Digital Subscriber Line (DSL) is a family of technologies that provides digital A digital system is a data technology that uses discrete values. By contrast, non-digital (or analog) systems use a continuous range of values to represent information. Although digital representations are discrete, the information represented can be either discrete, such as numbers, letters or icons, or continuous, such as sounds, images, and data transmission over the wires of a local telephone network An access network is that part of a communications network which connects subscribers to their immediate service provider. It is contrasted with the core network, for example the Network Switching Subsystem in GSM. The access network may be further divided between feeder plant or distribution network, and drop plant or edge 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 Asymmetric Digital Subscriber Line is one form of the Digital Subscriber Line technology, 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 - (ADSL), the most commonly installed technical varieties of DSL. DSL service is delivered simultaneously with regular telephone Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in most parts of the world. The name is a retronym, and is a reflection of the telephone service still available after the advent of more advanced forms of telephony such as ISDN, on the same telephone line 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. This is possible because DSL uses a higher frequency. These frequency bands are subsequently separated by filtering.

The data throughput of consumer DSL services typically ranges from 384 KB/s to 20 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. In ADSL, the data throughput in the upsream direction, (i.e. in the direction to the service provider) is lower, hence the designation of asymmetric service. The downstream and upstream data rates are equal in 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 (SDSL) service.

Voice and data

DSL uses a second, higher frequency band A frequency range or frequency band is a range of wave frequencies. It most often refers to either a range of frequencies in sound or a range of frequencies in electromagnetic radiation, which includes light and radio waves (greater than 25 kHz) above the low frequency regime (5 kHz and below) used by voice communications. On the customer premises, a 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 is installed on each outlet for telephone handsets to remove the high frequency band, eliminating interference with the operation of the telephone set, and enabling simultaneous use.

History

Implementation of Digital Subscriber Line technology originally was part of the Integrated Services Digital Network Integrated Services Digital Network is a set of communications standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the public switched telephone network. It was first defined in 1984 in the CCITT red book (ISDN) specification published in 1984 by the CCITT and ITU as part of Recommendation I.120, later reused as ISDN Digital Subscriber Line (IDSL). Engineers have developed higher-speed DSL facilities such as High bit rate Digital Subscriber Line (HDSL) and 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 (SDSL) to provision traditional Digital Signal 1 Digital signal 1 is a T-carrier signaling scheme devised by Bell Labs. DS1 is a widely used standard in telecommunications in North America and Japan to transmit voice and data between devices. E1 is used in place of T1 outside of North America, Japan, and South Korea. Technically, DS1 is the logical bit pattern used over a physical T1 line; (DS1) services over standard copper pair facilities. Consumer-oriented Asymmetric Digital Subscriber Line Asymmetric Digital Subscriber Line is one form of the Digital Subscriber Line technology, 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 - (ADSL), first tested at Bellcore in 1988, was designed to operate on existing lines already conditioned for BRI ISDN services, which itself is a switched digital service (non-IP), though most incumbent local exchange carriers An ILEC, short for incumbent local exchange carrier, is a local telephone company in the United States that was in existence at the time of the break up of AT&T into the Regional Bell Operating Companies , also known as the "Baby Bells." The ILEC is the former Bell System or Independent Telephone Company responsible for providing (ILECs) provision Rate-Adaptive Digital Subscriber Line (RADSL) to work on virtually any available copper pair facility—whether conditioned for BRI or not.

