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Integrated Services Digital Network (ISDN) is a set of communication standards for digital telephone connection and the transmission of voice and data over a digital line, and is a development of the plain old telephone service (POTS). These digital lines are commonly telephone lines and exchanges established by the government. They are used instead of the traditional circuits of the classic switched telephone network since they can integrate data and speech on the same line. Before ISDN, it was not possible for ordinary telephone lines to provide fast transmission over a single line.
ISDN was designed to run on digital telephone systems that were already in place. As such, it meets telecom’s digital voice network specifications. Originally, it was largely used by businesses that needed to support many desk phones and fax machines. However, it took so long for ISDN to be standardized that it was never fully deployed in the telecommunications networks it was intended for.
ISDN was formally standardized in 1988 and gained some significant popularity in the 1990s as a faster (128 Kbps) alternative to the 56 Kbps dial-up connection for internet access. However, as soon as telecom companies switched from analog to digital infrastructures, modern long-distance networking and broadband internet technologies eventually made it an obsolete technology.
Techopedia Explains Integrated Services Digital Network
ISDN can simultaneously transmit all kinds of data over a single telephone line. As such, voice and data are no longer separated as they were in earlier technologies, which used separate lines for different services. ISDN is a circuit-switched telephone network system, but it also allows access to packet-switched networks.
ISDN is also used with specific protocols, such as Q.931, where it acts as the network, data link and physical layers in the OSI model. Therefore, in broad terms, ISDN is actually a suite of transmission services on the first, second and third layers of the OSI model. [See also ISDN replacement – Sessions Initiated Protocol (SIP)]
There are three different ISDN iterations, although the third never obtained mainstream use. They are:
Basic Rate Interface (BRI-ISDN)
Basic Rate Interface (BRI) is the entry-level alternative that was generally used as the standard internet access option. It is capable of reaching data transmission speeds of up to 128 Kbps and was generally advertised for small business and home use. However, since the ISDN works over preexisting copper telephone lines, the speed rarely truly reached the 128 Kbps advertised by telecom companies. Also known as ISDN2, the BRI carried data over two 64 Kbps bearer channels (hence the name) known as B channels, while control information was handled by a 16 Kbps channel. The 16 kbps channel, known as the D (data) channel is used for protocol negotiation.
Primary Rate Interface (PRI-ISDN)
Primary Rate Interface (PRI) was a higher speed ISDN mostly intended for enterprise use as it supports full T1 speeds of 1.544 Mbps and up to 2.048 Mbps on E1 (E-carrier system). Instead of the two channels used for BRI, PRI makes full use of 23 parallel 64 Kbps bearer channels. E1 lines can support 30 bearer channels and were mostly used in Europe and Asia. They were also known as ISDN30 to differentiate them from ISDN2.
Broadband ISDN (B-ISDN)
Broadband ISDN (B-ISDN) is a very advanced form of ISDN that was designed to improve its performance even further over the PRI-ISDN. B-ISDN could run over fiber optic cables using ATM switching technology to transmit hundreds of Mbps of data both in download and upload. However, it was overtaken by ADSL technologies which made it obsolete before it could become a mainstream technology. It never saw any substantial practical application but still survives as a low-level, entry-level layer for DSL technologies and in WiMAX.
