When the whole telecom network, switching equipment and the transmission have been converted into a digital network, the connection between end-user and the exchange (called the local loop) has remained analog, which means that the communication between the exchange and the end-user occurs through analog signals. That is the reason there was a need for modems to convert digital signals from computers into analog signals, so that it is possible to transmit data over, say, the Internet. Now, with the introduction of ISDN (Integrated Services Digital Network), the digital has entered the equipment of the telephone user. This digital connection gives a faster and more secure digital connection for data communications, besides a good quality voice service.
Integrated Services Digital Network (ISDN) is one of those key advancements in communications technology that have revolutionized the field of telecommunications. ISDN can deliver all types of communications facilities including voice, fax, data, video, sound etc over a single or common high-speed digital network using the existing copper phone lines.
Apart from being digital, the name ISDN also tells us that it is for integrated services. What it means to a customer is amazing. Until now, over a single cable pair (two wires), only a single call or service (telephone call or fax or modem) has been possible. While the introduction of ISDN, up to eight devices, i.e. either 8 ISDN sets or combinations of ISDN, faxes, analog phones or PC, routers etc, can be connected to the single pair from the exchange with an ISDN device called NT (Network Terminator). Out of these eight devices, two devices can work simultaneously. To the subscriber it means they can make telephone calls while browsing the Internet or sending fax etc at the same time. Each device can have its own directory numbers as well.
In the case of Internet access, ISDN provides bandwidth capabilities comparable to that of leased line but on use-as-required basis, unlike always-online nature of the leased lines. The ISDN can be used for many purposes including faster internet connections most suitable for those users who do not need round-the-clock access but require high-speed Internet access whenever they use it (up to 128kbps uncompressed in both directions) connection to the internet, connection between two LANs, video conferencing etc.