Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. 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.

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