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{{short description|DSL service where downstream bandwidth exceeds upstream bandwidth}}
{{about|the data communications technology|the gene|ADSL (gene)}}
{{more citations needed|date=August 2013}}
[[File:ADSL router with Wi-Fi (802.11 b-g).jpg|thumb|A [[residential gateway|gateway]] is commonly used to make an ADSL connection]]
'''Asymmetric digital subscriber line''' ('''ADSL''') is a type of [[digital subscriber line]] (DSL) technology, a data communications technology that enables faster data transmission over [[Copper wire|copper]] [[telephone line]]s than a conventional [[voiceband modem]] can provide. ADSL differs from the less common [[symmetric digital subscriber line]] (SDSL). In ADSL, [[Bandwidth (computing)|bandwidth]] and [[bit rate]] are said to be asymmetric, meaning greater toward the customer premises ([[downstream (networking)|downstream]]) than the reverse ([[upstream (networking)|upstream]]). Providers usually market ADSL as an [[Internet access]] service primarily for [[download]]ing content from the Internet, but not for serving content accessed by others.
==Overview==
[[Image:D-Link DSL-10MF-NZ and DSL-11MF-NZ 20080103.jpg|thumb|Modern [[DSL filter|ADSL filter/splitter]] (left) and filter (right)]]
ADSL works by using spectrum above the band used by voice [[telephone call]]s.<ref>ANSI T1.413-1998 "Network and Customer Installation Interfaces – Asymmetric Digital Subscriber Line (ADSL) Metallic Interface." (American National Standards Institute  1998)</ref> With a [[DSL filter]], often called ''splitter'', the frequency bands are isolated, permitting a single telephone line to be used for both ADSL service and telephone calls at the same time. ADSL is generally only installed for short distances from the [[telephone exchange]] (the [[Last mile (telecommunications)|last mile]]), typically less than {{Convert|4|km|1}},<ref>Data and Computer Communications, William Stallings, {{ISBN|0-13-243310-9}}, {{ISBN|978-0-13-243310-5}}</ref> but has been known to exceed {{Convert|8|km|0}} if the originally laid [[wire gauge]] allows for further{{clarify|date=October 2016}} distribution.
At the telephone exchange, the line generally terminates at a [[digital subscriber line access multiplexer]] (DSLAM) where another frequency splitter separates the voice band [[Signal (electronics)|signal]] for the conventional [[Telecommunications network|phone network]]. Data carried by the ADSL are typically routed over the [[telephone company]]'s data network and eventually reach a conventional [[Internet Protocol]] network.
There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more [[crosstalk]] from other circuits at the DSLAM end (where the wires from many local loops are close to each other) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL.
The marketing reasons for an asymmetric connection are that, firstly, most users of internet traffic will require less data to be uploaded than downloaded. For example, in normal web browsing, a user will visit a number of web sites and will need to download the data that comprises the web pages from the site, images, text, sound files etc. but they will only upload a small amount of data, as the only uploaded data is that used for the purpose of verifying the receipt of the downloaded data (in very common [[transmission control protocol|TCP]] connections) or any data inputted by the user into forms etc. This provides a justification for internet service providers to offer a more expensive service aimed at commercial users who host websites, and who therefore need a service which allows for as much data to be uploaded as downloaded. File sharing applications are an obvious exception to this situation. Secondly internet service providers, seeking to avoid overloading of their backbone connections, have traditionally tried to limit uses such as file sharing which generate a lot of uploads.
==Operation==
[[File:Lantiq XWAY VRX288 V1.1.png|thumb|[[Digital subscriber line|DSL]] [[System on a chip|SoC]]]]
Currently, most ADSL communication is [[Duplex (telecommunications)|full-duplex]]. Full-duplex ADSL communication is usually achieved on a wire pair by either frequency-division duplex (FDD), [[Duplex (telecommunications)#Echo cancellation|echo-cancelling duplex (ECD)]], or [[time-division duplex]] (TDD). FDD uses two separate frequency bands, referred to as the upstream and downstream bands. The [[Upstream (networking)|upstream]] band is used for communication from the end user to the telephone central office. The [[Downstream (networking)|downstream]] band is used for communicating from the central office to the end user.
[[File: ADSL frequency plan.svg |thumb|[[Frequency plan]] for ADSL Annex A, with [[frequency-division multiplexing]].<ref>{{cite web |url=http://tele.sj.ifsc.edu.br/~msobral/RCO2/docs/adsl.pdf |title=DSL Annex A |archive-url=https://web.archive.org/web/20180923073736/http://tele.sj.ifsc.edu.br/~msobral/RCO2/docs/adsl.pdf |archive-date=23 September 2018 |url-status=dead}}</ref> Red area is the frequency range used by normal voice telephony ([[PSTN]]), the green (upstream) and blue (downstream) areas are used for ADSL.]]
