Editing Digital subscriber line (section)

==History==
Initially, it was believed that ordinary phone lines could only be used at modest speeds, usually less than 9600 bits per second. In the 1950s, ordinary twisted-pair telephone cable often carried 4&nbsp;[[MHz]] television signals between studios, suggesting that such lines would allow transmitting many megabits per second. One such circuit in the United Kingdom ran some {{convert|10|mi|km}} between the [[BBC]] studios in [[Newcastle-upon-Tyne]] and the [[Pontop Pike transmitting station]]. However, these cables had other impairments besides [[Gaussian noise]], preventing such rates from becoming practical in the field. The 1980s saw the development of techniques for [[broadband]] communications that allowed the limit to be greatly extended. A patent was filed in 1979 for the use of existing telephone wires for both telephones and data terminals that were connected to a remote computer via a digital data carrier system.<ref>{{cite web |title= Audio and full duplex digital data carrier system |author1=John E. Trombly |author2=John D. Foulkes |author3=David K. Worthington |work= US Patent 4,330,687 |url=https://patents.google.com/patent/US4330687 |date= May 18, 1982 |publication-date=March 14, 1979}}</ref>

The motivation for digital subscriber line technology was the [[Integrated Services Digital Network]] (ISDN) specification proposed in 1984 by the CCITT (now [[ITU-T]]) as part of Recommendation [[I.120]], later reused as [[ISDN digital subscriber line]] (IDSL). Employees at Bellcore (now [[Telcordia Technologies]]) developed [[asymmetric digital subscriber line]] (ADSL) by placing wide-band digital signals at frequencies above the existing [[baseband]] analog voice signal carried on conventional [[twisted pair]] cabling between [[telephone exchange]]s and customers.<ref>{{cite web |title= EE535 Homework 3 |first=Ronald|last=Shamus |publisher= Worcester Polytechnic Institute |url= http://www.ece.wpi.edu/courses/ee535/hwk96/hwk3cd96/rshamus/rshamus.html |archive-url= https://web.archive.org/web/20000412185056/http://www.ece.wpi.edu/courses/ee535/hwk96/hwk3cd96/rshamus/rshamus.html |url-status= dead |archive-date= April 12, 2000 |access-date= September 15, 2011 }}</ref> A patent was filed by AT&T Bell Labs on the basic DSL concept in 1988.<ref>{{cite patent |title=Method and apparatus for wideband transmission of digital signals between, for example, a telephone central office and customer premises |inventor=Richard D. Gitlin |inventor2=Sailesh K. Rao |inventor3=Jean-Jacques Werner |inventor4=Nicholas Zervos |country=US |number=4924492 |url=https://patents.google.com/patent/US4924492A/ |pubdate= May 8, 1990}}</ref>

[[Joseph W. Lechleider]]'s contribution to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of symmetric DSL.<ref>{{cite journal | author = Joseph W. Lechleider |date=August 1991 | title = High Bit Rate Digital Subscriber Lines: A Review of HDSL Progress | journal = IEEE Journal on Selected Areas in Communications | volume = 9 | issue = 6 | pages = 769–784 | doi = 10.1109/49.93088}}</ref> This allowed Internet service providers to offer efficient service to consumers, who benefited greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and [[Bit-interleaving|interleaved channel]]. Fast channel is preferred for [[streaming multimedia]], where an occasional ''dropped [[bit]]'' is acceptable, but lags are less so. Interleaved channel works better for file transfers, where the delivered data must be error-free but latency (time delay) incurred by the retransmission of error-containing packets is acceptable.

Consumer-oriented ADSL was designed to operate on existing lines already conditioned for [[Basic Rate Interface]] ISDN services. Engineers developed high-speed DSL facilities such as [[high bit rate digital subscriber line]] (HDSL) and [[symmetric digital subscriber line]] (SDSL) to provision traditional [[Digital Signal 1]] (DS1) services over standard copper pair facilities.

