ISDN, though not popular in US, it is still one of the most popular data communication networks in UK, India and many other European countries. ISDN evolved from the IDN (Integrated Digital Network) in which both the transmission media and switches employs digital techniques. In this system, speech signals are converted to digital signal at transmitting and receiving stations. This system has easy maintenance and supervision carried out by computer, thus enabling more flexibility in terms of remote concentration & subscriber units and results in savings of circuit costs. Now, building on the IDN, ISDN services are launched.
ISDN technology is used to send voice and data including video, graphics and sound in the form of digital signals over communication lines in a PSTN. This technology includes the standard digital transmission protocols, adaptors and communication lines. To the users making an ordinary phone call, ISDN offers no perceivable difference in speed, but on a data call, the difference in speed becomes remarkable when compared to other data networks.
A typical ISDN network integrates non-integrated systems such as telephony, telex, cable TV, various company databases, virtual workgroups supported by MS Windows, telecommuting systems, video and teleconferencing systems etc into one integrated network which can be accessed by different customer premises equipment.
In ISDN, the subscriber-loop from the local exchange will be terminated at the network terminal. From the network terminal, the signals will be distributed to information sockets. These information sockets will be distributed in the premises just like power sockets are distributed for providing electricity. On these information sockets, different terminals for voice and non-voice can be attached.