With commonly deployed ADSL over [[Plain old telephone service|POTS]] (Annex A), the band from 26.075&nbsp;[[kHz]] to 137.825&nbsp;kHz is used for upstream communication, while 138–1104&nbsp;kHz is used for downstream communication. Under the usual [[discrete multitone modulation]] (DMT) scheme, each of these is further divided into smaller frequency channels of 4.3125&nbsp;kHz. These frequency channels are sometimes termed ''bins''. During initial training to optimize transmission quality and speed, the [[DSL modem|ADSL modem]] tests each of the bins to determine the [[signal-to-noise ratio]] at each bin's frequency. Distance from the [[telephone exchange]], cable characteristics, interference from [[AM broadcasting|AM radio stations]], and local interference and electrical noise at the modem's location can adversely affect the [[signal-to-noise ratio]] at particular frequencies. Bins for frequencies exhibiting a reduced signal-to-noise ratio will be used at a lower throughput rate or not at all; this reduces the maximum link capacity but allows the modem to maintain an adequate connection. The DSL modem will make a plan on how to exploit each of the bins, sometimes termed "bits per bin" allocation. Those bins that have a good signal-to-noise ratio (SNR) will be chosen to transmit signals chosen from a greater number of possible encoded values (this range of possibilities equating to more bits of data sent) in each main clock cycle. The number of possibilities must not be so large that the receiver might incorrectly decode which one was intended in the presence of noise. Noisy bins may only be required to carry as few as two bits, a choice from only one of four possible patterns, or only one bit per bin in the case of ADSL2+, and very noisy bins are not used at all. If the pattern of noise versus frequencies heard in the bins changes, the DSL modem can alter the bits-per-bin allocations, in a process called "bitswap", where bins that have become noisier are only required to carry fewer bits and other channels will be chosen to be given a higher burden.
The data transfer capacity the DSL modem therefore reports is determined by the total of the bits-per-bin allocations of all the bins combined. Higher signal-to-noise ratios and more bins being in use gives a higher total link capacity, while lower signal-to-noise ratios or fewer bins being used gives a low link capacity. The total maximum capacity derived from summing the bits-per-bin is reported by DSL modems and is sometimes termed ''sync rate''. This will always be rather misleading: the true maximum link capacity for user data transfer rate will be significantly lower because extra data are transmitted that are termed ''[[protocol overhead]]'', reduced figures for [[Point-to-Point Protocol over ATM|PPPoA]] connections of around 84–87 percent, at most, being common. In addition, some ISPs will have traffic policies that limit maximum transfer rates further in the networks beyond the exchange, and traffic congestion on the Internet, heavy loading on servers and slowness or inefficiency in customers' computers may all contribute to reductions below the maximum attainable. When a wireless access point is used, low or unstable wireless signal quality can also cause reduction or fluctuation of actual speed.
In fixed-rate mode, the sync rate is predefined by the operator and the DSL modem chooses a bits-per-bin allocation that yields an approximately equal error rate in each bin.<ref name="troiani">{{cite web |url=http://web.tiscali.it/fabiotroiani/index_file/How_ADSL_work2.html |title=Thesis in Electronics Engineering (DU) on ADSL system with DMT modulation in respect of the Standard ANSI T1.413 |year=1999 |work=DSL Knowledge Center |last=Troiani |first=Fabio |access-date=2014-03-06}}</ref> In variable-rate mode, the bits-per-bin are chosen to maximize the sync rate, subject to a tolerable error risk.<ref name="troiani"/> These choices can either be conservative, where the modem chooses to allocate fewer bits per bin than it possibly could, a choice that makes for a slower connection, or less conservative in which more bits per bin are chosen in which case there is a greater risk case of error should future signal-to-noise ratios deteriorate to the point where the bits-per-bin allocations chosen are too high to cope with the greater noise present. This conservatism, involving a choice of using fewer bits per bin as a safeguard against future noise increases, is reported as the signal-to-noise ratio ''margin'' or ''SNR margin''.
The telephone exchange can indicate a suggested SNR margin to the customer's DSL modem when it initially connects, and the modem may make its bits-per-bin allocation plan accordingly. A high SNR margin will mean a reduced maximum throughput, but greater reliability and stability of the connection. A low SNR margin will mean high speeds, provided the noise level does not increase too much; otherwise, the connection will have to be dropped and renegotiated (resynced). ADSL2+ can better accommodate such circumstances, offering a feature termed ''seamless rate adaptation'' (SRA), which can accommodate changes in total link capacity with less disruption to communications.
[[File:ADSL spectrum Fritz Box Fon WLAN.png|thumb|right|[[Frequency spectrum]] of modem on ADSL line]]
Vendors may support the usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk problems that affect other lines in the same bundle.
