ISDN - Overview

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.

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. DASS II 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.

Basic Rate ISDN (ISDN 2)
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 combine 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.

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.

Primary Rate ISDN (ISDN 30)
The Primary Rate Interface (PRI), or ISDN 30, uses the same basis as a Megastream. In the UK this is a 2Mbps link or 'pipe' providing 30 B channels and using 2 D channels for timing and control. The USA and Japan use a similar system with slower data rates and a different data encoding system (T1). On this platform a range of protocols have been and are being developed to allow interfacing between the network and connected equipment.

The high capacity of the PRI means that its use is primarily for PBX connection to the network using one of the standard protocols.

DASS Signalling
The Digital Access Signalling System (DASS) is the UK proprietary standard created by BT to provide ISDN services in the UK. The first incarnation DASS1 is now obsolete and has been replaced by DASS II. This itself will become obsolete over the coming years as Q.931, a European standard becomes widely adopted in the EEC.

DASS II is purely a protocol used to control data flow across the ISDN doing a similar job to the protocols used with modems when sending the data over the analogue PSTN. It is more highly specified and controlled and operates at higher speeds.

Euro ISDN (Q931)
The Euro-ISDN, Q.931, is a further protocol development building upon the features and facilities offered by DASS II and developed for implementation on networks throughout the EEC. It will eventually replace DASS II in the UK and the systems used by other EEC member countries to become the single ISDN standard used throughout Europe.

Users of previous protocols will in most cases be able to continue with their current equipment with the necessary conversions being performed by the network equipment to allow their traffic across the Euro-ISDN network.

DPNSS Signalling
Digital Private Network Signalling System (DPNSS) is, in a way, the 'odd man out' in the list of ISDN protocols. It is not formally regulated, but is a voluntary standard developed by the exchange and large PBX manufacturers, over the last decade, in conjunction with BT to allow interconnection between their equipment over the ISDN network.

The specification carries details of approximately 50 services and functions which if followed will operate over systems produced by the involved parties and even allow for equipment from the same manufacturer to be transparently connected over a digital link using 'private' functions. In practice any given manufacturer will implement a core of essential services and around 6 optional functions completely plus some 'private' ones for their own equipment. A subset is selected by each manufacturer since these facilities are all inter-related and to use all of them would be too difficult to engineer. However the subsets vary between manufacturers and the DPNSS protocol will limit inter-connection facilities to those supported in common between linked PBX equipment.

If one of the involved manufacturers develops a new facility which is considered to be of general interest and practical use it will be included as a standard and controlled function in the DPNSS definition for all parties to use and draw upon.

Q-Sig is the Euro ISDN based equivalent of DPNSS. Unlike DPNSS it is a regulated standard, but like DPNSS the feature list is optional after the basic of call set up and handling have been implemented, so the interoperation of any two switches will be dependant upon the common features of the implementations of the connected telephone systems.

Whilst there are a number of existing DPNSS connected networks in operation which will expand and keep up a demand for DPNSS, the Q-Sig protocol will eventually supersede it as new networks adopt it as their communications standard in coming years.

ISDN - more detailed Technical Description


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Last revised: October 30, 2022