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Integrated Services Digital Network (ISDN) is a digital telephony technology that supports multiple 64-kbps channels (known as bearer channels [B channels]) on a single connection. ISDN was popular back in the 1980s and was used to connect private branch exchanges (PBX), which are telephone switches owned by and operated by a company, to a central office. ISDN has the capability to carry voice, video, or data over its B channels. ISDN also offers a robust set of signaling protocols: Q.921 for Layer 2 signaling and Q.931 for Layer 3 signaling. These signaling protocols run on a separate channel in an ISDN circuit (known as the delta channel, data channel, or D channel).

A PRI circuit is an ISDN circuit built on a T1 or E1 circuit. Recall that a T1 circuit has 24 channels. Therefore, if a PRI circuit is built on a T1 circuit, the ISDN PRI circuit has 23 B channels and one 64-kbps D channel. The 24th channel in the T1 circuit is used as the ISDN D channel (the channel used to carry the Q.921 and Q.931 signaling protocols, which are used to set up, maintain, and tear down connections).

Also, recall that an E1 circuit has 32 channels, with the first channel being reserved for framing and synchronization and the seventeenth channel used for signaling. Therefore, an ISDN PRI circuit built on an E1 circuit has 30 B channels and one D channel, which is the seventeenth channel.

Some ISDN circuits are four-wire circuits and some are two-wire. Also, some devices in an ISDN network might not natively be ISDN devices, or they might need to connect to a four-wire ISDN circuit or a two-wire ISDN circuit.

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