There is a direct relationship between the number of channels available and the throughput capacity of the ADSL connection. The exact data capacity per channel depends on the [[modulation]] method used.
ADSL initially existed in two versions (similar to [[VDSL]]), namely [[carrierless amplitude phase modulation|CAP]] and DMT. CAP was the ''de facto'' standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installations at the time. However, DMT was chosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called ''G.dmt'' and ''G.lite'' respectively). Therefore, all modern installations of ADSL are based on the DMT modulation scheme.
===Interleaving and fastpath===
ISPs (but users rarely, apart from Australia where it's the default<ref name="telstra">{{cite web |url=https://crowdsupport.telstra.com.au/t5/Home-Broadband-KB/How-to-optimise-your-gaming-performance/ta-p/54568 |title=How to optimise your gaming performance}}</ref>) have the option to use [[Bit-interleaving|interleaving]] of packets to counter the effects of [[burst noise]] on the telephone line. An interleaved line has a depth, usually 8 to 64, which describes how many [[Reed–Solomon codeword]]s are accumulated before they are sent. As they can all be sent together, their [[forward error correction]] codes can be made more resilient. Interleaving adds [[latency (engineering)|latency]] as all the packets have to first be gathered (or replaced by empty packets) and they, of course, all take time to transmit. 8 frame interleaving adds 5&nbsp;ms [[round-trip time]], while 64 deep interleaving adds 25&nbsp;ms. Other possible depths are 16 and 32.
"Fastpath" connections have an interleaving depth of 1, that is one packet is sent at a time. This has a low latency, usually around 10&nbsp;ms (interleaving adds to it, this is not greater than interleaved) but it is extremely prone to errors, as any burst of noise can take out the entire packet and so require it all to be retransmitted. Such a burst on a large interleaved packet only blanks part of the packet, it can be recovered from error correction information in the rest of the packet. A "fastpath" connection will result in extremely high latency on a poor line, as each packet will take many retries.
==Installation problems==
ADSL deployment on an existing [[plain old telephone service]] (POTS) telephone line presents some problems because the DSL is within a frequency band that might interact unfavorably with existing equipment connected to the line. It is therefore necessary to install appropriate frequency filters at the customer's premises to avoid interference between the DSL, voice services, and any other connections to the line (for example intruder alarms). This is desirable for the voice service and essential for a reliable ADSL connection.
In the early days of DSL, installation required a technician to visit the premises. A [[DSL filter|splitter]] or ''microfilter'' was installed near the [[demarcation point]], from which a dedicated data line was installed. This way, the DSL signal is separated as close as possible to the central office and is not attenuated inside the customer's premises. However, this procedure was costly, and also caused problems with customers complaining about having to wait for the technician to perform the installation. So, many DSL providers started offering a "self-install" option, in which the provider provided equipment and instructions to the customer. Instead of separating the DSL signal at the demarcation point, the DSL signal is [[Electronic filter|filtered]] at each telephone outlet by use of a low-pass filter for voice and a high-pass filter for data, usually enclosed in what is known as a [[DSL filter|microfilter]]. This microfilter can be plugged by an end user into any telephone jack: it does not require any rewiring at the customer's premises.
Commonly, microfilters are only low-pass filters, so beyond them only low frequencies (voice signals) can pass. In the data section, a microfilter is not used because digital devices that are intended to extract data from the DSL signal will, themselves, filter out low frequencies. Voice telephone devices will pick up the entire spectrum so high frequencies, including the ADSL signal, will be "heard" as noise in telephone terminals, and will affect and often degrade the service in fax, dataphones and modems. From the point of view of DSL devices, any acceptance of their signal by POTS devices mean that there is a degradation of the DSL signal to the devices, and this is the central reason why these filters are required.
A side effect of the move to the self-install model is that the DSL signal can be degraded, especially if more than 5 voiceband (that is, POTS telephone-like) devices are connected to the line. Once a line has had DSL enabled, the DSL signal is present on all telephone wiring in the building, causing [[attenuation]] and echo. A way to circumvent this is to go back to the original model, and install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer, and may not work on some household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use.
DSL signals may be degraded by older telephone lines, surge protectors, poorly designed microfilters, [[repetitive electrical impulse noise]], and by long telephone extension cords. Telephone extension cords are typically made with small-gauge, multi-strand copper conductors which do not maintain a noise-reducing pair twist. Such cable is more susceptible to electromagnetic interference and has more attenuation than solid twisted-pair copper wires typically wired to telephone jacks. These effects are especially significant where the customer's phone line is more than 4&nbsp;km from the DSLAM in the telephone exchange, which causes the signal levels to be lower relative to any local noise and attenuation. This will have the effect of reducing speeds or causing connection failures. [[FTTx]] usually has no surge problems, for example, caused by [[lightning]].
