- Abbreviation is T1/Primary Rate Interface. An ISDN line with 23 B channels, each with a speed of 64 Kbps, and one 64 Kbps D channel. This is equivalent to a T1 or DS1 line. In Europe, PRI has 30 B channels and one D channel, so it is equivalent to an E1 line. Multiple B channels can be combined for the desired or needed bandwidth. Also called primary rate Interface, PRI/T1, or ISDN-PRI.