Integrated Services Digital Network

Integrated Services Digital Network is an international standard for a communication network. It is often shortened to ISDN. It is used for telephone lines. Before ISDN, telephone lines were mainly seen as a way for people to make phone calls, so the main idea was to use circuit switching. When a phone call is made, a line is set up between the two participants. This phone call is analogue. With ISDN, this view changes. As the signal is now digital ISDN can be used to offer a number of services, which use the telephone line to transport data. There were different services, like telex or teleprinters before ISDN, but they used their own networks. At specific locations, there were gateways between these networks. With ISDN, this changes. There is only one network, and it offers different services. Some of them are for voice, to make phone calls, others are for data, for example to send telefaxes.
Channels.
ISDN offers a number of data channels, and a signalling channel. The data channels carry the actual service data. The two data channels are completely independent and can be used independently. The usual setup has two such data channels. That way, it is possible to send a fax through one channel, while doing a phone call over the other channel, for example. If the main focus is on a high quality voice connection, the two channels can be combined into one single channel carrying twice the amount of a single channel enhancing the quality dramatically.
For larger companies it is possible to extend the number of data channels to 30 with still one signalling channel.