ISDN User Part
| SS7 protocols by OSI layer | |
| Application | TCAP, CAP, ISUP, ... |
|---|---|
| Network | MTP Level 3 + SCCP |
| Data link | MTP Level 2 |
| Physical | MTP Level 1 |
The ISDN (Integrated Services Digital Network) User Part or ISUP is part of the Signaling System No. 7 (SS7) which is used to set up telephone calls in the public switched telephone network (PSTN). It is specified by the ITU-T as part of the Q.76x series.
When a telephone call is set up from one subscriber to another, several telephone exchanges could be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a switch will signal call-related information like called party number to the next switch in the network using ISUP messages.
The telephone exchanges may be connected via E1 or T1 trunks which transport the speech from the calls. These trunks are divided into 64 kbit/s timeslots, and one timeslot can carry exactly one call. Regardless of what facilities are used to interconnect switches, each circuit between two switches is uniquely identified by a circuit identification code (CIC) that is included in the ISUP messages. The exchange uses this information along with the received signaling information (especially the called party number) to determine which inbound and outbound circuits should be connected together to provide an end to end speech path.
In addition to call related information, ISUP is also used to exchange status information for, and permit management of, the available circuits. 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.
Different ISUP variants exist. ITU-T specifies the variant used in the international network. In Europe ETSI releases its own ISUP specification which is close that of the ITU-T. ITU-T ISUP is used for international connections and is the base for some national ISUP variants. Most countries have their own variation of ISUP to cover national requirements. 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. Also, RBOCs support Telcordia procedures not fully specified by ANSI.) Some countries outside the NANP support ANSI-based variants (e.g. Mexico).
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