Specify the length of the messages for which the signaling service should optimize in a transport class on the CLASSLEN keyword of the CLASSDEF statement or, after IPL, on the SETXCF START,CLASSDEF or the SETXCF MODIFY,CLASSDEF command.

The class length determines the size of the XCF message buffers used to hold messages sent in that transport class. MVS™ selects the smallest buffer that will hold messages of the size specified on the CLASSLEN keyword. In general, specify a class length equal to the length of the messages most frequently sent by the group(s) in the transport class.

Specify the length of the messages (CLASSLEN) for a transport class based on the group(s) you assign to the transport class. Therefore, you must know the characteristics of the messages the groups send.

Selecting an appropriate class length involves making compromises between storage use and signaling performance. If messages are usually shorter than the message buffer, storage is wasted; if longer, performance is degraded.

A message whose length does not exceed the buffer length receives the best performance the signaling service can provide. Messages longer than the buffer length might require additional processing, such as the preparation of additional message buffer space or the sending of additional signals to deliver the oversized messages.

If there are many messages longer than the buffer length and the message traffic patterns warrant it, the signaling service dynamically increases the size of the buffers in the class to avoid the additional processing overhead to deliver these messages. This dynamic adjustment of buffer sizes means that specifying a small class length does not necessarily mean that larger signals will suffer performance degradation. However, if increasing the buffer size would cause the signaling service to exceed the maximum message buffer space allowed for messages in a particular transport class for a particular receiving system, these adjustments are not made. (See Planning the message buffer space.)