

You can define a logical path either by issuing the type-2 command CREATE MSNAME or by coding the MSNAME stage-1 system definition macro. Because of the macro and command names, the term MSNAME is often used as a synonym for the term logical link path.

To save logical link paths that are created or updated by using type-2 commands across IMS cold starts, either export the modified logical link path definitions to the IMSRSC repository or code the changes to the MSC resources into stage-1 system definition macros.

Examples

Consider the MSNAME macro definitions of two logical link paths:

MSNAME  SYSID=(2,1)
MSNAME  SYSID=(3,1)

The first definition says that messages that use this logical link path are processed in the remote system whose local SYSID is 2. The second definition says that messages that use this logical link path are processed in the remote system whose local SYSID is 3. By using these definitions, the IMS system definition process assigns SYSID 1 to the IMS being defined and recognizes two remote systems with SYSIDs of 2 and 3. If a third path is defined with SYSID=(5,4), IMS would also assign SYSID 4 to the local system.

Transactions are assigned to logical link paths in the APPLCTN macro definition.

In the next example, consider the following application definitions, each with one transaction code defined:

APPLCTN     PSB=A
  TRANSACT  CODE=A
APPLCTN     PSB=B,SYSID=(2,1)
  TRANSACT  CODE=B
APPLCTN     PSB=C,SYSID=(3,1)
  TRANSACT  CODE=C

The SYSID keyword identifies the logical link path to be used for the transactions that are associated with the application. Transaction A is considered to be a local transaction, because the absence of the SYSID keyword indicates transaction A is only processed by the IMS being defined. Transactions B and C are remote transactions. Relating the application definitions to MSNAME definitions, IMS would return responses from transactions B and C to the IMS defined as SYSID 1, unless the application program specified an alternative destination for the response.

If messages that originate in the local system refer to any logical terminals in a remote system, the logical link definition must also include NAME macros to identify those remote logical terminals, unless directed routing is used.