Integrated Services Digital Network (ISDN) is an architecture that uses digital technology to support integrated voice, data, video, and image services through standard interfaces over standard twisted-pair telephone wire. Because it uses exclusively digital signals, ISDN can carry multiple virtual lines on a single copper telephone line.

ISDN is available in two sizes: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI carries data and voice traffic up to a total of 128 Kbps of bandwidth. PRI offers more bandwidth and more virtual line—up to 1.5 Mbps.

ISDN Drawbacks
Like DSL, ISDN technology cannot be used to send data over long distances. ISDN users need to be within 18,000 feet of the central office to receive a clear, undistorted signal. Until recently, ISDN service was available on demand, with users having to dial up each time they wanted to go online. Always-On/Dynamic ISDN (AO/DI) is a recent development permitting end users to maintain a cost-effective "virtual permanent" connection to a network or service provider, allowing the user to continuously receive data, news, and email.

Cost Comparison
In many cases, it's difficult for a user to order ISDN, procure the necessary equipment, and install the equipment so it functions properly. And, since ISDN does not operate with current network technologies (such as modems), customers are forced to abandon existing equipment and switch to a new technology. Monthly charges for ISDN also tend to make it rather expensive to maintain, though costs for service vary dramatically from carrier to carrier, and state to state.