Concept of ISDN

Integrated Services Digital Network(ISDN) is launched by telephone companies and used to provide large scale digital services to subscribers.ISDN provides digitized voice and data to subscribers over conventional local wiring.ISDN uses the same type of twisted pair copper wiring as the analog telephone system.If we discuss from the subscribers point of view then ISDN offers three separate digital channels designated B,B and D(usually written 2B+D).The two B channels operates at a speed of 64 Kbps,are intended to carry digitized voice and data or compresses videos, on the other hand the D channel which operates at 16 Kbps,is intended as a control channel.In general,a subscriber uses the D channel to request services which are then supplied over the B channels.The subscriber can also use the D channel to manage a session that is in progress or to terminate a session.Finally,both the B channels can be combined or bonded together to produce a single channel with effective data rate at 128 Kbps.The 2B+D channels are known as ISDN Basic Rate Interface(BRI).Infact,ISDN uses a form of time division multiplexing to provide the illusion of multiple channels of data to travel over a single pair of wires.