ISDN Has emerged as a powerful tool worldwide for provisioning of different services like voice, data and image transmission over the telephone line through the telephone network. ISDN is being viewed as the logical extension of the digitalization of telecommunication network and most developed countries are in different stages of implementing ISDN.
An ISDN subscriber can establish two simultaneous independent calls (except when the terminal equipment is such that it occupies two 'B' channels for one call itself like in video conferencing etc.) on existing pair of wires of the telephone line (Basic rate ISDN) where as only one call is possible at present on the analog line / telephone connection. The two simultaneous calls in ISDN can be of any type like speech, data, image etc. The call setup time for a call between two ISDN subscribers is very short, of the order of 1 to 2 seconds. ISDN also supports a whole new set of additional facilities, called Supplementary Services.