Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneousdigitaltransmission of voice, video, data, and other network services over the digitalised circuits of thepublic switched telephone network.[1] Work on the standard began in 1980 atBell Labs and was formally standardized in 1988 in theCCITT "Red Book".[2] 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.[3] ISDN has largely been replaced withdigital subscriber line (DSL) systems of much higher performance.
Prior to ISDN, the telephone system consisted of digital links likeT1/E1 on the long-distance lines between telephone company offices and analog signals oncopper 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 likemodems 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" (PSDC).[3] This would allow call routing to be completed in an all-digital system, while also offering a separate data line. TheBasic Rate Interface, or BRI, is the standard last-mile connection in the ISDN system, offering two64 kbit/s "bearer" lines and a single16 kbit/s "data" channel for commands and data.
Although ISDN was successful in a few countries such as Germany, on a global scale the system was largely ignored and garnered the industry nickname "innovation(s) subscribers didn't need."[4] It found a use for a time for small-office digital connection, using the voice lines for data at64 kbit/s, sometimes "bonded" to128 kbit/s, but the introduction of56 kbit/s modems undercut its value in many roles. It also found use invideoconference systems, where the direct end-to-end connection was desirable. TheH.320 standard was designed around its64 kbit/s data rate. 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.
Since its introduction in 1881, thetwisted 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 thecrossbar switches that had largely replaced earlier concepts by the 1950s.[3]
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 of64 kbit/s as a standard for voice lines (or56 kbit/s in some systems). In 1962, Robert Aaron of Bell introduced the T1 system, which carried1.544 Mbit/s 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 at64 kbit/s and a separate8 kbit/s 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 (AMI), which reached only a few percent of thetheoretical capacity of the line but was appropriate for 1960s electronics.[4]
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 localend 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 offiber 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.[4]
Around 1978, Ralph Wyndrum, Barry Bossick and Joe Lechleider ofBell 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 about160 kbit/s of data over a distance of 4 to 5 miles (6.4 to 8.0 km). That would be enough to carry two voice-quality lines at64 kbit/s as well as a separate16 kbit/s line for data. At the time,modems were normally300 bit/s and1200 bit/s would not become common until the early 1980s and the2400 bit/s standard would not be completed until 1984. In this market,16 kbit/s represented a significant advance in performance in addition to being a separate channel that coexists with voice channels.[4]
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 useecho 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 to1.5 Mbit/s performance using this concept. The suggestion was literally laughed off the table (His boss told him to "sit down and shut up"[4]) but the echo cancellation concept that was taken up by Joe Lechleider eventually came to win the debate.[4]
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 theBell System was broken up and the US center for development moved to theAmerican National Standards Institute (ANSI) T1D1.3 committee. Thomas Starr of the newly formedAmeritech led this effort and eventually convinced the ANSI group to select the2B1Q standard proposed by Peter Adams ofBritish Telecom. This standard used an 80 kHz base frequency and encoded two bits per baud to produce the160 kbit/s 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.[5]
From an economic perspective, theEuropean Commission sought to liberalize and regulate ISDN across theEuropean Economic Community.[6] TheCouncil of the European Communities adopted Council Recommendation86/659/EEC[7] in December 1986 for its coordinated introduction within the framework of CEPT.ETSI (the European Telecommunications Standards Institute) was created by CEPT in 1988 and would develop the framework.
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 theMeridian Norstar took over telephone lines whilelocal area networks likeEthernet provided performance around10 Mbit/s 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, introducing9600 bit/s systems in the late 1980s and14.4 kbit/s in 1991, which significantly eroded ISDN's value proposition for the home customer.[5]
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 2 miles (3.2 km). Anotherstandards war broke out, but in 1991 Lechleider's1.6 Mbit/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 achieve2.048 Mbit/s.[8] By the mid-1990s, thesePrimary Rate Interface (PRI) lines had largely replaced T1 and E1 between telephone company offices.
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 ofcrosstalk 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.[8]
This work in the early 1990s eventually led to theADSL concept, which emerged in 1995. An early supporter of the concept wasAlcatel, who jumped on ADSL while many other companies were still devoted to ISDN. Krish Prabu 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 (DSLAM), 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.[9]
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.