The development of DSL, like many other forms of communication Communication is a process of transferring information from one entity to another. Communication processes are sign-mediated interactions between at least two agents which share a repertoire of signs and semiotic rules. Communication is commonly defined as "the imparting or interchange of thoughts, opinions, or information by speech, writing,, can be traced back to Claude Shannon Claude Elwood Shannon , an American mathematician and electronic engineer, is known as "the father of information theory"'s seminal 1948 paper: A Mathematical Theory of Communication "A Mathematical Theory of Communication" is an influential 1948 article by mathematician Claude E. Shannon. Employees at Bellcore Telcordia Technologies, formerly Bell Communications Research, Inc. or Bellcore, is a telecommunications research and development company based in the United States created as part of the 1982 Modification of Final Judgment that broke up American Telephone & Telegraph (now Telcordia Technologies) developed ADSL in 1988 by placing wide-band digital signals above the existing baseband In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting close to zero. Baseband can often be analog voice signal carried between telephone company A telephone company is a service provider of telecommunications services such as telephony and data communications access. Most of the largest telcos, whatever their origins, are or were at one time nationalized or state-regulated monopolies[citation needed]. These monopolies are often referred to, primarily in Europe, as PTTs central offices 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 customers on conventional twisted pair Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources; for instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and crosstalk between neighboring pairs. It was invented by Alexander Graham Bell and he cabling facilities.^[1]

Contrary to its name, while a DSL circuit provides digital service, it is actually not a digital signal. The underlying technology of transport across DSL facilities uses high-frequency sinusoidal The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields. Its most basic form as a function of time is: carrier wave In telecommunications, a carrier wave, or carrier is a waveform that is modulated (modified) with an input signal for the purpose of conveying information. This carrier wave is usually of much higher frequency than the input signal. The purpose of the carrier is usually either to transmit the information through space as an electromagnetic wave ( modulation, which is an analog signal transmission. A DSL circuit terminates at each end in a 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 which modulates patterns of bits A bit is the basic unit of information in computing and telecommunications; it is the maximum amount of information that can be stored by a device or other physical system that can normally exist in only two distinct states. These may be the two stable positions of an electrical switch, two distinct voltage or current levels allowed by a circuit, into certain high-frequency impulses for transmission to the opposing modem. Signals received from the far-end modem are demodulated to yield a corresponding bit pattern that the modem retransmits, in digital form, to its interfaced equipment, such as a computer, router, switch, etc. Unlike traditional dial-up modems, which modulate bits into signals in the 300–3400 Hz baseband (voice service), DSL modems modulate frequencies from 4000 Hz to as high as 4 MHz. This frequency band separation enables DSL service and plain old telephone service Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in most parts of the world. The name is a retronym, and is a reflection of the telephone service still available after the advent of more advanced forms of telephony such as ISDN, (POTS) to coexist on the same copper pair facility. Generally, higher bit rate transmissions require a wider frequency band, though the ratio of bit rate to bandwidth are not linear due to significant innovations in digital signal processing Digital signal processing is concerned with the representation of signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing. DSP includes subfields like: audio and speech signal processing, sonar and radar signal processing, sensor and digital modulation methods In electronics, modulation is the process of varying one or more properties of 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.

Early DSL service required a dedicated dry loop, but when the U.S. Federal Communications Commission The Federal Communications Commission is an independent agency of the United States government, created, directed and empowered by Congressional statute (see 47 U.S.C. § 151 and 47 U.S.C. § 154), and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, (FCC) required ILECs to lease their lines to competing DSL service providers, shared-line DSL became available. Also known as DSL over Unbundled Network Element, this unbundling of services allows a single subscriber to receive two separate services from two separate providers on one cable pair. The DSL service provider's equipment is collocated in the same central office as that of the ILEC supplying the customer's pre-existing voice service. The subscriber's circuit is then rewired to interface with hardware supplied by the ILEC which combines a DSL frequency and POTS frequency on a signal copper pair facility.

On the subscriber's end of the circuit, inline low-pass DSL filters 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 (splitters) are installed on each telephone to filter the high-frequency "hiss" that would otherwise be heard. Conversely, high-pass filters already incorporated in the circuitry of DSL modems filter out voice frequencies. Although ADSL and RADSL modulation do not use the voice-frequency band, nonlinear elements in the phone could otherwise generate audible intermodulation and may impair the operation of the data modem in the absence of low-pass filters.

