Editing Digital subscriber line (section)

==Operation==
{{unreferenced section|date=June 2023}}

Telephones are connected to the [[telephone exchange]] via a [[local loop]], which is a physical pair of wires. The local loop was originally intended mostly for the transmission of speech, encompassing an audio frequency range of 300 to 3400 [[hertz]] ([[commercial bandwidth]]). However, as long-distance [[Trunking|trunk]]s were gradually converted from analog to digital operation, the idea of being able to pass data through the local loop (by using frequencies above the voiceband) took hold, ultimately leading to DSL.

The [[local loop]] connecting the telephone exchange to most subscribers has the capability of carrying frequencies well beyond the 3400&nbsp;Hz upper limit of [[Plain old telephone service|POTS]]. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused [[Bandwidth (signal processing)|bandwidth]] of the local loop by creating 4312.5&nbsp;Hz wide channels starting between 10 and 100&nbsp;kHz, depending on how the system is configured. Allocation of channels continues to higher frequencies (up to 1.1&nbsp;MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an [[analog modem]] would on a POTS connection. More usable channels equate to more available bandwidth, which is why distance and line quality are a factor (the higher frequencies used by DSL travel only short distances).

The pool of usable channels is then split into two different frequency bands for [[upstream (networking)|upstream]] and [[downstream (networking)|downstream]] traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual [[Communication channel|channel]]s are [[Channel bonding|bonded]] into a pair of virtual circuits, one in each direction. Like analog modems, DSL [[transceiver]]s constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable. Once upstream and downstream circuits are established, a [[subscriber]] can connect to a service such as an [[Internet service provider]] or other network services, like a corporate [[MPLS]] network.

The underlying technology of transport across DSL facilities uses [[modulation]] of high-frequency [[carrier wave]]s, an analog signal transmission. A DSL circuit terminates at each end in a [[modem]] which modulates patterns of [[bit]]s into certain high-frequency impulses for transmission to the opposing modem. Signals received from the far-end modem are demodulated to yield a corresponding bit pattern that the modem passes on, in digital form, to its interfaced equipment, such as a computer, router, switch, etc.

Unlike traditional dial-up modems, which modulate bits into signals in the 300–3400&nbsp;Hz audio baseband, DSL modems modulate frequencies from 4000&nbsp;Hz to as high as 4&nbsp;MHz. This frequency band separation enables DSL service and [[plain old telephone service]] (POTS) to coexist on the same cables, known as voice-grade cables.<ref>{{cite web | url=https://books.google.com/books?id=fFrjSBw0w14C&dq=dsl+voice+grade&pg=PA230 | title=PC Mag | date=10 February 1998 }}</ref> On the subscriber's end of the circuit, inline [[DSL filter]]s are installed on each telephone to pass voice frequencies but filter the high-frequency signals that would otherwise be heard as hiss. Also, nonlinear elements in the phone could otherwise generate audible [[intermodulation]] and may impair the operation of the data modem in the absence of these [[low-pass filter]]s. DSL and RADSL modulations do not use the voice-frequency band so  [[high-pass filter]]s are incorporated in the circuitry of DSL modems filter out voice frequencies.
[[File:T-DSL Modem.jpg|thumb|right|A DSL modem]]

Because DSL operates above the 3.4&nbsp;kHz voice limit, it cannot pass through a [[loading coil]], which is an inductive coil that is designed to counteract loss caused by shunt capacitance (capacitance between the two wires of the twisted pair). Loading coils are commonly set at regular intervals in POTS lines. Voice service cannot be maintained past a certain distance without such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of loading coil placement. Because of this, phone companies endeavor to remove loading coils on copper loops that can operate without them. Longer lines that require them can be replaced with fiber to the neighborhood or node ([[FTTN]]).

Most residential and small-office DSL implementations reserve low frequencies for POTS, so that (with suitable filters and/or splitters) the existing voice service continues to operate independently of the DSL service. Thus POTS-based communications, including [[fax machine]]s and [[dial-up modem]]s, can share the wires with DSL. Only one DSL modem can use the [[subscriber line]] at a time. The standard way to let multiple computers share a DSL connection uses a [[Router (computing)|router]] that establishes a connection between the DSL modem and a local [[Ethernet]], [[Power line communication|powerline]], or [[Wi-Fi]] network on the customer's premises.

The theoretical foundations of DSL, like much of [[communication]] technology, can be traced back to [[Claude Shannon]]'s seminal 1948 paper, "[[A Mathematical Theory of Communication]]". Generally, higher bit rate transmissions require a wider frequency band, though the ratio of [[bit rate]] to [[symbol rate]] and thus to bandwidth are not linear due to significant innovations in [[digital signal processing]] and [[Digital modulation#Digital modulation methods|digital modulation methods]].

===Naked DSL===
{{main|Naked DSL}}

[[Naked DSL]] is a way of providing only DSL services over a [[local loop]]. It is useful when the customer does not need the traditional [[telephony]] voice service because voice service is received either on top of the DSL services (usually [[VoIP]]) or through another network (E.g., [[mobile telephony]]). It is also commonly called an [[unbundled network element]] (UNE) in the United States; in Australia it is known as a unconditioned local loop (ULL);<ref>[http://whirlpool.net.au/wiki/ull ULL (unconditioned local loop)]. Whirlpool.net.au. Retrieved on 2013-09-18.</ref> in Belgium it is known as "raw copper" and in the UK it is known as Single Order GEA (SoGEA).<ref>{{cite web |url=https://www.btwholesale.com/assets/documents/Fibre_Broadband/BTW_Next_Generation_Fibre_Jan_2017.pdf |archive-url=https://web.archive.org/web/20171019055322/https://www.btwholesale.com/assets/documents/Fibre_Broadband/BTW_Next_Generation_Fibre_Jan_2017.pdf |archive-date=2017-10-19 |title=Next Generation Fibre}}</ref>

It started making a comeback in the United States in 2004 when [[Qwest]] started offering it, closely followed by [[Speakeasy (ISP)|Speakeasy]]. As a result of [[AT&T Corporation|AT&T]]'s merger with [[SBC Communications|SBC]],<ref>{{cite web |url=http://www.sbc.com/gen/press-room?pid=4800&cdvn=news&newsarticleid=21853 |title=Federal Communications Commission Approves SBC/AT&T Merger |website=www.sbc.com |date=October 31, 2005}}</ref> and [[Verizon]]'s merger with [[MCI Inc.|MCI]],<ref>{{cite web |url=http://www.verizonbusiness.com/about/news/merger/ |title=Verizon MCI merger |archive-url=https://web.archive.org/web/20070402010500/http://www.verizonbusiness.com/about/news/merger/ |archive-date=April 2, 2007}}</ref> those telephone companies have an obligation to offer naked DSL to consumers.