What is ISDN

ISDN (Integrated Services Digital Network) is a system of digital phone connections that has been designed for sending voice, video, and data simultaneously over digital or ordinary phone lines, with a much faster speed and higher quality than an analog system can provide. ISDN is basically a set of protocol for making and breaking circuit switched connections as well as for advanced call features for the customers. ISDN is the international communication standard for data transmission along telephone lines and has transmission speeds up to 64 Kbps per channel.
ISDN uses two channels for communication which are the Bearer Channel or the B channel and the Delta Channel of the D Channel. The B channel is used for the data transmission and the D channel is used for signaling and control, though data can be transmitted through the D cannels as well. ISND has two access options, the Basic Rate Interface, also known as the BRI or the Basic Rate Access or BRA and Primary Rate Interface or Primary Rate Access. Basic Rate Interface is made up of two B channels with a bandwidth of 64 Kbit/s and a D channel with a bandwidth with 16 Kbit/s. The Basic Rate Interface is also known as 2B+D.

Primary Rate Interface has a greater number of B channels, which varies from nation to nation across the globe, and a D channel with a bandwidth of 64 Kbit/s. For example, in North America and Japan a PRI is represented as 23B+D (a total bit rate of 1.544 Mbit/s) while it is 30B+D in Australia and Europe (equivalent to a bit rate of 2.048 Mbit/s).

A technique called bipolar with eight-zero substitution technique is used to transfer calls through the data channels - the B channels - with the signaling channels (D channels) being exclusively used for call set up and management. Once the call had been set up, a 64 Kbit/s synchronous bidirectional B channel transfer the data between the ends, which lasts until the call ends. Theoretically, there can be as many calls as there are data channels, the choice of same or different end point not withstanding. Also, it is possible to multiplex a number of bearer channels (B channels) to produce a single higher bandwidth channel, using a process called B channel bonding.

ISDN has become a relatively old technology, but it isn't obsolete. ISDN is a technology that is often used behind the scenes as a component of more recent technology. Hopefully ISDN will continue to evolve so that it can continue to make an impact in the technological world.


Posted by: Wasim Javed