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 its 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.
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
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
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.