Maximum frame size of token ring

This topic contains a few guidelines and tips for specifying the maximum frame size.

First, here is some background information to help you understand the reasons for the guidelines and tips.

Larger frames require less processing overhead than the same data that is divided up among many smaller frames. Therefore, the use of larger frame sizes can improve the performance of your system. However, the frame size that you select can be reduced if other entities within the communications path cannot support the frame size.

Maximum frame size fields:

You can use these fields to specify the maximum frame size (the parentheses show location of the field):

• Maximum frame size (line description) affects all devices that use the line description.
• Source service access point (SSAP) maximum frame size (line description) affects only devices that use the SSAP and line description.
• Maximum frame size (controller description) affects all line descriptions that are associated with the controller description.

If you specify a maximum frame size in two or more of these fields, the system selects the smallest size. To ease the frame size configuration of your line and controller descriptions, use these guidelines:

• Use the largest possible frame size that the network connection supports.
• Specify the maximum frame size only in the Maximum frame size field of the line description. The system then uses this value for all other maximum frame size fields.
• Specify a size smaller than the maximum frame size of the input/output adapter (IOA) if you receive a frame rejection error.

Reduction of selected maximum frame size:

Other entities within the communications path can reduce your maximum frame size if they cannot support it. Some of these entities are:

• Remote IOAs or other devices: Some support only smaller maximum frame sizes.
• Line or controller description: These descriptions on the remote system sometimes specify a smaller maximum frame size.
• Devices: Some do not support maximum frame size negotiations.

Solution:

If negotiation does not produce a common frame size, a frame reject (FRMR) condition occurs, and your system sends message CPA58E2 to message queue QSYSOPR. If you cannot change the frame size of the other entities, use an acceptable maximum frame size value in your line or controller description.