Introduction To Line Telecommunications


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7. Integrated Services Digital Network
  7.1 - Basic Rate ISDN (ISDN 2) / I.420 7.5 - DPNSS Signalling
  7.2 - Primary Rate ISDN (ISDN 30) 7.6 - Q-Sig
  7.3 - DASS Signalling 7.7 - ADSL
7.4 - Euro ISDN (I.421) 7.8 - Cable Modems

7. Integrated Services Digital Network

The Integrated Services Digital Network (ISDN) concept was formally defined by the CCITT in 1979. It has been created to be a single network to carry all communication services now and in the foreseeable future. To this end it’s specification was drawn up to allow for current and envisioned future developments in telecommunications and to provide a common basis for their implementation on the ISDN.

The development leading to the creation of the digital network began in the 1960s. It was driven by the need to carry more traffic on the existing network which was becoming swamped by the number of users taking advantage of the telephone. It was during this time that the telephone moved from being a rare luxury to an everyday item and then a necessity for businesses and homes. The first steps were to introduce mutilplexing which lead to development of digital transmission techniques. These drove the need to update the transmission media used to cope with the increased speed of transmission , capacity and the increased demands. The media developed include Co-axial cables, optic fibres and microwave links all of which have become commonplace in the network of today and without which it would be unable to function.

The networks today are all built upon this technology and converted to analogue for subscribers using the PSTN. It is therefore simple to extend the fully digital service to customer premises. Many recent services do just this (see ADSL later).

Variations on the ISDN are now in use in most developed countries although there are differences between their exact implementations. The concept is essentially the same.

ISDN services provide a number of Bearer or B channels which carry the traffic and Data or D channels used to control data flow, signal timings and transmit routing information. The number of these channels and the services they provide is determined by the interface used by the customer at any given site and the equipment used.

Signalling over the ISDN is achieved using an appropriate protocol e.g. I.421 which signals using message and data information packets. The contents and functioning of these signals is beyond the scope of this book and will not be covered in detail.

7.1 - Basic Rate ISDN (ISDN 2) / I.420

This is the simplest form of connection to the ISDN network. It is intended for small CRA, single device connection and is now installed in some homes, where the users require specialist communications facilities which cannot be supported on the PSTN.

The Basic Rate Interface (BRI), also known as ISDN 2, I.421 or S-Bus, is a 144Kbps service providing the user with 2 B channels each 64Kbps and 1 D channel of 16Kbps. The B channels can be combined to give a single 128Kbps channel for high-speed data applications. Each B channel can be thought of an exchange line so if both are in use each will incur a call charge. Multiple B channels can be combined to provide increased bandwidth. For example some videoconferencing systems use 6 B channels to provide a 384Kbps link. Each of these channels will incur call charges, making this combination an expensive option for long term use.

Within the specification for the BRI are contention protocols which allow up to 8 compatible ISDN devices to be attached simultaneously in a similar way to PSTN extension parallel connection. However the BRI will only allow a single device access to the channels at one time and other devices are restricted from using channels which are busy, being used by another connected device.

Applications for the BRI available now include video telephones, direct network data connections at higher speeds than the PSTN and even a radio presenter who runs his show and even plays the music down a link to his studio for broadcast with no loss of facility or sound quality. New and innovative applications are being developed all the time.

CLI, DDI and similar services can be provided over BRI circuits making them a viable alternative to small numbers of analogue lines and providing facilities similar to the higher capacity Primary Rate services.



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Introduction To Line Telecommunications
Copyright Panasonic Business Systems UK Ltd 2000