Layer 3 messages that are longer than the length of frames that the data link layer can support may be partitioned into several segments.
Message segmentation shall only be used when the message length exceeds N201 (defined in Recommendation Q.921 [3]).
The architectural relationship to other Q.931 functions is shown in Figure H.1. These procedures apply only within a specific data link connection and do not impact the procedures in operation on other parallel data link connections.
In order to support expressed needs for applications requiring message lengths of 10 000 octets, or greater,procedures to support those applications are under study. These procedures will consider backward compatibility and methods to allow information on other call references to be interleaved with segments of a long message. The specifics of these procedures are for further study.
Figure H.1/Q.931 – Logical architecture containing segmentation function