Asymmetric Digital Subscriber Line (ADSL) is one form of the Digital Subscriber Line Digital Subscriber Line is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term Digital Subscriber Line is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly technology, a data communications technology that enables faster data transmission over copper Copper is a chemical element with the symbol Cu (Latin: cuprum) and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is rather soft and malleable, and a freshly exposed surface has a pinkish or peachy color. It is used as a thermal conductor, an electrical conductor, a building material, and a telephone lines A telephone line or telephone circuit is a single-user circuit on a telephone communications system. Typically this refers to the physical wire or other signaling medium connecting the user's telephone apparatus to the telecommunications network, and usually also implies a single telephone number for billing purposes reserved for that user than a conventional voiceband In electronics, voiceband means the typical human hearing frequency range that is from 20 Hz to 20 kHz. In telephony, it means the frequency range normally transmitted by a telephone line, generally about 200–3600 Hz. Frequency-division multiplexing in telephony normally uses 4 kHz carrier spacing. The roll-off rate, or rate at which the modem A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog can provide. 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.^[1] A splitter, or DSL filter A DSL filter is an analog low-pass filter installed between analog devices and a POTS telephone line, in order to prevent interference between such devices and a DSL service operating on the same line. Without DSL filters, signals or echoes from analog devices at the top of their frequency range can result in reduced performance and connection, allows a single telephone connection to be used for both ADSL service and voice calls at the same time. ADSL can generally only be distributed over short distances from the central office In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make telephone calls "work" in the sense of making connections and relaying the, typically less than 4 kilometres (2 mi),^[2] but has been known to exceed 8 kilometres (5 mi) if the originally laid wire gauge Wire gauge is a measurement of how large a wire is, either in diameter or cross sectional area. This determines the amount of electric current a wire can safely carry, as well as its electrical resistance and weight per unit of length. Wire gauge is applicable to both electrical and non-electrical wires, being important to electrical wiring and to allows for farther distribution.

At the telephone exchange the line generally terminates at a Digital Subscriber Line Access Multiplexer A Digital Subscriber Line Access Multiplexer allows telephone lines to make faster connections to the Internet. It is a network device, located in the telephone exchanges of the service providers, that connects multiple customer Digital Subscriber Lines (DSLs) to a high-speed Internet backbone line using multiplexing techniques. By placing remote (DSLAM) where another frequency splitter separates the voice band signal In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity for the conventional phone network A telecommunications network is a collection of terminals, links and nodes which connect together to enable telecommunication between users of the terminals. Networks may use circuit switching or message switching. Each terminal in the network must have a unique address so messages or connections can be routed to the correct recipients. The. Data carried by the ADSL are typically routed over the telephone company's data network and eventually reach a conventional Internet Protocol The Internet Protocol is a protocol used for communicating data across a packet-switched internetwork using the Internet Protocol Suite, also referred to as TCP/IP network.

Operation

Currently, most ADSL communication is full-duplex A duplex communication system is a system composed of two connected parties or devices that 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-cancelling duplex (ECD), or time-division duplex (TDD). FDD 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 26.000 kHz The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of sine wave, particularly those used in radio and audio applications to 137.825 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. These frequency channels are sometimes termed bins. During initial training, the ADSL modem A Digital Subscriber Line modem is a device used to connect a computer or router to a telephone circuit that has Digital Subscriber Line service configured. Like other modems, it is a type of transceiver. It is also called a DSL Transceiver or ATU-R (for ADSL Transceiver Unit-Remote). The acronym NTBBA, which stands for network termination broad tests each of the bins to establish the signal-to-noise ratio Signal-to-noise ratio is a measure used in science and engineering to quantify how much a signal has been corrupted by noise. It is defined as the ratio of signal power to the noise power corrupting the signal. A ratio higher than 1:1 indicates more signal than noise. While SNR is commonly quoted for electrical signals, it can be applied to any at each bin's frequency. The distance from the telephone exchange In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls. A central office is the physical building used to house inside plant equipment including telephone switches, which make telephone calls "work" in the sense of making connections and relaying the and the characteristics of the cable mean that some frequencies may not propagate well, and noise on the copper wire, interference from AM radio stations AM radio began with the first, experimental broadcast on Christmas Eve of 1906 by Canadian experimenter Reginald Fessenden, and was used for small-scale voice and music broadcasts up until World War I. San Francisco, California radio station KCBS claims to be the direct descendant of KQW, founded by radio experimenter Charles "Doc" and local interference and electrical noise at the customer end mean that relatively high levels of noise are present at some frequencies both effects the signal-to-noise ratio Signal-to-noise ratio is a measure used in science and engineering to quantify how much a signal has been corrupted by noise. It is defined as the ratio of signal power to the noise power corrupting the signal. A ratio higher than 1:1 indicates more signal than noise. While SNR is commonly quoted for electrical signals, it can be applied to any in some bins (at some frequencies) may be good or completely inadequate. A bad signal-to-noise ratio measured at certain frequencies will mean that those bins will not be used, resulting in a reduced maximum link capacity, but with an otherwise functional ADSL 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 more noisy 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-bins is reported by DSL modems and is sometimes termed sync rate. This will always be rather misleading, as 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 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.