Integrated services refers to ISDN's ability to deliver at minimum two simultaneous connections, in any combination of data, voice,video, andfax, over asingle line. Multiple devices can be attached to the line, and used as needed. That means an ISDN line can take care of what were expected to be most people's complete communications needs (apart frombroadband Internet access and entertainmenttelevision) at a much higher transmission rate, without forcing the purchase of multiple analog phone lines. It also refers to integrated switching and transmission[10] in thattelephone switching andcarrier wave transmission are integrated rather than separate as in earlier technology.
In ISDN, there are two types of channels,B (for "bearer") andD (for "data").B channels are used for data (which may include voice), andD channels are intended for signaling and control (but can also be used for data).
There are two ISDN implementations. Basic Rate Interface (BRI), also called basic rate access (BRA) — consists of two B channels, each with bandwidth of 64kbit/s, and one D channel with a bandwidth of16 kbit/s. Together these three channels can be designated as 2B+D. Primary Rate Interface (PRI), also called primary rate access (PRA) in Europe — contains a greater number of B channels and a D channel with a bandwidth of64 kbit/s. The number of B channels for PRI varies according to the nation: in North America and Japan it is 23B+1D, with an aggregate bit rate of 1.544Mbit/s (T1); in Europe, India and Australia it is 30B+2D, with an aggregate bit rate of2.048 Mbit/s (E1).Broadband Integrated Services Digital Network (BISDN) is another ISDN implementation and it is able to manage different types of services at the same time. It is primarily used withinnetwork backbones and employsATM.
Another alternative ISDN configuration can be used in which the B channels of an ISDN BRI line arebonded to provide a total duplex bandwidth of128 kbit/s. This precludes use of the line for voice calls while the internet connection is in use. The B channels of several BRIs can be bonded, a typical use is a 384K videoconferencing channel.
Usingbipolar with eight-zero substitution encoding technique, call data is transmitted over the data (B) channels, with the signaling (D) channels used for call setup and management. Once a call is set up, there is a simple64 kbit/s synchronous bidirectional data channel (actually implemented as two simplex channels, one in each direction) between the end parties, lasting until the call is terminated. There can be as many calls as there are bearer channels, to the same or different end-points. Bearer channels may also bemultiplexed into what may be considered single, higher-bandwidth channels via a process called B channel BONDING, or via use of Multi-Link PPP "bundling" or by using an H0, H11, or H12 channel on a PRI.
The D channel can also be used for sending and receivingX.25 data packets, and connection to X.25 packet network, this is specified inX.31. In practice, X.31 was only commercially implemented in the UK, France, Japan and Germany.
A set ofreference points are defined in the ISDNstandard to refer to certain points between thetelco and the end-user equipment.
R – defines the point between non-ISDN terminal equipment,terminal equipment 2 (TE2), and aterminal adapter (TA) which provides translation to and from such a device
S – defines the point between ISDN terminal equipment,terminal equipment 1 (TE1), or a TA and aNetwork Termination Type 2 (NT2) device
Most NT-1 devices can perform the functions of the NT2 as well, and so the S and T reference points are generally collapsed into the S/T reference point.
In North America, the NT1 device is consideredcustomer premises equipment (CPE) and must be maintained by the customer, thus, the U interface is provided to the customer. In other locations, the NT1 device is maintained by the telco, and the S/T interface is provided to the customer. In India, service providers provide U interface and an NT1 may be supplied by Service provider as part of service offering.
The entry level interface to ISDN is theBasic Rate Interface (BRI), a 128kbit/s service delivered over a pair of standard telephone copper wires.[11] The144 kbit/s overall payload rate is divided into two64 kbit/sbearer channels ('B' channels) and one16 kbit/s signaling channel ('D' channel or data channel). This is sometimes referred to as 2B+D.[12]
The interface specifies the following network interfaces:
TheU interface is a two-wire interface between the exchange and anetwork terminating unit, which is usually thedemarcation point in non-North American networks.
TheT interface is a serial interface between a computing device and aterminal adapter, which is the digital equivalent of a modem.
TheS interface is a four-wire bus that ISDN consumer devices plug into; the S & T reference points are commonly implemented as a single interface labeled 'S/T' on aNetwork termination 1 (NT1).
TheR interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.
BRI-ISDN is very popular in Europe but is much less common in North America. It is also common in Japan — where it is known as INS64.[13][14]
The other ISDN access available is thePrimary Rate Interface (PRI), which is carried overT-carrier (T1) with 24 time slots (channels) in North America, and overE-carrier (E1) with 32 channels in most other countries. Each channel provides transmission at a64 kbit/s data rate.
