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Integrated Services Digital Network (ISDN) supports integrated voice, text, images, video, and sound, over a single service at rates up to 128 kbps. Voice call features include Caller ID, call forwarding, call conferencing, and multiple incoming calls on a single line.

ISDN transmits digital data, so the equipment does not use modems (although many vendors refer to ISDN equipment as ISDN modems). ISDN actually uses specially designed equipment, most of which can extend ISDN services to other ISDN equipment on the same line or to ordinary analog phone equipment, such as phones, fax machines, or even modems.

ISDN uses its own signaling standard that defines the way data frames are formatted and the way packets are moved across the network. It also uses a number of protocol standards that integrate ISDN into the mainstream of the telecommunications network.

• # Basic Rate Interface (BRI), for the small office, home office (SOHO), includes two 64 kbps channels (called B channels) used for data or voice transmission, and a 16 kbps channel (called a D channel) used to control the B channels.
• # Primary Rate Interface (PRI) has much more capacity than BRI and is slightly different in North America than in Europe. In North American, PRI consists of 23 B channels and a single 64 kbps D channel with an overall rate of 1.544 megabits per second. In Europe, PRI has 30 B channels and a 64 kbps D channel with an overall rate of 2.048 megabits per second. (If you do the math, you'll see that it doesn't quite add up. This is because a few bits are used for framing.)

ISDN supplies on-demand digital communication as opposed to dedicated digital communication. On demand means that the line comes up (dials out) when needed, and drops (hangs up) after the line is idle for a specified period of time (assuming your ISDN device includes an idle timer), so you are only charged for the time actually on line. ISDN can be set up to start a call with one B channel, but then when you need more 'bandwidth', the second B channel comes up automatically and drops off when demand goes down. You are charged each time you dial out and each time the second B channel dials out. Additionally, your Internet Service Provider may charge for each B channel.

When you are transmitting data or receiving data calls on your ISDN line, the other end must also be ISDN. Incoming calls must be from another ISDN line in order to transmit text, video, files, images, or any other type of data. If your ISDN equipment has phone jacks, incoming and outgoing voice calls are always analog—plain old telephone service, also called POTS.