Integrated Services Digital Network (ISDN) is an upgraded phone line that can be used for faster Internet access and for regular voice calls. Using one line, you can talk on the phone while you’re surfing the Web. ISDN is all digital, which means that data doesn’t have to be converted to an analog signal for transmission.
The ISDN service intended for residential use is Basic Rate Interface (BRI). On one ISDN line, BRI provides two 64Kbps channels, or B channels, and one 16Kbps channel, or D channel.
The D channel is mostly used for signaling—for instance, to indicate that the line is busy.
The B channels are where the action is. When the B channels are combined, you have a 128Kbps line to the Internet. That’s over twice the speed of the fastest analog modem, 56Kbps. If you want to talk on the phone or send a fax, your Internet access drops down to one 64Kbps B channel while the other B channel is used for voice.
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