A PRI was and still is 23B+D, not 24 2B+D lines. 23B+D (ISDN PRI or Primary Rate Interface) is 23x64 kbps bearer channels and a 64 kbps Delta channel carried on a B8ZS T1 You could carry additional groups of 24 B channels using NFAS, or Non-Facility Associated Signalling where the D channel on the first PRI carried the call setup and teardown information for the additional B channels on up to 20 PRI circuits. 2B+D (ISDN BRI or Basic Rate Interface) 2x64 kbps bearer channels and a 16 kbps Delta channel carried on an 144 kbps 2B1Q U interface. At 02:58 PM 27/01/2020, bzs@theworld.com wrote:
FWIW bulk dial-up lines were often brought in as PRIs which were 24 ISDN 2B+D lines on basically a T1 (1.544mbps) and then you could break those out to serial lines.