Editing ISDN (section)

{{Short description|Set of digital telephony standards}}
{{other uses}}
{{Distinguish|text=[[Software-defined networking|SDN (Software-defined networking)]]}}
{{Use American English|date=September 2024}}

{{Infobox technology standard
| title = ISDN
| long_name = Integrated Services Digital Network
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| native_name = <!-- Name in local language. If more than one, separate using {{plain list}} -->
| native_name_lang = <!-- ISO 639-1 code e.g. "fr" for French. If more than one, use {{lang}} inside native_name items instead -->
| status = 
| year_started = {{End date and age|1980}}
| first_published = {{End date and age|1988}}{{Efn|There is not a single ISDN specification, but mere a collection of various incompatible dialects.}}
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| organization = [[ITU-T]]
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| predecessor = [[Plain old telephone service]] with [[dial-up Internet access]]
| successor = [[Broadband]] such as [[Digital subscriber line|DSL]] or [[FTTx]]
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{{IPstack}}
[[File:ISDN telefono.jpg|thumb|ISDN telephone]]

'''Integrated Services Digital Network''' ('''ISDN''') is a set of communication standards for simultaneous [[Digital data|digital]] [[Transmission (telecommunications)|transmission]] of voice, video, data, and other network services over the digitalised circuits of the [[public switched telephone network]].<ref>{{Cite book|last=Dr. rer. nat. Peter Bocker|title=ISDN The Integrated Services Digital Network: Concepts, Methods, Systems.|publisher=Springer Berlin Heidelberg|year=1988|isbn=978-3-662-08036-8}}</ref> Work on the standard began in 1980 at [[Bell Labs]] and was formally standardized in 1988 in the [[ITU-T|CCITT]] "Red Book".<ref>{{cite journal|journal=IEEE Journal on Selected Areas in Communications|date=May 1986|volume= 4|issue= 3|pages = 320–25 | issn= 0733-8716|doi= 10.1109/JSAC.1986.1146333| title= CCITT Recommendations on the ISDN: A Review | last1 = Decina| first1= M |last2= Scace| first2= E}}</ref> 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.{{sfn|Cioffi|2011|p=30}} ISDN has largely been replaced with [[digital subscriber line]] (DSL) systems of much higher performance.

Prior to ISDN, the telephone system consisted of digital links like [[T-carrier|T1]]/[[E-carrier|E1]] on the long-distance lines between telephone company offices and analog signals on [[Twisted pair|copper telephone wires]] to the customers, the "[[Last mile (telecommunications)|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 [[modem]]s 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).{{sfn|Cioffi|2011|p=30}} 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 {{nowrap|64 kbit/s}} "bearer" lines and a single {{nowrap|16 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."{{sfn|Cioffi|2011|p=31}} It found a use for a time for small-office digital connection, using the voice lines for data at {{nowrap|64 kbit/s}}, sometimes "bonded" to {{nowrap|128 kbit/s}}, but the introduction of {{nowrap|56 kbit/s}} modems undercut its value in many roles. It also found use in [[videoconference]] systems, where the direct end-to-end connection was desirable. The [[H.320]] standard was designed around its {{nowrap|64 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.