Older ADSL standards can deliver 8 Mbit/s In telecommunications, bit rate or data transfer rate is the average number of bits, characters, or blocks per unit time passing between equipment in a data transmission system. This is typically measured in multiples of the units bit per second or byte per second to the customer over about 2 km (1·25 miles) of unshielded twisted-pair Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources; for instance, electromagnetic radiation from Unshielded Twisted Pair (UTP) cables, and crosstalk between neighboring pairs copper wire. As of 2009^[update], the latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM 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. Distances greater than 2 km (1.25 miles) significantly reduce the bandwidth In computer networking and computer science, bandwidth, digital bandwidth, or network bandwidth is a measure of available or consumed data communication resources expressed in bit/s or multiples of it usable on the wires, thus reducing the data rate. ADSL loop extenders increase these distances substantially.

Operation

Basic technology

Telephone engineers initially^[when?] developed the local loop In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network. At the edge of the carrier access network in a traditional PSTN (Public Switched Telephone Network) scenario, the local loop terminates in a of the public switched telephone network The public switched telephone network is the network of the world's public circuit-switched telephone networks. Originally a network of fixed-line analog telephone systems, the PSTN is now almost entirely digital in its core and includes mobile as well as fixed telephones (PSTN) to carry "plain old telephone service Plain old telephone service is the voice-grade telephone service that remains the basic form of residential and small business service connection to the telephone network in most parts of the world. The name is a retronym, and is a reflection of the telephone service still available after the advent of more advanced forms of telephony such as ISDN," (POTS) voice communication and signaling: no requirement for data communication as we know it today existed. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz 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, which is regarded as the range required for human speech to be clearly audible. This is known as 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 or commercial bandwidth. The local 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 (United Kingdom) or central office (United States) generally digitizes speech signals by using an analog-to-digital converter An analog-to-digital converter is a device which converts continuous signals to discrete digital numbers. The reverse operation is performed by a digital-to-analog converter (DAC) sampling at a rate of 8000 samples per second (and which captures 8-bit A bit or binary digit is the basic unit of information in computing and telecommunications; it is the amount of information that can be stored by a digital device or other physical system that can usually exist in only two distinct states. These may be the two stable positions of an electrical switch, two distinct voltage or current levels allowed values producing a 64 kbit/s In telecommunications, bit rate or data transfer rate is the average number of bits, characters, or blocks per unit time passing between equipment in a data transmission system. This is typically measured in multiples of the units bit per second or byte per second data stream). According to the Nyquist theorem, if an input audio signal injected into such an analog-to-digital converter contains frequency components higher than half of the sampling frequency, then such high frequency components will be aliased In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled. It also refers to the distortion or artifact that results when the signal reconstructed from samples is different from the original continuous signal by the system, and so must be blocked at the input by an appropriate 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 in order to prevent such effects. Therefore in this case input frequencies above 4000Hz will not be allowed to pass into the system and so attempting to achieve end-to-end communications by injecting arbitrarily high frequencies into the POTS through the normal voice path is not an option.

For a long time engineers believed it impossible to push a conventional phone-line beyond low-speed limits (typically under 9600 bit/s). In the 1950s ordinary twisted-pair telephone-cable often carried 4 MHz television signals between studios,^[where?] suggesting that the such lines would allow transmitting many megabits per second.^{[citation needed]} However, these cables had other impairments besides Gaussian noise, preventing such rates from becoming practical in the field. The 1980s saw the development of techniques for broadband The term broadband refers to a telecommunications signal of greater bandwidth, in some sense, than another standard or usual signal. Different criteria for "broad" have been applied in different contexts and at different times communications that allowed the limit to be greatly extended.

