Integrated Services Digital Network are the standards for data transmission over the Plain Old Telephone System or Public Switched Telephone Network which are circuit switched networks. Circuit-switched networks differ from packet-switched ones by requiring a dedicated communications path, or circuit, before two hosts begin to communicate.
The ISDN standard provides data transfer speed in multiples of 64kbps. The Integrated part of the term refers to the fact that it is possible to deliver 2 or more concurrent connections over one line.
A Basic Rate Interface (BRI) line is the 'starter' ISDN connection level that has a bandwidth of 128kbps over a pair of copper telephone lines. In addition to the dual 64 kbps channels there is an additional 16kbps signaling channel.
A Primary Rate Inetrface (PRI) offers much higher bandwidths by using higher multiples of 64kbps Bearer or 'B' channels.
A T1 carrier line, as is used in the US & Canada consists of 23 Bearer channels & 1 Signaling (or 'D') channel. The E1 Line , used in European countries, consists of 30 B channels and 1 D channel.
Where quality and reliable connections are needed over lower price points, ISDN is preferred and as such are used by the broadcast industry.
Consumers generally prefer DSL or ADSL connections (Asynchronous Digital Subscriber Line)