Outbound and local message buffers can be further segregated by transport class. Thus, the supply of message buffers in one transport class for a particular system does not affect the supply of message buffers available to any other system in the same transport class.

Figure 1 shows SYSA's message buffers with the local and outbound buffers segregated by class (TC3, TC4 and TC1, TC2) and the outbound buffers segregated by system (SYSB, SYSC, SYSD). SYSA's inbound message buffers are segregated by system only (SYSB, SYSC, SYSD).

Figure 1. SYSA with Inbound, Local and Outbound Message Buffers

The message buffer space available to outbound messages in a transport class is the sum of the message buffer space contributed by the transport class plus the message buffer space contributed by each outbound signaling path in that class. If the available message buffer space is not adequate, then messages are rejected.

Because the message buffer space available for communicating with another system depends on the contribution of message buffer space by the outbound signaling paths connected to that system, the loss of a signaling path can cause a reduction in the available buffer space. Moreover, the operator can use commands to change the amount of message buffer space contributed by a signaling path or a transport class definition. Therefore, it is possible for the amount of message buffer space in use to exceed the buffer space limit. In this situation, MVS™ rejects message requests until the buffer space in use is consistent with the new buffer space limit.

The total message buffer space available to local messages in a transport class is the sum of the message buffer space contributed by the transport class (specified or defaulted on the MAXMSG parameter of the CLASSDEF or COUPLE statement) plus any message buffer space specified on the LOCALMSG statement.

You want to specify sufficient message buffer space to support the message traffic for each system in each transport class that is used for delivering messages to that system. When specifying buffer space for a transport class, consider the size and frequency of the messages, as well as the performance of the signaling paths and systems involved in the message transfer.