{{DSL technologies}}
'''Asymmetric Digital Subscriber Line''' ('''ADSL''') is a form of [[Digital subscriber line|DSL]], a data communications technology that enables faster data transmission over [[copper]] [[telephone line]]s than a conventional [[voiceband]] [[modem]] can provide. It does this by utilizing frequencies that are not used by a voice [[telephone call]]. A splitter - or [[DSL filter|microfilter]] - allows a single telephone connection to be used for both ADSL service and voice calls at the same time. Because phone lines vary in quality and were not originally engineered with DSL in mind, it can generally only be used over short distances, typically less than 3mi (5 km).{{Fact|date=October 2007}}
At the telephone exchange the line generally terminates at a [[DSLAM]] where another frequency splitter separates the voice band [[Signal (electrical engineering)|signal]] for the conventional [[Telecommunications network|phone network]]. Data carried by the ADSL is typically routed over the telephone company's data network and eventually reaches a conventional [[Internet Protocol|internet network]].
In the [[UK]] under [[British Telecom]] the data network in question is its [[Asynchronous Transfer Mode|ATM]] network which in turn sends it to its [[Internet Protocol|IP]] network IP Colossus.
== Explanation ==
[[Image:!dsl router.jpg|thumb|200px|A Modem Router is commonly used to connect to an ADSL]]
The distinguishing characteristic of ADSL over other forms of DSL is that the volume of data flow is greater in one direction than the other, i.e. it is [[asymmetric]]. Providers usually market ADSL as a service for consumers to connect to the [[Internet]] in a relatively passive mode: able to use the higher speed direction for the "[[download]]" from the Internet but not needing to run servers that would require high speed in the other direction.
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 [[Digital subscriber line access multiplexer|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. On the marketing side, limiting upload speeds limits the attractiveness of this service to business customers, often causing them to purchase higher cost [[Digital Signal 1]] services instead. In this fashion, it segments the digital communications market between business and home users
== How ADSL works ==
=== On the wire ===
[[Image:ADSL frequency plan.svg|thumb|350px|Frequency plan for ADSL. The red area is the frequency range used by normal voice telephony (PSTN), the green (upstream) and blue (downstream) areas are used for ADSL.]]
Currently, most ADSL communication is [[full duplex]]. Full duplex ADSL communication is usually achieved on a wire pair by either frequency division multiplex (FDM), echo canceling duplex (ECD), or time division duplexing (TDD). FDM uses two separate frequency bands, referred to as the upstream and downstream bands. The upstream band is used for communication from the end user to the telephone central office. The downstream band is used for communicating from the central office to the end user. With standard ADSL (annex A), the band from 25.875 [[kHz]] to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication. Each of these is further divided into smaller frequency channels of 4.3125 kHz. During initial training, the [[DSL modem|ADSL modem]] tests which of the available channels have an acceptable [[signal-to-noise ratio]]. The distance from the [[telephone exchange]], noise on the copper wire, or interference from [[AM broadcasting|AM radio stations]] may introduce errors on some frequencies. By keeping the channels small, a high error rate on one frequency thus need not render the line unusable: the channel will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connection.
Vendors may support 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 issues 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.
A common error is to attribute the ''A'' in ''ADSL'' to the word [[asynchronous]]. ADSL technologies use a [[synchronous]] framed protocol for data transmission on the wire.
=== Modulation ===
ADSL initially existed in two flavors (similar to [[VDSL]]), namely [[Carrierless Amplitude Phase Modulation|CAP]] and [[Orthogonal frequency-division multiplexing|DMT]]. CAP was the ''de facto'' standard for ADSL deployments up until [[1996]], deployed in 90 percent of ADSL installs 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.
