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