Controlling the bandwidth of MSC links

You can control the bandwidth of CTC, MTM, and VTAM® MSC link types by increasing MSC link buffer sizes and turning MSC bandwidth mode on and off.

TCP/IP physical link types always run in bandwidth mode. Bandwidth mode cannot be turned off for TCP/IP links.

When bandwidth mode is off, MSC sends a maximum of one message or response per I/O send or write operation. When bandwidth mode is on, IMS consolidates the following messages and responses in the same buffer:
  • All messages that are queued and ready to be sent
  • Any responses that are owed for messages received

Increasing the link buffer size allows more messages and responses to be sent simultaneously. Valid MSC link buffer sizes are 1024 bytes to 65536 bytes. The physical link defines the default buffer size for logical links associated with it.

You can set the default value by specifying the BUFSIZE keyword on the MSPLINK macro during system definition, or you can set it dynamically by specifying the BUFSIZE keyword on the CREATE MSPLINK command. You can then modify the buffer sizes for each logical link in an online IMS system by using the UPDATE MSLINK or the UPDATE MSPLINK command; however for any one logical link you must specify the same buffer size in both IMS systems at each end of the logical link.

To change bandwidth mode and link buffer sizes, use either the type-2 command UPDATE MSLINK or the type-1 command /UPDATE MSLINK. To display the status of bandwidth mode and the size of the buffers, use the type-2 command QUERY MSLINK or the type-1 command /DISPLAY LINK OPTION BUFSIZE.

You cannot set bandwidth mode during system definition. It must be set with a command. After you establish a bandwidth mode and buffer size, the mode and size selected remain in effect across link restarts and IMS warm starts. Bandwidth mode is set off and the buffer size is restored to the SYSDEF value across an IMS cold start.