3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.8 Integrated services digital network
Integrated Services Digital Network (ISDN) is a digital circuit-switched technology that can transport voice and data simultaneously over the same physical connection. Connections are made on demand by dialing the telephone number of another ISDN circuit. This type of service is known as dial-on-demand service. ISDN can be ordered in either Basic Rate Interface (BRI) or Primary Rate Interface (PRI).

The BRI offers 144 kilobits per second (Kbps) in a format known as 2B+D. The 2B refers to two 64-Kbps B (for bearer) channels that can be linked together, or bonded, to form one logical connection at 128 kbps.

The D channel is a 16-kbps control channel used for call setup, takedown, and other control functions. Originally the ISDN BRI was distinctly a remote-access technology, as opposed to a true, multiuser, WAN transmission facility. Today, this transmission technology is frequently used as a relatively low-cost backup facility for dedicated-line networks. As a backup facility, ISDN may be pressed into service whenever a dedicated line either fails or becomes severely congested.

Typically the PRI is delivered over a T1 facility at a gross transmission rate of 1.544 Mbps. This is usually channelized into 23 64-Kbps B channels and one 64-Kbps D channel. Higher-rate H channels of either 384, 1536, or 1920 Kbps can be used rather than, or in combination with, the B and D channels. ISDN PRI in Europe, Australia, and other parts of the world provides 30 B channels plus one 64-kbps D channel and a total interface rate of 2.048 Mbps. The PRI physical-layer specification is ITU-T I.431.