With the E1 carrier, the available channels are divided into 30 bearer (B) channels, one data (D) channel, and one timing and alarm channel. This scheme is often referred to as 30B+2D.[15]
In North America, PRI service is delivered via T1 carriers with only one data channel, often referred to as 23B+D, and a total data rate of1544 kbit/s.Non-Facility Associated Signalling (NFAS) allows two or more PRI circuits to be controlled by a singleD channel, which is sometimes called23B+D + n*24B. D-channel backup allows for a second D channel in case the primary fails. NFAS is commonly used on aDigital Signal 3 (DS3/T3).
Even though many network professionals use the termISDN to refer to the lower-bandwidth BRI circuit, in North America BRI is relatively uncommon whilst PRI circuits serving PBXs are commonplace.
The bearer channel (B) is a standard64 kbit/s voice channel of 8 bits sampled at 8 kHz withG.711 encoding. B-channels can also be used to carry data, since they are nothing more than digital channels.
Most B channels can carry a64 kbit/s signal, but some were limited to 56K because they traveled overRBS lines. This was commonplace in the 20th century, but has since become less so.
X.25 can be carried over the B or D channels of a BRI line, and over the B channels of a PRI line. X.25 over the D channel is used at many point-of-sale (credit card) terminals because it eliminates the modem setup, and because it connects to the central system over a B channel, thereby eliminating the need for modems and making much better use of the central system's telephone lines.
X.25 was also part of an ISDN protocol called "Always On/Dynamic ISDN", or AO/DI. This allowed a user to have a constant multi-link PPP connection to the internet over X.25 on the D channel, and brought up one or two B channels as needed.
ISDN is a core technology in the telephone industry. Atelephone network can be thought of as a collection of wires strung between switching systems. The common electrical specification for the signals on these wires isT1 orE1. Between telephone company switches, the signaling is performed viaSS7. Normally, a PBX is connected via a T1 withrobbed bit signaling to indicate on-hook or off-hook conditions andMF andDTMF tones to encode the destination number. ISDN is much better because messages can be sent much more quickly than by trying to encode numbers as long (100ms per digit) tone sequences. This results in faster call setup times. Also, a greater number of features are available and fraud is reduced.
In common use, ISDN is often limited to usage toQ.931 and related protocols, which are a set ofsignaling protocols establishing and breaking circuit-switched connections, and for advancedcalling features for the user.[16] Another usage was the deployment ofvideoconference systems, where a direct end-to-end connection is desirable. ISDN uses theH.320 standard foraudio coding andvideo coding.
ISDN is also used as a smart-network technology intended to add new services to thepublic switched telephone network (PSTN) by giving users direct access to end-to-end circuit-switched digital services and as a backup or failsafe circuit solution for critical use data circuits.
One of ISDNs successful use-cases was in thevideoconference field, where even small improvements in data rates are useful, but more importantly, its direct end-to-end connection offers lower latency and better reliability than packet-switched networks of the 1990s. TheH.320 standard foraudio coding andvideo coding was designed with ISDN in mind, and more specifically its64 kbit/s basic data rate. includingaudio codecs such asG.711 (PCM) andG.728 (CELP), anddiscrete cosine transform (DCT)video codecs such asH.261 andH.263.[17][18]
ISDN is used heavily by the broadcast industry as a reliable way of switching low-latency, high-quality, long-distance audio circuits. In conjunction with an appropriatecodec using MPEG or various manufacturers' proprietary algorithms, an ISDN BRI can be used to send stereo bi-directional audio coded at128 kbit/s with 20 Hz – 20 kHz audio bandwidth, although commonly theG.722 algorithm is used with a single64 kbit/s B channel to send much lower latency mono audio at the expense of audio quality. Where very high quality audio is required multiple ISDN BRIs can be used in parallel to provide a higher bandwidth circuit switched connection.BBC Radio 3 commonly makes use of three ISDN BRIs to carry320 kbit/s audio stream for live outside broadcasts. ISDN BRI services are used to link remote studios, sports grounds andoutside broadcasts into the mainbroadcast studio. ISDN via satellite is used by field reporters around the world. It is also common to use ISDN for the return audio links to remote satellite broadcast vehicles.