The local loop In telephony, the local loop is the physical link or circuit that connects from the demarcation point of the customer premises to the edge of the carrier or telecommunications service provider's network. At the edge of the carrier access network in a traditional PSTN (Public Switched Telephone Network) scenario, the local loop terminates in a connecting the telephone exchange to most subscribers has the capability of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth Bandwidth is typically measured in hertz, and may sometimes refer to passband bandwidth, sometimes to baseband bandwidth, depending on context. Passband bandwidth is the difference between the upper and lower cutoff frequencies of, for example, an electronic filter, a communication channel, or a signal spectrum. In case of a lowpass filter or of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog An Analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are meaningful. Analog is usually thought of in an modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor (the higher frequencies used by DSL travel only short distances). The pool of usable channels is then split into two different frequency bands for upstream and downstream The term downstream has several possible meanings: In geography, downstream means literally away from the source of a stream or river, and in meteorology, away from the source of an air parcel or mass, along the normal direction of water or air flow. By analogy with this, see also: traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable.

One of Lechleider's^[2] contributions to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of symmetric DSL. This allowed Internet Service Providers to offer efficient service to consumers, who benefitted greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where the delivered data must be error free but latency incurred by the retransmission of errored packets is acceptable.

Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a properly installed and working load coil, while voice service cannot be maintained past a certain distance without such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this, phone companies are endeavoring to remove load coils on copper loops that can operate without them, and conditioning lines to avoid them through the use of fiber to the neighborhood or node FTTN.

The commercial success of DSL and similar technologies largely reflects the advances made in electronics, that, over the past few decades, have been getting faster and cheaper even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. Several factors contributed to the popularization of DSL technology:

• Until the late 1990s, the cost of digital signal processors for DSL was prohibitive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of Very-large-scale integration (VLSI) technology, the cost of the equipment associated with a DSL deployment (a Digital subscriber line access multiplexer (DSLAM) at one end and a DSL modem at the other end) lowered significantly.
• A DSL connection can be deployed over existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cable over the same route and distance. This is true both for ADSL and SDSL variations.
• In the case of ADSL, competition in Internet access caused subscription fees to drop significantly over the years, thus making ADSL more economical than dial up access. Telephone companies were pressured into moving to ADSL largely due to competition from cable companies, which use DOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to popularize ADSL technology.

Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that (with suitable filters and/or splitters) the existing voice service continues to operate independent of the DSL service. Thus POTS-based communications, including fax machines and analog modems, can share the wires with DSL. Only one DSL "modem" can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection uses a router that establishes a connection between the DSL modem and a local Ethernet, Powerline, or Wi-Fi network on the customer's premises.

Once upstream and downstream channels are established, a subscriber can connect to a service such as an Internet service provider.

Naked DSL

Dry-loop DSL or "naked DSL", which does not require the subscriber to have traditional land-line telephone service, started making a comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC,^[3] and Verizon's merger with MCI,^[4] those telephone companies have an obligation to offer naked DSL to consumers.

Even without the regulatory mandate, however, many ILECs offer naked DSL to consumers. The number of telephone landlines in the US dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has grown to 195 million (277 million as of 2010).^[5] This lack of demand for landline service has resulted in the expansion of naked DSL availability.

Naked DSL products are also marketed in some other countries eg. Australia.

Typical setup and connection procedures

Physical connection must come first. On the customer side, the DSL Transceiver, or ATU-R, or more commonly known as a DSL modem, is hooked up to a phone line. The telephone company(telco) connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single box. The location of the DSLAM depends on the telco, but it cannot be located too far from the user because of attenuation, the loss of data due to the large amount of electrical resistance encountered as the data moves between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.

