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ISDN: An Overview

ISDN, Integrated Services Digital Network is a digital network interface to the telephone network. For many years, the original telephone networks have been undergoing a process of upgrading from analog to digital technology. This upgrading began internally in the network. Even if networks are digital internally, many users still have analog equipment, and the signals undergo analog-to-digital conversion. By offering ISDN, the network provider moves the digital interface out to the end user. This means that the end user must have equipment with a digital ISDN interface. This interface offers several new possibilities compared with an analog modem. The three most important ones for data transfer are:

The network provider terminates the network connection at the user in a box on the wall designated NT1- NetworkTermination 1.On the user side, the NT1 has a socket (RJ45)to connect the user's hardware. This point is called a T-point or S-point. (S-pointmeans that, between the network supplier and the user, there is an exchange (PBX)owned by the user, designated NT2.)

S Bus

Electrically,the S bus consists of two loops, a send loop and a receive loop. On these loops, frames, called B 1,B2, and D, are sent and received. From a logical point of view, we can say that the S bus has two data channels B 1 and B2, each with a capacity of 64 Kbps. An ISDN device ("El) which uses one of the B channels can therefore send and receive at up to 64 Kbps.

Two Channels

Since there are two B-channels, two devices can be active at the same time. For example, an ISDN telephone can be used on channel B1 and a PC with an ISDN connection on channel B2, at the same time. One device can also use both B-channels, so that a computer can transfer data at 2 x 64 = 128 Kbps.


The capacity of the D-channel is 16 Kbps. This channel is used for signaling,ATE 1 sends data to the network about requested connection or disconnection via this channel. So, here, signalling and data traffic are separate on different channels. Setting up ISDN connections between computers wlll in practice be much faster than using a modem. Setting up a modem connection can take 10-30 seconds, while an ISDN connection is normally established in 1-2 seconds or less.

Published on Wed 12 April 2006 by Tracey Mann in Networking with tag(s): isdn