ISDN interfaces can be either Basic Rate Interface (BRI) or Primary Rate Interface (PRI). A PRI differs
from a BRI mainly due to the number of channels it offers. ISDN channels are usually divided into two
different types: the bearer (B) channel, which carries the data, and the D channel, which are used to make
signalling requests.
Each B channel has a maximum throughput of 64 kbps and can carry encoded pulse code modulation
(computerM) digital voice, video, or data. They are used mainly for circuit-switched data communications
such as High-Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP). However, they can also
carry packet-switched data communications.
The router uses the D channel to dial destination phone numbers. It has a bandwidth of 16 kbps for BRI or
64 kbps for PRI. Although the D channel is used mainly for signalling, it too can also carry packet-switched
data.
The different types of ISDN lines are often described with a phrase that implies the number of each type of
channel. For example, BRIs are referred to as 2B+D, meaning two B channels and one D channel.