ISDN
Integrated Services Digital Network is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Work on the standard began in 1980 at Bell Labs and was formally standardized in 1988 in the CCITT "Red Book". By the time the standard was released, newer networking systems with much greater speeds were available, and ISDN saw relatively little uptake in the wider market. One estimate suggests ISDN use peaked at a worldwide total of 25 million subscribers at a time when 1.3 billion analog lines were in use. ISDN has largely been replaced with digital subscriber line systems of much higher performance.
Prior to ISDN, the telephone system consisted of digital links like T1/E1 on the long-distance lines between telephone company offices and analog signals on copper telephone wires to the customers, the "last mile". At the time, the network was viewed as a way to transport voice, with some special services available for data using additional equipment like modems or by providing a T1 on the customer's location. What became ISDN started as an effort to digitize the last mile, originally under the name "Public Switched Digital Capacity". This would allow call routing to be completed in an all-digital system, while also offering a separate data line. The Basic Rate Interface, or BRI, is the standard last-mile connection in the ISDN system, offering two "bearer" lines and a single "data" channel for commands and data.
ISDN remains successful globally, but in the US the system was largely ignored and garnered the industry nickname "innovation subscribers didn't need." The underlying ISDN concepts found wider use as a replacement for the T1/E1 lines it was originally intended to extend, roughly doubling the performance of those lines.
History
Digital lines
Since its introduction in 1881, the twisted pair copper line has been installed for telephone use worldwide, with well over a billion individual connections installed by the year 2000. Over the first half of the 20th century, the connection of these lines to form calls was increasingly automated, culminating in the crossbar switches that had largely replaced earlier concepts by the 1950s.As telephone use surged in the post-WWII era, the problem of connecting the massive number of lines became an area of significant study. Bell Labs' seminal work on digital encoding of voice led to the use of as a standard for voice lines. In 1962, Robert Aaron of Bell introduced the T1 system, which carried of data on a pair of twisted pair lines over a distance of about one mile. This was used in the Bell network to carry traffic between local switch offices, with 24 voice lines at and a separate line for signaling commands like connecting or hanging up a call. This could be extended over long distances using repeaters in the lines. T1 used a very simple encoding scheme, alternate mark inversion, which reached only a few percent of the theoretical capacity of the line but was appropriate for 1960s electronics.
By the late 1970s, T1 lines and their faster counterparts, along with all-digital switching systems, had replaced the earlier analog systems for most of the western world, leaving only the customer's equipment and their local end office using analog systems. Digitizing this "last mile" was increasingly seen as the next problem that needed to be solved. However, these connections now represented over 99% of the total telephony network, as the upstream links had increasingly been aggregated into a smaller number of much higher performance systems, especially after the introduction of fiber optic lines. If the system was to become all-digital, a new standard would be needed that was appropriate for the existing customer lines, which might be miles long and of widely varying quality.
Standardization
Around 1978, Ralph Wyndrum, Barry Bossick and Joe Lechleider of Bell Labs began one such effort to develop a last-mile solution. They studied a number of derivatives of the T1's AMI concept and concluded that a customer-side line could reliably carry about of data over a distance of. That would be enough to carry two voice-quality lines at as well as a separate line for data. At the time, modems that were normally and would not become common until the early 1980s and the standard would not be completed until 1984. In this market, represented a significant advance in performance in addition to being a separate channel that coexists with voice channels.A key problem was that the customer might only have a single twisted pair line to the location of the handset, so the solution used in T1 with separate upstream and downstream connections was not universally available. With analog connections, the solution was to use echo cancellation, but at the much higher bandwidth of the new concept, this would not be so simple. A debate broke out between teams worldwide about the best solution to this problem; some promoted newer versions of echo cancellation, while others preferred the "ping pong" concept where the direction of data would rapidly switch the line from send to receive at such a high rate it would not be noticeable to the user. John Cioffi had recently demonstrated echo cancellation would work at these speeds, and further suggested that they should consider moving directly to performance using this concept. The suggestion was literally laughed off the table and his boss told him to "sit down and shut up." But the echo cancellation concept that was taken up by Joe Lechleider eventually came to win the debate.
