The

ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Wo ...

Services User Part ( ISUP) Circuit Identification Code (CIC) is part of the Signaling System #7 which is used to set up

telephone call A telephone call is a connection over a telephone network A telephone network is a telecommunications network that connects telephones, which allows telephone calls between two or more parties, as well as newer features such as fax and interne ...

s in Public Switched Telephone NetworksITU-T Q-Series Recommendations
/ref> as part of the Initial Address Message (IAM). When a

telephone A telephone is a telecommunications device that permits two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most efficiently the human voice, into e ...

call is set up from one subscriber to another, many

telephone exchange A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...

s will be involved, possibly across international boundaries. To allow a call to be set up correctly, where ISUP is supported, a

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type of ...

will signal call-related information like called or

calling party The calling party (in some contexts called the "A-Number") is a person who (or device that) initiates a telephone call. The person who, or device that, receives a telephone call is the called party (or callee or B-party). In some countries, it is ...

number to the next

in the network using ISUP messages. The CIC provides information about where the voice part of the call is carried - on which trunk and in which timeslot.

References

Integrated Services Digital Network Signaling System 7 {{telephony-stub