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Chapter 17. How a Modem (Really) Works > Integrated Services Digital Network

Integrated Services Digital Network

As originally conceived, Integrated Services Digital Network (ISDN) was the replacement technology for the analog network. It provides a total bandwidth of 144 Kbps, parceled into three channels—two 64-Kbps “bearer” channels (or B-channels) for transmitting digital information (voice, data, or video) and one 16-Kbps “signaling” channel (or D-channel) for transmitting call setup information and packet-switched data. ISDN is a dialup technology that uses your existing twisted-pair copper telephone lines to transmit and receive data while allowing for simultaneous use of your telephone for voice calls. The two B-channels are independent from one another and can be used to transmit voice or data to two independent destinations. ISDN comes in two variants: BRI and PRI. The version described previously is ISDN BRI (Base Rate Interface).

The other version is ISDN PRI (Primary Rate Interface) and is installed over a high-speed circuit T1 (T1s run at 1.544 Mbps). ISDN PRI provides the user with 23 B-channels and one 64-Kbps signaling channel, or if multiple PRI circuits are installed, they can share the signaling channel and use all 24 B-channels on the additional circuits. ISDN PRI is typically used by ISPs (you most likely dial into them with your analog modem or ISDN Terminal Adapter) and large corporations for voice and data communication needs.


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