Meanwhile, the debate over the encoding scheme itself was also ongoing. As the new standard was to be international, this was even more contentious as several regional digital standards had emerged in the 1960s and 70s and merging them was not going to be easy. To further confuse issues, in 1984 the Bell System was broken up and the US center for development moved to the American National Standards Institute T1D1.3 committee. Thomas Starr of the newly formed Ameritech led this effort and eventually convinced the ANSI group to select the 2B1Q standard proposed by Peter Adams of British Telecom. This standard used an 80 kHz base frequency and encoded two bits per baud to produce the base rate. Ultimately Japan selected a different standard, and Germany selected one with three levels instead of four, but all of these could interchange with the ANSI standard.
From an economic perspective, the European Commission sought to liberalize and regulate ISDN across the European Economic Community. The Council of the European Communities adopted Council Recommendation in December 1986 for its coordinated introduction within the framework of CEPT. ETSI was created by CEPT in 1988 and would develop the framework.
Rollout
With digital-quality voice made possible by ISDN, offering two separate lines and continuous data connectivity, there was an initial global expectation of high customer demand for such systems in both the home and office environments. This expectation was met with varying degrees of success across different regions.In the United States, many changes in the market led to the introduction of ISDN being tepid. During the lengthy standardization process, new concepts rendered the system largely superfluous. In the office, multi-line digital switches like the Meridian Norstar took over telephone lines while local area networks like Ethernet provided performance around which had become the baseline for inter-computer connections in offices. ISDN offered no real advantages in the voice role and was far from competitive in data. Additionally, modems had continued improving, introducing systems in the late 1980s and in 1991, which significantly eroded ISDN's value proposition for the home customer.
Conversely, in Europe, ISDN found fertile ground for deployment, driven by regulatory support, infrastructural needs, and the absence of comparable high-speed communication technologies at the time. The technology was widely embraced for its ability to digitalize the "last mile" of telecommunications, significantly enhancing the quality and efficiency of voice, data, and video transmission over traditional analog systems.
Meanwhile, Lechleider had proposed using ISDN's echo cancellation and 2B1Q encoding on existing T1 connections so that the distance between repeaters could be doubled to about. Another standards war broke out, but in 1991 Lechleider's "High-Speed Digital Subscriber Line" eventually won this process as well, after Starr drove it through the ANSI T1E1.4 group. A similar standard emerged in Europe to replace their E1 lines, increasing the sampling range from 80 to 100 kHz to achieve. By the mid-1990s, these Primary Rate Interface lines had largely replaced T1 and E1 between telephone company offices.
Replacement by ADSL
Lechleider also believed this higher-speed standard would be much more attractive to customers than ISDN had proven. Unfortunately, at these speeds, the systems suffered from a type of crosstalk known as "NEXT", for "near-end crosstalk". This made longer connections on customer lines difficult. Lechleider noted that NEXT only occurred when similar frequencies were being used, and could be diminished if one of the directions used a different carrier rate, but doing so would reduce the potential bandwidth of that channel. Lechleider suggested that most consumer use would be asymmetric anyway, and that providing a high-speed channel towards the user and a lower speed return would be suitable for many uses.This work in the early 1990s eventually led to the ADSL concept, which emerged in 1995. An early supporter of the concept was Alcatel, who jumped on ADSL while many other companies were still devoted to ISDN. Krish Prabu, COO of Alcatel, stated that "Alcatel will have to invest one billion dollars in ADSL before it makes a profit, but it is worth it." They introduced the first DSL Access Multiplexers, the large multi-modem systems used at the telephony offices, and later introduced customer ADSL modems under the Thomson brand. Alcatel remained the primary vendor of ADSL systems for well over a decade.
ADSL quickly replaced ISDN as the customer-facing solution for last-mile connectivity. ISDN has largely disappeared on the customer side, remaining in use only in niche roles like dedicated teleconferencing systems and similar legacy systems.