MSC physical links
A physical link is how the IMS systems connect to one other through access methods or hardware.
You can define physical links in online IMS systems by using the CREATE MSPLINK command.
You can also define physical links during IMS system definition by using the MSPLINK stage-1 system definition macro.
To update physical links in online IMS systems, use the UPDATE MSPLINK command.
To save physical links that are created or updated by using type-2 commands across IMS cold starts, either export the physical link definitions to the IMSRSC repository or code the changes to the MSC resources into stage-1 system definition macros.
A maximum of 1018 physical links are allowed in each IMS in a Multiple Systems Coupling (MSC) network.
Multiple Systems Coupling supports the following types of physical links:
- Channel-to-channel (CTC) adapter
- Usually used only when the IMS systems are in the same data
center.
The CTC adapter is a channel-to-channel hardware connection. You can assign only one logical link to a physical link that uses the CTC connection type.
- Memory-to-memory (MTM)
- Used when the IMS systems are in the same logical
partition.
The MTM link is a software link between IMS subsystems that are running in the same logical partition. You can assign only one logical link to a physical link that uses the MTM connection type.
- TCP/IP
- Usually used when the IMS systems are in different data
centers. IMS Connect manages the TCP/IP connections and
protocols for the physical links.
The TCP/IP connection and networking protocols are the protocols that are used by the internet. IMS Connect manages the TCP/IP connections and protocols for MSC. Communications between MSC and IMS Connect are managed by the Structured Call Interface (SCI) component of the IMS Common Service Layer (CSL) in an IMSplex. You can assign multiple logical links to a physical link that uses the TCP/IP connection type. TCP/IP physical links always operate in MSC bandwidth mode and require a slightly larger buffer size than the other physical link types.
- VTAM®
- Usually used when the IMS systems are in different data
centers.
VTAM is an access method that usually uses a teleprocessing media connection. You can assign multiple logical links to a physical link that uses the VTAM connection type.
From an MSC perspective, the operation of TCP/IP physical links and VTAM physical links is similar. Apart from differences in system definition and buffer size requirements, the IMS and z/OS® components that support each connection type present the most significant operational differences.
Both TCP/IP and VTAM physical link types can be used either as your primary physical link type, or as a backup link type in case the other link type fails for any reason.
Depending on various factors, such as network traffic and the distance between the two connected IMS systems, a TCP/IP physical link is likely to provide better performance than a VTAM physical link.
The following figure illustrates the types of physical links.
