Before the coming of age of ISDN, original analogy telecommunication services, also known as Plain Old Telephone System (POTS) were the only source of telecommunication worldwide. POTS was the originally acronymic for Post Office Telephone Service/System but changed when post offices stopped offering telephone services. POTS comprised mainly of copper wires which linked the subscriber’s home to the central switch office. This form of telecommunication had its limitations. Long distance calls had to be routed through operators and switchboards which made them highly unreliable and time consuming. There was also the issue of static inference known as line noise which disturbed communication.
In the 1960s, the telecommunication industry began working on converting its analogue systems to transmit ‘packets’ of digitized data by digital switching. The United Nations’ International Telephone and Telegraph Consultative Committee (CCITT, French acronym), now known as International Telecommunications Union Telecommunications (ITU-T) actively pushed and encouraged the research for Integrated Services Digital Network (ISDN) by starting a movement to recommend and provide standards for international digitization of the telecommunication services. This was initiated in 1984.
Two major US networks, Northern Telecomm and AT&T took the first steps towards implementing ISDN, but it did not interoperate with existing equipments and software available to telecommunication networks, this led to a setback for ISDN worldwide. By 1990, due to a world wide effort, National ISDN 1 (NI-1) was made compatible with existing proprietary equipments for telecommunication services, this way people did not need to switch brand or buy software for the network. This set the standards and procedure for future digital telecommunication technology to be used by everyone.
ISDN has resulted in better voice quality and Internet access due to its packet-switch connection. With ISDN, voice and data are carried by a bearer channel (B channel) with a bandwidth of 64 kb/s, sometimes 56 kb/s as opposed to telephone line which carry 52 kb/s. A data channel (D channel) is used for controlling network services and signalling to construct, break connections and carry data over the bearer channels. This carries a bandwidth of 16 kb/s or 64 kb/s.
There are two services offered by ISDN. This is known as the BRI and PRI. BRI – Basic Rate Interface consists of two 64 kb/s bearer channels and one 16 kb/s data channel, totaling 144 kb/s; as the name implies, BRI usually has enough bandwidth for individual users. PRI – Primary Rate Interface is used for clients that require greater bandwidth. It usually comprised of 23 64 kb/s B channels and one 64 kb/s D channel, totaling 1.536 Mb/s. Sometimes, the PRI service can have 30 B channels and one D channel for a total of 1.984 Mb/s.
For now, the future of ISDN lies with Broadband ISDN (BISDN). This entails transferring data, voice and video all at the same time using fibre optic telephone line which can carry data rates from 155 Mb/s to 622 Mb/s and beyond. This is a major topic for research and development.