MSC logical links

A logical link relates a physical link to the transactions and terminals that can use that physical link.

Each IMS in an MSC network has one or more defined logical links.

A maximum of 1018 logical links are allowed for each IMS in the MSC network.

Two IMS systems that are defined to communicate with each other, each through a specific logical link, are called partner systems.

To establish connection between two IMS systems, each partner must have a logical-link definition. The two logical-link definitions must specify the same partner identifications and be assigned to the same physical link. The IMS system definition process assigns a number to each defined logical link. Logical link numbers are assigned sequentially, beginning with 1, in the order in which the links are defined. A logical link can be reassigned to a different physical link, but the two IMS systems must always communicate through a logical link partnership.

If you use the MSLINK stage-1 system definition macro, IMS also assigns a default name to each logical link, unless you specify a different logical link name in the label field on the MSLINK macro. Default logical link names are DFSLxxxx, where xxxx is the logical link number. You can display or modify the logical link name by using the type-2 commands QUERY MSLINK and UPDATE MSLINK.

If you use the type-2 CREATE MSLINK command, you must specify the MSPLINK keyword to assign the logical link to a physical link. Otherwise, the logical link is not assigned to a physical link.

Type-1 commands require you to specify the logical link number to identify the target logical link. Type-2 commands require you to specify the logical link name to identify the target logical link.

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

If you use the IMSRSC repository to store dynamically defined MSC resources, ensure that automation and operational procedures that issue commands for MSC resources use type-2 commands, which specify link names, instead of type-1 commands, which specify link numbers. For example, instead of using the /RSTART LINK 10 command to start a link, use the UPDATE MSLINK NAME(logicallinkname) START(COMM) command. During stage-1 system generation, the IMS system assigns numbers to logical links in the order in which the links are generated. However, the numbers for links are not stored in the IMSRSC repository. If logical links are referenced by using link numbers and are automatically imported from the IMSRSC repository, the numbers of the links are likely to change at the next IMS cold start.

With TCP/IP and VTAM® physical link types, multiple logical links can use a physical link. When defining each physical link, you can specify how many logical links can share a physical link by using the SESSIONS keyword. The term session comes from VTAM and is generally synonymous with the term logical link. For VTAM link types, an active session is a logical link between partner systems.

The IMS system definition process does not require that a physical link be specified for each logical link. You can assign a physical link to the logical link online by using either the type-1 IMS command /MSASSIGN LINK or the type-2 IMS command UPDATE MSLINK NAME (linkname)SET(MSPLINK(msplinkname)). No communication between partners can occur until the assignment is made.