Integrated Services Digital Network
ISDN is a digital phone line standard. It uses a pair of copper wires just like regular analog phone lines, but it cannot use the same wiring since analog lines have load coils that prevent digital data transmission. Also, ISDN cables cannot be over 10 miles (16 Km) in length from the phone company to your home because of noise level.
ISDN lines can only be used by ISDN phones, ISDN fax machines and ISDB modems. It is possible to install regular (analog) devices to ISDN lines through the use of an adapter (usually the phone company installs this device).
ISDN lines allow a connection speed of up to 128 Kbps and you will need an Internet Service Provider (ISP) that allows ISDN dial-up connections.
There are two service levels: Basic Rate Interface (BRI), for domestic and small office use and Primary Rate Interface (PRI), for big companies. Both use three channels: two B-type channels, which transfer data at 64 Kbps rate, and one D-type channel, which transfers control data at 16 Kbps. Since it has two data channels, it is possible to talk on the phone and be on-line at the same time.
To connect at 128 Kbps it uses both 64 Kbps channels. If someone calls you and you want to pick up the phone, one of the channels must be disconnected, making the transfer rate to drop to 64 Kbps. You can go back to 128 Kbps after you finish your phone call.
Depending on the phone company, each channel can be billed as an individual phone line, costing you the double if you are connected at 128 Kbps.
There is a new ISDN technology called B-ISDN (Broadband ISDN) that uses fiber optics and ATM transmission.
ISDN started to fade away when new broadband technologies came to market, especially ADSL, which provides higher transfer rates.