The choices the DSL modem make can also be either conservative, where the modem chooses to allocate fewer bits per bin than it possibly could, a choice which 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 to 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.

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 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 In electronics, modulation is the process of varying one or more properties of a high frequency periodic waveform, called the carrier signal, with respect to a modulating signal. This is done in a similar fashion as a musician may modulate a tone from a musical instrument by varying its volume, timing and pitch. The three key parameters of a method used.

ADSL initially existed in two versions (similar to VDSL Very-high-bitrate DSL is a DSL technology providing faster data transmission (up to 52 Mbit/s downstream and 16 Mbit/s upstream) over a single flat untwisted or twisted pair of copper wires. These fast speeds mean that VDSL is capable of supporting high bandwidth applications such as HDTV, as well as telephone services (voice over IP) and general), namely CAP Carrierless amplitude phase modulation is a non-standard variation of quadrature amplitude modulation (QAM). Instead of modulating the amplitude of two carrier waves, CAP generates QAM signal by combining two PAM signals filtered through two filters designed so that their impulse responses form a Hilbert pair and DMT Orthogonal frequency-division multiplexing , essentially identical to coded OFDM (COFDM) and discrete multi-tone modulation (DMT), is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data is divided into several. 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.

Interleaving and fastpath

Some^[which?] ADSL connections use interleaving of packets to counter the effects of noise bursts on the telephone line. Each packet to be sent (usually an Ethernet packet) is split into segments, that are sent over a longer period of time interleaved with data from previous and following packets. This allows error correction algorithms to recover the packets even if all data is lost during the burst. A negative side effect of interleaving is an increase of latency by tens of milliseconds. An ADSL profile with interleaving turned off is referred to as fastpath.

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 interference 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 A DSL filter is an analog low-pass filter installed between analog devices and a POTS telephone line, in order to prevent interference between such devices and a DSL service operating on the same line. Without DSL filters, signals or echoes from analog devices at the top of their frequency range can result in reduced performance and connection or microfilter was installed near the demarcation point In telephony, the demarcation point is the point at which the telephone company network ends and connects with the wiring at the customer premises. A demarcation point is also referred to as the demarc, DMARC, MPOE, 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 Electronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both. Electronic filters can be: at each phone 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 microfilter A DSL filter is an analog low-pass filter installed between analog devices and a POTS telephone line, in order to prevent interference between such devices and a DSL service operating on the same line. Without DSL filters, signals or echoes from analog devices at the top of their frequency range can result in reduced performance and connection. This microfilter can be plugged directly into any phone jack, and does not require any rewiring at the customer's 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 In physics, attenuation is the gradual loss in intensity of any kind of flux through a medium. For instance, sunlight is attenuated by dark glasses, X-rays are attenuated by lead, and light and sound are attenuated while passing through seawater 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, radio frequency interference, electrical 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 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.

ADSL standards

See also

• Broadband Internet access Broadband Internet access, often shortened to just broadband, is a high data rate Internet access—typically contrasted with dial-up access using a 56k modem
• Digital Subscriber Line Access Multiplexer A Digital Subscriber Line Access Multiplexer allows telephone lines to make faster connections to the Internet. It is a network device, located in the telephone exchanges of the service providers, that connects multiple customer Digital Subscriber Lines (DSLs) to a high-speed Internet backbone line using multiplexing techniques. By placing remote
• ADSL loop extender can be used to expand the reach and rate of ADSL services.
• Low-pass filter A low-pass filter is a filter that passes low-frequency signals but attenuates signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter when used in audio applications. A low-pass filter is the opposite and ADSL splitter.
• Symmetric Digital Subscriber Line SDSL is a rate-adaptive Digital Subscriber Line variant with T1/E1-like data rates (T1: 1.544 Mbit/s, E1: 2.048 Mbit/s). It runs over one pair of copper wires, with a maximum range of 10,000 feet (3,000 m). It cannot co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth
• Rate-Adaptive Digital Subscriber Line (RADSL)
• Flat rate A flat fee, also referred to as a flat rate or a linear rate, refers to a pricing structure that charges a single fixed fee for a service, regardless of usage. Rarely, it may refer to a rate that does not vary with usage or time of use. Another term used is "flate", a hybrid of "flat" and "rate"
• Attenuation distortion Attenuation distortion is the distortion of an analog signal that occurs during transmission when the transmission medium does not have a flat frequency response across the bandwidth of the medium or the frequency spectrum of the signal
• ADSL max
• List of device bandwidths This is a list of device bit rates, or the physical layer information rates net bit rates or 'useful' bit rates, at which digital devices can communicate over various kinds of buses and networks. The distinction between a bus and a communications network cable (external, between boxes and rarely relying on more than four wires) can be arbitrary
• Single-pair high-speed digital subscriber line Single-Pair High-speed Digital Subscriber Line 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. Compared to ADSL, SHDSL employs frequencies that include those used by traditional Plain old telephone service (POTS) to provide equal (SHDSL)

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