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Information about Asymmetric Digital Subscriber Line

DSL technologies
Technology ITU-standard
ADSLANSI T1.413 Issue 2
ITU G.992.1 (G.DMT)
ITU G.992.2 (G.Lite)
ADSL2ITU G.992.3/4
ITU G.992.3 Annex J
ITU G.992.3 Annex L
ADSL2+ITU G.992.5
ITU G.992.5 Annex L
ITU G.992.5 Annex M
HDSLITU G.991.1
HDSL2 
IDSL 
MSDSL 
PDSL 
RADSL 
SDSL 
SHDSLITU G.991.2
UDSL 
VDSLITU G.993.1
VDSL2ITU G.993.2
Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines 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 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 ADSL in mind, it can generally only be used over short distances, typically less than 3mi (5 km).

At the telephone exchange the line generally terminates at a DSLAM where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL is typically routed over the telephone company's data network and eventually reaches a conventional internet network.

Explanation

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 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

ADSL 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 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 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 CAP and 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

Standard nameCommon name Downstream rate  Upstream rate 
ANSI T1.413-1998 Issue 2ADSL8 Mbit/s1.0 Mbit/s
ITU G.992.1ADSL (G.DMT)12 Mbit/s1.3 Mbit/s
ITU G.992.1 Annex AADSL over POTS12 Mbit/s1.3 MBit/s
ITU G.992.1 Annex BADSL over ISDN12 Mbit/s1.8 MBit/s
ITU G.992.2ADSL Lite (G.Lite)4.0 Mbit/s0.5 Mbit/s
ITU G.992.3/4ADSL212 Mbit/s1.0 Mbit/s
ITU G.992.3/4 Annex JADSL212 Mbit/s3.5 Mbit/s
ITU G.992.3/4 Annex L[1]RE-ADSL25 Mbit/s0.8 Mbit/s
ITU G.992.5ADSL2+24 Mbit/s1.0 Mbit/s
ITU G.992.5 Annex L<ref name="adsl-annex-l" />RE-ADSL2+24 Mbit/s1.0 Mbit/s
ITU G.992.5 Annex MADSL2+M24 Mbit/s3.5 Mbit/s


Enlarge picture
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 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 maximums. Note also that because Digital subscriber line access multiplexers and 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

Enlarge picture
ADSL Router by UTStarcom
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 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 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 (poorly designed) household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use.

Footnotes

1. ^ 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.

See also

External links

DSL or xDSL, is a family of technologies that provide digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, although in recent years, many have adopted digital subscriber line
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International Telecommunication Union

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Formation May 17 1865
Headquarters Geneva, Switzerland
Membership United Nations
Official languages French, English, Spanish, Russian, Arabic, Chinese
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ANSI T1.413 defines the requirements for the single Asymmetric Digital Subscriber Line (ADSL) for the interface between the telecommunications network and the customer installation in terms of their interaction and electrical characteristics.
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In telecommunications, ITU G.992.1 (better known as G.DMT) is an ITU standard for ADSL using discrete multitone modulation. G.DMT full-rate ADSL expands the usable bandwidth of existing copper telephone lines, delivering high-speed data communications at rates up to 12
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In telecommunications, ITU G.992.2 (better known as G.Lite) is an ITU standard for ADSL using discrete multitone modulation.

G.Lite offers a maximum of 1.5 Mbit/s downstream and 512 kbit/s upstream and does not require the use of phone line splitters.
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ITU G.992.3 is an ITU (International Telecommunication Union) standard, also referred to as ADSL2. It optionally extends the capability of basic ADSL in data rates to 12 Mbit/s downstream and 3.
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ITU G.992.3 is an ITU (International Telecommunication Union) standard, also referred to as ADSL2. It optionally extends the capability of basic ADSL in data rates to 12 Mbit/s downstream and 3.
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India has over 2.5 million DSL subscribers, 80% of which are on the ADSL2+ standard. Indian DSL penetration has been limited by the absence of personal computers in Indian homes. Only an estimated 3 million of the 200 million Indian homes own a personal computer.
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ITU G.992.5 is an ITU (International Telecommunication Union) standard, also referred to as ADSL2+ or ADSL2Plus.
Commercially it is notable for its maximum theoretical speed of 24 Mbit/s.
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ITU G.992.5 is an ITU (International Telecommunication Union) standard, also referred to as ADSL2+ or ADSL2Plus.
Commercially it is notable for its maximum theoretical speed of 24 Mbit/s.
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This is an ITU (International Telecommunication Union) standard, also referred to as RE ADSL2+. This extends the capability of basic ADSL by doubling the number of downstream bits.
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This is an ITU (International Telecommunication Union) standard, also referred to as 'ADSL2+M'. This extends the capability of basic ADSL2 by doubling the number of upstream bits. The data rates can be as high as 24 Mbit/s downstream and 3.
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High bit rate Digital Subscriber Line (HDSL) was the first DSL technology that uses a higher frequency spectrum of copper, twisted pair cables. HDSL was developed in the USA, as a better technology for high-speed, synchronous circuits typically used to interconnect local
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ITU G.991.1 is an ITU standard for DSL that defines HDSL.

