Circuit identification code

The ISDN Services User Part (ISUP) Circuit Identification Code (CIC) in the Initial Address Message (IAM) consists of a range of 0 to 65,535. On the signalling path the CIC provides information about where the voice part of the call is carried - on which trunk and in which timeslot.

The Bearer Independent Call Control (BICC) Call Instance Code (CIC) in the IAM consists of a range of 0 to 4,294,967,295.

There can be multiple trunk groups to a given point code using a combination of ISUP and BICC signaling. This means, for example, that CIC 100 on an ISUP trunk group can be replicated using CIC 100 on a BICC trunk group to the same point code. The far end switch distinguishes the difference by referencing the Service Indicator (SI). ISUP uses SI=05 and BICC uses SI=13 in North America.

You can find more details in the ISUP wiki.

CIC is the information identifying a circuit between two exchanges that is being setup for the purpose of transmission of voice signals during a voice call in PSTN. The CIC is sent in the SS7 ISUP (ISDN User Part) messages, for example, IAM, ACM, ANM etc. to carry the information about the circuit, that is being setup for voice signal transmission. Usually CIC is of 14 bits, that means it can be used to identify 2^14 = 16384, circuits between two exchanges in PSTN. Basically CIC determines on which trunk and on that trunk on which time-slot the voice-signals are to be transmitted for the call being setup using the messages carrying this CIC value.