In many countries, such as the UK and Australia, ISDN has displaced the older technology of equalised analogue landlines, with these circuits being phased out by telecommunications providers. Use of IP-based streaming codecs such asComrex ACCESS andipDTL is becoming more widespread in the broadcast sector, using broadband internet to connect remote studios.[19]
Telstra provides the business customer with the ISDN services. There are five types of ISDN services which are ISDN2, ISDN2 Enhanced, ISDN10, ISDN20 and ISDN30. Telstra changed the minimum monthly charge for voice and data calls. In general, there are two group of ISDN service types; The Basic Rate services – ISDN 2 or ISDN 2 Enhanced. Another group of types are the Primary Rate services, ISDN 10/20/30.[22] Telstra announced that the new sales of ISDN product would be unavailable as of 31 January 2018.[23] The final exit date of ISDN service and migration to the new service was on 31 May 2022.[24]
Orange offers ISDN services under their product name Numeris (2 B+D), of which a professional Duo and home Itoo version is available. ISDN is generally known as RNIS in France and has widespread availability. The introduction of ADSL is reducing ISDN use[when?] for data transfer and Internet access, although it is still common in more rural and outlying areas, and for applications such asbusiness voice andpoint-of-sale terminals. In 2023, Numeris services will enter a phase-out process. They will be replaced by VoIP services.
InGermany, ISDN was very popular with an installed base of 25 million channels (29% of all subscriber lines in Germany as of 2003 and 20% of all ISDN channels worldwide). Due to the success of ISDN, the number of installed analog lines was decreasing.Deutsche Telekom (DTAG) offered both BRI and PRI. Competing phone companies often offered ISDN only and no analog lines. However, these operators generally offered free hardware that also allows the use of POTS equipment, such as NTBAs ("Network Termination for ISDN Basic rate Access": small devices that bridge the two-wire UK0 line to the four-wire S0 bus) with integratedterminal adapters. Because of the widespread availability of ADSL services, ISDN was primarily used for voice and fax traffic.
Until 2007 ISDN (BRI) andADSL/VDSL were often bundled on the same line, mainly because the combination of DSL with an analog line had no cost advantage over a combined ISDN-DSL line. This practice turned into an issue for the operators when vendors of ISDN technology stopped manufacturing it and spare parts became hard to come by. Since then phone companies started introducing cheaper xDSL-only products usingVoIP for telephony,[25] also in an effort to reduce their costs by operating separate data & voice networks.
Since approximately 2010, most German operators have offered more and more VoIP on top of DSL lines and ceased offering ISDN lines. New ISDN lines have been no longer available in Germany since 2018, existing ISDN lines were phased out from 2016 onwards and existing customers were encouraged to move to DSL-based VoIP products.Deutsche Telekom intended to phase-out by 2018[26] but announced the complete transition in 2020.[27] Other providers like Vodafone estimate to have their phase-out completed by 2022.
OTE, the incumbent telecommunications operator, offers ISDN BRI (BRA) services inGreece. Following the launch of ADSL in 2003, the importance of ISDN for data transfer began to decrease and is today limited to niche business applications with point-to-point requirements.
Bharat Sanchar Nigam Limited,Reliance Communications andBharti Airtel are the largest communication service providers, and offer both ISDN BRI and PRI services across the country. Reliance Communications and Bharti Airtel uses the DLC technology for providing these services. With the introduction of broadband technology, the load on bandwidth is being absorbed by ADSL. ISDN continues to be an important backup network for point-to-point leased line customers such as banks, e-Seva Centers,[28]Life Insurance Corporation of India, andSBI ATMs.
On April 19, 1988, Japanese telecommunications companyNTT began offering nationwide ISDN services trademarked INS Net 64, and INS Net 1500, a fruition of NTT's independent research and trial from the 1970s of what it referred to the INS (Information Network System).[29]
Previously, in April 1985, Japanese digital telephone exchange hardware made by Fujitsu was used to experimentally deploy the world's first I interface ISDN. The I interface, unlike the older and incompatible Y interface, is what modern ISDN services use today.
Since 2000, NTT's ISDN offering have been known asFLET's ISDN, incorporating the "FLET's" brand that NTT uses for all of its ISP offerings.
