ISDN User Part

In the case of no outbound circuit being available on a particular exchange, a release message is sent back to the preceding switches in the chain.

ANSI specifies variations of ISUP utilized under the North American Numbering Plan; however, some countries under the NANP differ in their support of some procedures (for example, LATA is meaningless within Canada.

While these variations of ISUP differ in subtle ways, the vast majority of ISUP message type, parameter type, and parameter field code-points, and related fundamental call processing procedures, agree across all variants.

[6] Below is a detailed exchange of ISUP messages involving two ISDN telecom switches.

Release cause codes are used to identify and debug any events occurring in ISDN User Part signaling.

Similarly Telecom operators trace for Causecodes to debug any call failures.

Note that some versions of ANSI ISUP permit a CIC with 14 significant bits instead of the 12 that are shown.

The routing label and circuit identification code are not included in the user data passed to SCCP.