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