See also

  • ITU-T
  • HDSL

External links

  • ITU-T Recommendation G.991.1: High bit rate Digital Subscriber Line (HDSL)
  • ITU-T Recommendations: Series G
  • ITU-T

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HDSL2 is the 2nd generation of HDSL with a 6db Noise Margin, put simply it is another way to provision a T-1 line, only this technology relies on fewer wires - two instead of four - and therefore costs less to set up. Source
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ISDN Digital Subscriber Line (IDSL) transmits data digitally (rather than analog) on a regular twisted pair copper telephone line, across existing ISDN lines, at a rate of 144 kbit/s, slightly higher than a bonded dual channel ISDN connection at 128kbit/s.
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MSDSL, or Multi-rate Symmetric DSL. is a Digital Subscriber Line technology with a maximum distance of 8,800 m (29,000 ft). It is capable of multiple transfer rates, as set by the Internet service provider, typically based on the service and/or price.
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Power line communication (PLC), also called power line carrier, mains communication, power line telecom (PLT), or power line networking (PLN
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''' ''' Rate-adaptive DSL (RADSL) is a variation of ADSL technology. With RADSL the modem adjusts the upstream speed of the connection depending upon the length and quality of the line between the DCE (Telephone Exchange) or DSLAM and the DTE (Modem), in an attempt to
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Symmetric Digital Subscriber Line (SDSL) is a Digital Subscriber Line (DSL) variant with E1-like data rates (72 to 2320 kbit/s). It runs over one pair of copper wires, with a maximum range of about 3 kilometers.
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Symmetric high-speed digital subscriber line (SHDSL) is a telecommunications technology for Digital Subscriber Line (DSL) subscriber lines. It describes a transmission method for signals on copper pair lines, being mostly used in access networks to connect subscribers to Telephone
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Symmetric high-speed digital subscriber line (SHDSL) is a telecommunications technology for Digital Subscriber Line (DSL) subscriber lines. It describes a transmission method for signals on copper pair lines, being mostly used in access networks to connect subscribers to Telephone
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VDSL or VHDSL (Very High Speed DSL) is a DSL technology providing faster data transmission over a single twisted pair of copper wires. These fast speeds mean that VDSL is capable of supporting new high bandwidth applications such as HDTV, as well as telephone services
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ITU G.993.1 is an ITU standard for DSL that defines VDSL.

See also

  • ITU-T
  • VDSL

External links

  • ITU-T Recommendation G.993.1: Very high speed digital subscriber line transceivers (VDSL)
  • ITU-T Recommendations: Series G
  • ITU-T

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VDSL2 (Very High Speed Digital Subscriber Line 2) is an access technology that exploits the existing infrastructure of copper wires that were originally deployed for POTS services.
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ITU G.993.2 is an ITU standard for DSL that defines VDSL2.

See also

  • ITU-T
  • VDSL

External links

  • ITU-T Recommendation G.993.2: Very high speed digital subscriber line transceivers 2 (VDSL2)
  • ITU-T Recommendations: Series G

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DSL or xDSL, is a family of technologies that provide digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, although in recent years, many have adopted digital subscriber line
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Electronegativity 1.90 (Pauling scale)
Ionization energies
(more) 1st: 745.5 kJmol−1
2nd: 1957.9 kJmol−1
3rd: 3666 kJmol−1

Atomic radius 135 pm
Atomic radius (calc.
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