ISDN is a network concept providing a integration of data, voice and video. Its based on 64Kbps digital Communication channel. ISDN is a generic term for any network which connects homes and business together with a service companies such as bank, air-lines, stock market etc using a digital network. From 1990 ISDN became a well convinced and planned area for development in the field of telecommunication. Telephone network requires two basic functions:

(i) Signaling –To establish and realize a call.

(ii) End-to-end transmission – To transfer the information between the users

ISDN uses these functions as separate and indeed provide different channels for these services. This is done through proper interface. The purpose is to provide access to various services that are possibly supported by different networks. The base rate interface(BRI) provides the users with two 64Kbps barrier(B) channels and one 16Kbps data(D) channels. The primary data rate interface(PRI) provides users with(23B+1D) channels in North America and Japan and (30B+1D) channels in Europe. Each B channel is bidirectional and provides 64Kbps end –to-end digital connection that can carry PCM voice or data. The primary function of D channel is to carry information for B channels. Types of ISDN are:

(i) Narrow band ISDN

(ii) Broad band ISDN

Narrow band has a smaller bandwidth and supports data rate upto 64Kbps only. The internet in HD(TV) and high speed data interconnected promoted the work on B-ISDN. Due to low bit rates the quality of service provided by the narrowband ISDN is poor. That means the quality of video Signals in the narrow band ISDN is poor. The quality of service is improved by using broadband ISDN. The broadband ISDN can support higher data rate due to use of optical fibre cables. The bandwidth of an optical fibre cable is very high. So the broadband ISDN can allow transmission of very high quality video images through it.

Merits:

• An ISDN user can establish two simultaneously independent telecom calls on existing pair of telephone wires.
• Two simultaneously calls may be of any type such as speech, data, image, video.