In Japan, the number of ISDN subscribers dwindled as alternative technologies such asADSL,cable Internet access, andfiber to the home gained greater popularity. On November 2, 2010, NTT announced plans to migrate their backend fromPSTN to theIP network from around 2020 to around 2025. For this migration, ISDN services will be retired, and fiber optic services are recommended as an alternative.[30]
On April 19, 1988, Norwegian telecommunications companyTelenor began offering nationwide ISDN services trademarked INS Net 64, and INS Net 1500, a fruition of NTT's independent research and trial from the 1970s of what it referred to the INS (Information Network System).[31]
In theUnited Kingdom,British Telecom (BT) provides ISDN2e (BRI) as well as ISDN30 (PRI). Until April 2006, they also offered services namedHome Highway and Business Highway, which were BRI ISDN-based services that offered integrated analogue connectivity as well as ISDN. Later versions of the Highway products also included built-inUSB sockets for direct computer access. Home Highway was bought by many home users, usually for Internet connection, although not as fast as ADSL, because it was available before ADSL and in places where ADSL does not reach.
In early 2015, BT announced their intention to retire the UK's ISDN infrastructure by 2025.[32]
ISDN-BRI never gained popularity as a general use telephone access technology in Canada and the US, and remains a niche product. The service was seen as "a solution in search of a problem",[33] and the extensive array of options and features were difficult for customers to understand and use. ISDN has long been known by derogatorybackronyms highlighting these issues, such asIt Still Does Nothing,Innovations Subscribers Don't Need, andI Still Don't kNow,[34][35] or, from the supposed standpoint of telephone companies,I Smell Dollars Now.[36]
Although various minimum bandwidths have been used in definitions ofBroadband Internet access, ranging up from64 kbit/s up to1.0 Mbit/s, the 2006 OECD report is typical by defining broadband as having download data transfer rates equal to or faster than256 kbit/s,[37] while the United States FCC, as of 2008, defines broadband as anything above768 kbit/s.[38][39] Once the term "broadband" came to be associated with data rates incoming to the customer at256 kbit/s or more, and alternatives likeADSL grew in popularity, the consumer market for BRI did not develop. Its only remaining advantage is that, while ADSL has a functional distance limitation and can useADSL loop extenders, BRI has a greater limit and can use repeaters. As such, BRI may be acceptable for customers who are too remote for ADSL. Widespread use of BRI is further stymied by some small North AmericanCLECs such asCenturyTel having given up on it and not providing Internet access using it.[40] However, AT&T in most states (especially the former SBC/SWB territory) will still install an ISDN BRI line anywhere a normal analog line can be placed.
This article needs to beupdated. Please help update this article to reflect recent events or newly available information.(December 2024)
ISDN-BRI is currently primarily used in industries with specialized and very specific needs. High-endvideoconferencing hardware can bond up to 8 B-channels together (using a BRI circuit for every 2 channels) to provide digital, circuit-switched video connections to almost anywhere in the world. This is very expensive, and is being replaced by IP-based conferencing, but where cost concern is less of an issue than predictable quality and where aQoS-enabled IP does not exist, BRI is the preferred choice.
Most modern non-VoIP PBXs use ISDN-PRI circuits. These are connected viaT1 lines with the central office switch, replacing older analog two-way anddirect inward dialing (DID) trunks. PRI is capable of deliveringCalling Line Identification (CLID) in both directions so that the telephone number of an extension, rather than a company's main number, can be sent. It is still commonly used inrecording studios and someradio programs, when avoice-over actor or host is in one studio conductingremote work, but the director and producer are in a studio at another location.[11] The ISDN protocol delivers channelized, not-over-the-Internet service, powerful call setup and routing features, faster setup and tear down, superior audio fidelity as compared toplain old telephone service (POTS), lower delay and, at higher densities, lower cost.
In 2013,Verizon announced it would no longer take orders for ISDN service in theNortheastern United States, signalling the beginning of the end for the technology in that region.[11]
^Dr. rer. nat. Peter Bocker (1988).ISDN The Integrated Services Digital Network: Concepts, Methods, Systems.Springer Berlin Heidelberg.ISBN978-3-662-08036-8.
^Robin, G; Treves, S (July 1979). "Pragmatic Introduction of Digital Switching and Transmission in Existing Networks".IEEE Transactions on Communications.27 (7): 1071.Bibcode:1979ITCom..27.1071R.doi:10.1109/TCOM.1979.1094494.
^"Disclaimer",Internet Access service offerings, CenturyTel, archived fromthe original on 2008-12-25, retrieved2009-06-07,You may not obtain Internet services over ISDN lines (BRI or PRI), dedicated circuits or special service circuits.
