ISDN (Integrated Services Digital Network) is an international network, like a telephone network, that supports rapid data transmission between many different types of personal communications devices, such as telephones, computers, and fax machines. With an ISDN line, you can send and receive both voice and other data, for example pictures, text, or video images simultaneously, at speeds up to 64 kilobits per second (kbps), per B (bearer) channel.
A typical ISDN line consists of two B channels and one D (signaling) channel which makes a BRI (Basic Rate Interface) ISDN line. The B channels are either 56 kbps, or 64 kbps, depending on the telephone company's equipment, and the D channel is 16 kbps. The total bandwidth for a BRI ISDN line is 144 kbps, however, the B channels are actually used for data and/or voice resulting in a bandwidth of 112 or 128 kbps.
The B channels can also be bonded together. Bonding refers to strapping the two B channels together using inverse multiplexing to produce what appears to be one 112 or 128 kbps channel.
Another type of ISDN line, has 23 B channels and one D channel, is the PRI (Primary Rate Interface) which is equivalent to a T1 line. Typically a PRI would be used by large corporations or an internet service provider.