Editing ISDN User Part (section)

{{SS7stack}}
The '''ISDN (Integrated Services Digital Network) User Part''' or '''ISUP''' is part of [[Signaling System No. 7]] (SS7), which is used to set up [[telephone call]]s in the [[public switched telephone network]] (PSTN). It is specified by the [[ITU-T]] as part of the Q.76x series.<ref>{{Citation | url = http://www.itu.int/rec/T-REC-Q/en/ | publisher = ITU-T | title = Q-Series Recommendations}}.</ref>

When a telephone call is set up from one subscriber to another, several [[telephone exchange]]s could be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a [[Telephone exchange|switch]] will signal call-related information like [[called party]] number to the next [[Telephone exchange|switch]] in the network using ISUP messages.

The [[telephone exchange]]s may be connected via [[T-carrier|T1]] or [[E-carrier|E1]] trunks which transport the speech from the calls. These trunks are divided into 64&nbsp;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.