In the North American telecommunications system, a T1 typically carries 24 individual timeslots. Each timeslot in turn carries a single telephone call. When a T1 circuit is used to carry Primary Rate ISDN one of the timeslots is used to carry the D channel. In an NFAS ( Non-Facility Associated Signalling) configuration, multiple T1 share a single D channel. There is an upper limit of 20 T1 in a single NFAS group. A single Primary Rate ISDN circuit is sometimes described as 23B + D. There are 23 bearer channels carrying voice, and one D channel carrying the Common Channel Signalling. A single full NFAS group can then be described as 479B + D (20 T1).