What is ISDN?
ISDN is a network, completely digital in nature, which can provide
a myriad of data and telecommunications services of end-to-end significance.
In its simplest form, ISDN is merely an enhancement to the telephone local
loop that will allow both voice and data to be carried over the same twisted
pair. It is a fully digital network, where all devices and applications
present themselves in a digital form. The picture below shows the difference
between a normal analog line and an isdn line.
Definition of ISDN
The International Telecommunication Union Telecommunication Standardization
Sector (ITU-T), which is largely responsible for today's international
ISDN standards, defines an ISDN as:
"a network, in general evolving from a telephony Integrated Digital
Network (IDN), that provides end-to-end digital connectivity to support
a wide range of services, including voice and non-voice services, to which
users have access by a limited set of standard multi-purpose user-network
interfaces."
History of ISDN
Analog Telephone
The early phone network consisted of a pure analog system that connected telephone users directly by an interconnection of wires. This system was very inefficient, very prone to breakdown and noise, and did not lend itself easily to long-distance connections.
Digital Telephone
In 1960 the telephone system gradually began converting its internal connections to a packet-based, digital switching system.
ITU
A standards movement was started by the International Telephone and
Telegraph Consultative Committee (CCITT), now known as the International
Telecommunications Union Telecommunication Standardization Sector(ITU-T).
ITU-T came out with international standards for ISDN. The ISDN standards
provide the rules for the user interface to the network, but they do not
describe the network itself. The standards also define the services that
may be offered by an ISDN, but they do not describe their implementation
or applications that utilize the services.