Older ADSL standards delivered {{nowrap|8 Mbit/s}} to the customer over about {{convert|2|km|mi|abbr=on}} of [[unshielded twisted pair|unshielded twisted-pair]] copper wire. Newer variants improved these rates. Distances greater than {{convert|2|km|mi|abbr=on}} significantly reduce the [[bandwidth (computing)|bandwidth]] usable on the wires, thus reducing the data rate. But [[ADSL loop extender]]s increase these distances by repeating the signal, allowing the [[local exchange carrier]] (LEC) to deliver DSL speeds to any distance.<ref>{{Cite web|url=http://www.strowger.com/|title=Home|website=www.strowger.com}}</ref>

[[File:Lantiq XWAY VRX288 V1.1.png|thumb|DSL [[System on a chip|SoC]]]]
Until the late 1990s, the cost of [[digital signal processor]]s for DSL was prohibitive. All types of DSL employ highly complex [[digital signal processing]] algorithms to overcome the inherent limitations of the existing [[twisted pair]] wires. Due to the advancements of [[very-large-scale integration]] (VLSI) technology, the cost of the equipment associated with a DSL deployment lowered significantly. The two main pieces of equipment are a [[digital subscriber line access multiplexer]] (DSLAM) at one end and a [[DSL modem]] at the other end.

It is possible to set up a DSL connection over an existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth [[fiber-optic]] cable over the same route and distance. This is true both for ADSL and SDSL variations. The commercial success of DSL and similar technologies largely reflects the advances made in electronics over the decades that have increased performance and reduced costs even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive.

These advantages made ADSL a better proposition for customers requiring [[Internet access]] than metered dial up, while also allowing voice calls to be received at the same time as a data connection. Telephone companies were also under pressure to move to ADSL owing to competition from cable companies, which use [[DOCSIS cable modem]] technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to the popularity of ADSL technology. Some of the first field trials for DSL were carried out in 1996.<ref>{{cite web | url=https://books.google.com/books?id=IxgEAAAAMBAJ&dq=isdn+adsl&pg=PA120 | title=Network World | date=16 September 1996 }}</ref>

Early DSL service required a dedicated [[dry loop]], but when the U.S. [[Federal Communications Commission]] (FCC) required [[incumbent local exchange carrier]]s (ILECs) to lease their lines to competing DSL service providers, shared-line DSL became available. Also known as DSL over [[unbundled network element]], this unbundling of services allows a single subscriber to receive two separate services from two separate providers on one cable pair. The DSL service provider's equipment is co-located in the same [[telephone exchange]] as that of the ILEC supplying the customer's pre-existing voice service. The subscriber's circuit is rewired to interface with hardware supplied by the ILEC which combines a DSL frequency and POTS signals on a single copper pair.

Since 1999, certain ISPs have been offering microfilters. These devices are installed indoors and serve the same purpose as DSL splitters, which are deployed outdoors: they divide the frequencies needed for ADSL and POTS phone calls.<ref>{{cite book | url=https://books.google.com/books?id=On_Hh23IXDUC&dq=adsl+microfilter&pg=PA309 | isbn=978-1-60267-000-6 | title=Network Dictionary | date=2007 | publisher=Javvin Technologies }}</ref><ref>{{cite web | url=https://books.google.com/books?id=PRcEAAAAMBAJ&dq=adsl+splitter+shelf&pg=PA12 | title=Network World | date=November 1999 }}</ref> These filters originated out of a desire to make self-installation of DSL service possible and eliminate early outdoor DSL splitters which were installed at or near the demarcation point between the customer and the ISP.<ref>{{cite book | url=https://books.google.com/books?id=Jjkd74jY47oC&dq=dsl+filter+rack&pg=PA7 | title=Implementation and Applications of DSL Technology | isbn=9781420013078 | last1=Golden | first1=Philip | last2=Dedieu | first2=Herve | last3=Jacobsen | first3=Krista S. | date=26 October 2007 | publisher=CRC Press }}</ref> 

By 2012, some carriers in the United States reported that DSL remote terminals with fiber [[backhaul (telecommunications)|backhaul]] were replacing older ADSL systems.<ref>{{cite web |url=http://gigaom.com/2012/04/24/the-dsl-death-march-continues/ |title=DSL Death March Continues |author=Om Malik |date=Apr 24, 2012 |website=Gigaom.com |access-date=2019-10-21 |archive-date=2013-06-02 |archive-url=https://web.archive.org/web/20130602151508/http://gigaom.com/2012/04/24/the-dsl-death-march-continues/ |url-status=dead }}</ref>