ISDN Basic Access – A Brief Overview

ISDN Basic Access – A Brief Overview

This document is a brief overview of ISDN Basic Access and written from viewpoint of Network Termination (NT1). The Integrated Services Digital Network (ISDN) is a set of digital transmission standards which are used for end-to-end digital connectivity. ‘‘Integrated Services’’ referring to its ability to sustain numerous applications. ‘‘Digital Network’’ relating to its end-to-end digital connection. In general, ISDN networks extend from the local telephone exchange to the remote user and include all the telecommunications and switching equipments in between. ISDN supports voice and data. In the past, video, audio, voice and data services required at least four separate networks. ISDN integrates all four over the same network.

System Concept and Overview

Network Termination (NT1) device has been designed for connecting Integrated Services Digital Network (ISDN) Voice or Data Termination Equipment (TE) to ISDN line provided by the telephone company. It is a layer 1 device, which converts the 2-wire public network U-interface to the limited distance 4-wire S interface on the customer premises. It connects ISDN Basic (2B+D) line from exchange end at one side (U-interface) and the ISDN Terminal Equipment (TE) at the other side (S-interface).

In order to convert subscriber connection from the analog signal to digital, the termination at each end of the copper loop must be changed slightly. Under the Basic Access rate, each subscriber will be entitled to the use of 2B channels and 1D channel, (2B+1D). For Basic Access, each B channel (B stands for Bearer) represents 64Kbps channel and the D channel ( D for Delta) represents a 16Kbps channel. A minimum transmission rate of 144Kbps full duplex transmission is therefore required for basic access transport.

ISDN Basic Access Reference model

The figure shows the ISDN reference model for basic access connection. The reference model shows a block diagram of how the communication link is established.

TE or Terminal Equipment can represent either a digital telephone, a personal computer, a fax machine, a printer, etc. There are two types of terminal equipment. The TE1 is a terminal which synchronized to ISDN network. This simply means that the terminal equipment was manufactured with the intention of being compatible with the ISDN network. The TE2 on other hand is a terminal which is not compatible with the ISDN network. It is the function of theta or Terminal Adapter to convert either asynchronous or synchronous data from a non-ISDN terminal into data which is synchronized with ISDN B or D channels. The data rate by stuffing extra bits into the bit stream in a prescribed manner.

The next block in this model is the Network Termination (NT). The NT terminates this network at the users end of a two-wire loop. It converts the “U” interface to the “S” interface and “T” interface and acts as the master end of the users line. Just like the terminal equipment, the NT is also Customers Premises Equipment (CPE).

The next block is Line Termination (LT). Typically LT consists of the “U” interface transceiver and power feeding functions on the ISDN Line Card.

There are several functions that S-interface fulfills. It connects up 8 TEs in parallel with NT. For a basic access rate 2B+D, 3 channels are available for use the 8 pieces of terminal equipment. Part of the S function is to arbitrate the use of these channels to however many TEs reside on the line. Since computers and like don’t need to be operated in real time, the S interface can control the use of the phone connection and the 7 other pieces of terminal equipment can share the other 2 channels. A user can maintain a voice connection on one of the B channels and simultaneously transmit data on the other B and D channels.


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