==ADSL standards==
{| class="wikitable" style="min-width: 75%;"
! Standard name || Common name || Downstream rate || Upstream rate
|-
| [[ANSI T1.413 Issue 2|ANSI T1.413-1998 Issue 2 ]] || ADSL || 8 Mbit/s || 1.0 Mbit/s
|-
| [[ITU G.992.1]] || ADSL ([[G.DMT]]) || 12 Mbit/s || 1.3 Mbit/s
|-
| [[ITU G.992.1 Annex A]] || ADSL over POTS || 12 Mbit/s || 1.3 MBit/s
|-
| [[ITU G.992.1 Annex B]] || ADSL over ISDN || 12 Mbit/s || 1.8 MBit/s
|-
| [[ITU G.992.2]] || ADSL Lite ([[G.Lite]]) || 4.0 Mbit/s || 0.5 Mbit/s
|-
| colspan="4" |
|-
| [[ITU G.992.3/4]] || ADSL2 || 12 Mbit/s || 1.0 Mbit/s
|-
| [[ITU G.992.3 Annex J|ITU G.992.3/4 Annex J]] || ADSL2 || 12 Mbit/s || 3.5 Mbit/s
|-
| [[ITU G.992.3/4 Annex L]]<ref name="adsl-annex-l">ADSL2 Annex L is also known as RE-ADSL2, where 'RE' stands for 'Reach Extended.' With this ADSL standard, the power of the lower frequencies used for transmitting data is boosted up to increase the reach of this signal up to 7 kilometers (23,000 ft). The upper frequency limit for RE-ADSL2 is reduced to 552 kHz to keep the total power roughly the same as annex A. Since RE-ADSL2 is intended for use on long loops there isn't much (any) usable bandwidth above 552 kHz anyway. Although this standard has been ratified by the [[ITU]], not all local loop network maintainers allow this protocol to be used on their network, simply because the extra power on the lower frequencies might cause problems for existing services due to crosstalk.</ref> || RE-ADSL2 || 5 Mbit/s || 0.8 Mbit/s
|-
| colspan="4" |
|-
| [[ITU G.992.5]] || ADSL2+ || 24 Mbit/s || 1.0 Mbit/s
|-
| [[ITU G.992.5 Annex L]]<ref name="adsl-annex-l" /> || RE-ADSL2+ || 24 Mbit/s || 1.0 Mbit/s
|-
| [[ITU G.992.5 Annex M]] || ADSL2+M || 24 Mbit/s || 3.5 Mbit/s
|}
[[Image:ADSL spectrum Fritz Box Fon WLAN.png|thumb|right|300px|Frequencies spectrum of a Fritz modem on an ADSL Belgian line.]]
Annexes J and M shift the upstream/downstream frequency split up to 276 kHz (from 138 kHz used in the commonly deployed annex A) in order to boost upstream rates. Additionally, the "all-digital-loop" variants of ADSL2 and ADSL2+ (annexes I and J) support an extra 256 kbit/s of upstream if the bandwidth normally used for [[Plain old telephone service|POTS]] voice calls is allocated for ADSL usage.
While the ADSL access utilizes the 1.1 MHz band, ADSL2+ utilizes the 2.2 MHz band.
The downstream and upstream rates displayed are theoretical maxima. Note also that because [[Digital subscriber line access multiplexer]]s and [[DSL modem|ADSL modems]] may have been implemented based on differing or incomplete standards some manufacturers may advertise different speeds. For example, [[Ericsson]] has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+.
==Installation issues==
Due to the way it uses the frequency spectrum, ADSL deployment presents some issues. It is necessary to install appropriate frequency filters at the customer's premises, to avoid interferences with the voice service, while at the same time taking care to keep a clean signal level for the ADSL connection.
In the early days of DSL, installation required a technician to visit the premises. A [[DSL filter|splitter]] was installed near the [[demarcation point]], from which a dedicated data line was installed. This way, the DSL signal is separated earlier and is not attenuated inside the customer premises. However, this procedure is costly, and also caused problems with customers complaining about having to wait for the technician to perform the installation. As a result, many DSL vendors started offering a self-install option, in which they ship equipment and instructions to the customer. Instead of separating the DSL signal at the demarcation point, the opposite is done: the DSL signal is "filtered off" at each phone outlet by use of a low pass filter, also known as [[DSL filter|microfilter]]. This method does not require any rewiring inside the customer premises.
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 devices are connected to the line. The DSL signal is now 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.
==Footnotes and references==
<div class="references-small">
<references/>
</div>
{{Refimprove|date=October 2007}}
==See also==
* [[DSL around the world]]
* [[Digital Subscriber Line]] for further details and other varieties
* [[Digital subscriber line access multiplexer]]
* [[ADSL loop extender]] can be used to expand the reach and rate of ADSL services.
* [[Low-pass filter|Filter]] and [[ADSL splitter|splitter]].
* [[Symmetric Digital Subscriber Line]]
* [[Rate-Adaptive Digital Subscriber Line]] (RADSL)
* [[British telephone sockets]]
* [[Broadband Internet access]]
* [[Flat rate]]
* [[Attenuation distortion]]
* [[ADSL max]]
* [[List of device bandwidths]]
==External links==
* [http://www.iol.unh.edu/training/dsl/ The UNH-IOL DSL Knowledge Base (advanced tutorials)]
* [http://www.internode.on.net/adsl2/graph/ ADSL, ADSL2 and ADSL2+ Speeds and Reach Compared]
* [http://www.esatclear.ie/~aodhoh/adsl/report.html ADSL Research Report]
* [http://www2.rad.com/networks/2005/adsl/main.htm ADSL Tutorial]
* [http://www.systemtek.co.uk/TechDocs/Telecom_General/ADSL_Tech_docs.htm Various ADSL Technical Information]
*[http://www.intel.com/personal/digital-life/broadband/ check your broadband speed (from intel)]
[[Category:ITU-T recommendations]]
[[Category:Digital Subscriber Line]]
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