==Transport protocols==
ADSL defines three "Transmission protocol-specific transmission convergence (TPS-TC)" layers:<ref>{{cite web|title=Recommendation ITU-T G.992.3 - Asymmetric digital subscriber line transceivers 2 (ADSL2)|url=http://www.itu.int/rec/T-REC-G.992.3-200904-I|work=SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS Digital sections and digital line system – Access networks|publisher=Telecommunication standardization sector of ITU|access-date=11 April 2012|date=April 2009}}</ref>
* [[STM-1|Synchronous Transport Module (STM)]], which allows the transmission of frames of the [[Synchronous Digital Hierarchy]] (SDH)
* [[Asynchronous Transfer Mode]] (ATM)
* [[Packet Transfer Mode]] (starting with ADSL2, see below)
In home installation, the prevalent transport protocol is ATM. On top of ATM, there are multiple possibilities of additional layers of protocols (two of them are abbreviated in a simplified manner as "[[PPPoA]]" or "[[PPPoE]]"), with [[Transmission Control Protocol|TCP]]/[[Internet Protocol|IP]] providing the connection to the [[Internet]].
==ADSL standards==
[[File:ADSL annex overview.svg|thumb|600px|right|Frequency plan for common ADSL standards and annexes. {{legend-table|lang=en|title=Legend
|#ff6600|POTS/ISDN
|#ffff00|Guard band
|#00ee00|Upstream
|#d40000|Downstream ADSL, ADSL2, ADSL2+
|#900000|Downstream ADSL2+ only
}}]]
{| class="wikitable sortable"
|-
! Version || Standard name || Common name || Downstream rate || Upstream rate || Approved in
|-
| rowspan=5 | ADSL
| [[ANSI T1.413 Issue 2|ANSI T1.413-1998 Issue 2]] || ADSL || <span style="display:none">08.0</span>8.0 Mbit/s || 1.0 Mbit/s || 1998
|-
| [[ITU G.992.2]] || ADSL Lite ([[G.lite]]) || <span style="display:none">01.5</span>1.5 Mbit/s || 0.5 Mbit/s || 1999-07
|-
| [[ITU G.992.1]] || ADSL ([[G.dmt]]) || <span style="display:none">08.0</span>8.0 Mbit/s || 1.3 Mbit/s ||1999-07
|-
| [[ITU G.992.1 Annex A]] || ADSL over POTS || 12.0 Mbit/s || 1.3 Mbit/s || 2001
|-
| [[ITU G.992.1 Annex B]] || ADSL over ISDN || 12.0 Mbit/s || 1.8 Mbit/s || 2005
|-
| rowspan=4 | ADSL2
| [[ITU G.992.3 Annex L]] || RE-ADSL2 || <span style="display:none">05.0</span>5.0 Mbit/s || 0.8 Mbit/s || 2002-07
|-
| [[ITU G.992.3]] || ADSL2 || 12.0 Mbit/s || 1.3 Mbit/s || 2002-07
|-
| [[ITU G.992.3 Annex J]] || ADSL2 || 12.0 Mbit/s || 3.5 Mbit/s || 2002-07
|-
| [[ITU G.992.4]] || Splitterless ADSL2 || <span style="display:none">01.5</span>1.5 Mbit/s || 0.5 Mbit/s ||2002-07
|-
| rowspan=2 | ADSL2+
| [[ITU G.992.5]] || ADSL2+ || 24.0 Mbit/s || 1.4 Mbit/s || 2003-05
|-
| [[ITU G.992.5 Annex M]] || ADSL2+M || 24.0 Mbit/s || 3.3 Mbit/s || 2008
|}
==See also==
* [[ADSL loop extender]] can be used to expand the reach and rate of ADSL services.
* [[Attenuation distortion]]
* [[Digital subscriber line access multiplexer]]
* [[Flat rate]]
* [[Internet access]]
* [[List of interface bit rates]]
* [[Low-pass filter]] and [[ADSL splitter]].
* [[Rate-Adaptive Digital Subscriber Line]] (RADSL)
* [[Single-pair high-speed digital subscriber line]] (SHDSL)
* [[Symmetric digital subscriber line]] (SDSL)
* [[VDSL]] (Very high-speed digital subscriber line)
==References==
{{Reflist}}
==External links==
* {{Commonscatinline|ADSL}}
{{DSL technologies}}
{{Internet access}}
[[Category:Asymmetry|Digital]]
[[Category:Digital subscriber line]]
[[Category:ITU-T recommendations]]
[[Category:Internet terminology]]
[[Category:Telecommunications-related introductions in 1998]]
[[Category:Telecommunication protocols]]
[[sv:Digital Subscriber Line#ADSL]]