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. ITU G9925 is an ITU ( International Telecommunication Union) standard also referred to as ADSL2+ or ADSL2Plus. ITU G9925 is an ITU ( International Telecommunication Union) standard also referred to as ADSL2+ or ADSL2Plus. This is an ITU ( International Telecommunication Union) standard also referred to as RE ADSL2+. ITU G9925 Annex M is an ITU ( International Telecommunication Union) standard also referred to as 'ADSL2+M' High bit rate Digital Subscriber Line ( HDSL) was the first DSL technology to use a higher Frequency spectrum of copper Twisted pair cables ITU G9911 is an ITU standard for DSL that defines HDSL. See also ITU-T HDSL 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 ISDN Digital Subscriber Line ( IDSL) uses ISDN -based technology to provide a data communication channel across existing copper telephone lines at a rate of 144 kbit/s MSDSL, or Multi-rate Symmetric DSL. is a Digital Subscriber Line technology with a maximum distance of 8800  m (29000  ft) Power line communication ( PLC) also known as power line carrier, mains communication, power line telecom ( PLT) or power line For RE-ADSL2 see ITU G9925 Annex L Rate-adaptive DSL ( RADSL) is a variation of ADSL technology Symmetric Digital Subscriber Line ( SDSL) is a Digital Subscriber Line (DSL variant with T1 / E1 -like data rates (72 to 2320 kbit/s Single-Pair high-speed digital subscriber line (SHDSL is a Telecommunications Technology for Digital Subscriber Line (DSL subscriber lines Single-Pair high-speed digital subscriber line (SHDSL is a Telecommunications Technology for Digital Subscriber Line (DSL subscriber lines Uni-DSL ( UDSL) is a DSL -technology developed by Texas Instruments which would provide at least 200 Mbit/s in aggregate on the downstream and upstream ITU G9931 is an ITU standard for DSL that defines VDSL. See also ITU-T VDSL 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 ITU G9932 is an ITU standard for DSL that defines VDSL2. See also ITU-T VDSL Copper (ˈkɒpɚ is a Chemical element with the symbol Cu (cuprum and Atomic number 29 A telephone line or telephone circuit (or just line or circuit within the Industry) is a single-user circuit on a Telephone In Electronics, voiceband means the typical human hearing frequency range that is from 20 Hz to 20 kHz Modem (from mo dulator- dem odulator is a device that modulates an analog carrier signal to encode Digital information It does this by utilizing frequencies that are not used by a voice telephone call. A telephone call is a connection over a Telephone network between the Calling party and the Called party. 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 DSL in mind, it can generally only be used over short distances, typically less than 3mi (5. 5 km) [William Stallings' book].

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. A Digital Subscriber Line Access Multiplexer ( DSLAM, often pronounced dee-slam) allows Telephone lines to make faster connections to the Internet In the fields of communications, Signal processing, and in Electrical engineering more generally a signal is any time-varying or spatial-varying quantity A telecommunications network is a network of Telecommunications links and nodes arranged so that messages may be passed from one part of the network to another over Data carried by the ADSL is typically routed over the telephone company's data network and eventually reaches a conventional internet network. The Internet Protocol ( IP) is a protocol used for communicating data across a Packet-switched Internetwork using the Internet Protocol In the UK under British Telecom the data network in question is its ATM network which in turn sends it to its IP network IP Colossus. The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located BT Group plc (formerly British Telecommunications plc) which trades as BT (ˌbiːˈtiː bee tee) (previously known as British Telecom and still In electronic digital data transmission systems the Network protocol Asynchronous Transfer Mode (ATM encodes data traffic into small fixed-sized cells The Internet Protocol ( IP) is a protocol used for communicating data across a Packet-switched Internetwork using the Internet Protocol

Explanation

A gateway is commonly used to make an ADSL connection. The modem in the picture is also a wireless access point, hence the antenna. In Computer networking, a wireless access point ( WAP or AP) is a device that allows wireless communication devices to connect to a Wireless network

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. The Internet is a global system of interconnected Computer networks To download is to receive data from a remote or central system such as a Webserver, FTP server, mail server or other similar systems

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. In Electronics, the term crosstalk ( XT) refers to any phenomenon by which a signal transmitted on one circuit or channel of a Transmission system A Digital Subscriber Line Access Multiplexer ( DSLAM, often pronounced dee-slam) allows Telephone lines to make faster connections to the Internet 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

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. A duplex Communication system is a system composed of two connected parties or devices which can communicate with one another in both directions Full duplex ADSL communication is usually achieved on a wire pair by either frequency division duplex (FDD), 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. The hertz (symbol Hz) is a measure of Frequency, informally defined as the number of events occurring per Second. 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. ADSL modem or DSL modem is a device used to connect a single Computer or Router to a DSL phone line in order to use an ADSL Signal-to-noise ratio (often abbreviated SNR or S/N) is an Electrical engineering concept also used in other fields (such as scientific Measurements The distance from the telephone exchange, noise on the copper wire, or interference from AM radio stations may introduce errors on some frequencies. In the field of Telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls 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. In Telecommunications, modulation is the process of varying a periodic Waveform, i

Modulation

ADSL initially existed in two flavours (similar to VDSL), namely CAP and DMT. Carrierless amplitude phase modulation ( CAP) is a non-standard variation of Quadrature amplitude modulation (QAM Orthogonal frequency-division multiplexing ( OFDM) — essentially identical to Coded OFDM ( COFDM) and Discrete multi-tone modulation ( 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

Frequencies spectrum of a Fritz modem on an ADSL Belgian line. ITU G9925 Annex M is an ITU ( International Telecommunication Union) standard also referred to as 'ADSL2+M'

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 maxima. 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. A Digital Subscriber Line Access Multiplexer ( DSLAM, often pronounced dee-slam) allows Telephone lines to make faster connections to the Internet ADSL modem or DSL modem is a device used to connect a single Computer or Router to a DSL phone line in order to use an ADSL For example, Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+. Ericsson ( Telefonaktiebolaget L M Ericsson) () one of the largest Swedish companies is a leading provider of telecommunication and data communication systems

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 splitter was installed near the demarcation point, from which a dedicated data line was installed. In Telephony, the demarcation point is the point at which the Telephone company network ends and connects with the wiring at the customer premises 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. In Physics, attenuation (in some context also called extinction) is the gradual loss in intensity of any kind of Flux through a medium 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

See also

• British telephone sockets
• Broadband Internet access
• 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. British telephone sockets were introduced in their current plug and socket form on November 19th 1981 by British Telecom to allow subscribers to connect their own telephones Broadband Internet access, often shortened to just Broadband, is high-speed Internet access—typically contrasted with dial-up access over a Modem A Digital Subscriber Line Access Multiplexer ( DSLAM, often pronounced dee-slam) allows Telephone lines to make faster connections to the Internet An ADSL loop extender or ADSL repeater is a device that a Telephone company can place midway between the subscriber and the central office to extend the distance and
• Filter and splitter. A low-pass filter is a filter that passes low- Frequency signals but Attenuates (reduces the Amplitude of signals with frequencies
• Symmetric Digital Subscriber Line
• Rate-Adaptive Digital Subscriber Line (RADSL)
• Flat rate
• Attenuation distortion
• ADSL max
• List of device bandwidths

External links

• Free calculator used by telephone companies to compute bandwidth vs. distance
• The UNH-IOL DSL Knowledge Base (advanced tutorials)
• ADSL, ADSL2 and ADSL2+ Speeds and Reach Compared
• ADSL Research Report
• ADSL Tutorial
• Various ADSL Technical Information
• History of DSL