When the DSL modem powers up it goes through a sync procedure. The actual process varies from modem to modem but generally involves the following steps:

1. The DSL transceiver performs a self-test.
2. The DSL transceiver checks the connection between the DSL transceiver and the computer. For residential variations of DSL, this is usually the Ethernet (RJ-45) port or a USB port; in rare models, a FireWire port is used. Older DSL modems sported a native ATM interface (usually, a 25 Mbit serial interface). Also, some variations of DSL (such as SDSL) use synchronous serial connections.
3. The DSL transceiver then attempts to synchronize with the DSLAM. Data can only come into the computer when the DSLAM and the modem are synchronized. The synchronization process is relatively quick (in the range of seconds) but is very complex, involving extensive tests that allow both sides of the connection to optimize the performance according to the characteristics of the line in use. External, or stand-alone modem units have an indicator labeled "CD", "DSL", or "LINK", which can be used to tell if the modem is synchronized. During synchronization the light flashes; when synchronized, the light stays lit, usually with a green color.

Modern DSL gateways have more functionality and usually go through an initialization procedure very similar to a PC boot up. The system image is loaded from the flash memory; the system boots, synchronizes the DSL connection and establishes the IP connection between the local network and the service provider, using protocols such as DHCP or PPPoE. The system image can usually be updated to correct bugs, or to add new functionality.

The accompanying figure is a schematic of a simple DSL connection. The right side the shows a DSLAM residing in the telco’s central office. The left side shows the customer premises equipment with an optional router. This router manages a local area network (LAN) off of which are connected some number of PCs. With many service providers, the customer may opt for a modem which contains a wireless router. This option (within the dashed bubble) often simplifies the connection.

Equipment

The customer end of the connection consists of a terminal adaptor or in layman's terms "DSL modem". This converts data between the digital signals used by computers and the voltage signal of a suitable frequency range which is then applied to the phone line.

In some DSL variations (for example, HDSL), the terminal adapter connects directly to the computer via a serial interface, using protocols such as ethernet or V.35. In other cases (particularly ADSL), it is common for the customer equipment to be integrated with higher level functionality, such as routing, firewalling, or other application-specific hardware and software. In this case, the equipment is referred to as a gateway.

Some kinds of DSL technology require installation of appropriate filters to separate, or "split", the DSL signal from the low frequency voice signal. The separation can take place either at the demarcation point, or with filters installed at the telephone outlets inside the customer premises. Either way has its practical and economical limitations. See ADSL for more information about this.

At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. In the case of ADSL, the voice component is also separated at this step, either by a filter integrated in the DSLAM or by a specialized filtering equipment installed before it. The DSLAM terminates all connections and recovers the original digital information.

Protocols and configurations

Many DSL technologies implement an Asynchronous Transfer Mode (ATM) layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use Point-to-Point Protocol (PPP) or Asynchronous Transfer Mode (ATM) (Point-to-Point Protocol over Ethernet (PPPoE) or Point-to-Point Protocol over ATM (PPPoA)), while authenticating with a userid and password and using Point-to-Point Protocol (PPP) mechanisms to provide network details.

DSL technologies

The line-length limitations from telephone exchange to subscriber impose more restrictions on higher data-transmission rates. Technologies such as VDSL provide very high speed, short-range links as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures). Technologies likes GDSL can further increase the data rate of DSL. Fiber Optic technologies exist today that allow the conversion of copper based ISDN, ADSL and DSL over fiber optics.

DSL technologies (sometimes summarized as xDSL) include:

• ISDN Digital Subscriber Line (IDSL), uses ISDN based technology to provide data flow that is slightly higher than dual channel ISDN.
• High Data Rate Digital Subscriber Line (HDSL / HDSL2), was the first DSL technology that uses a higher frequency spectrum of copper, twisted pair cables.
• Symmetric Digital Subscriber Line (SDSL / SHDSL), the volume of data flow is equal in both directions.
• Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardised replacement for early proprietary SDSL.
• Asymmetric Digital Subscriber Line (ADSL), the volume of data flow is greater in one direction than the other.
• Asymmetric Digital Subscriber Line 2 (ADSL2), an improved version of ADSL
• Asymmetric Digital Subscriber Line 2 Plus (ADSL2+), A version of ADSL2 that doubles the data rates by using twice the spectrum.
• Asymmetric Digital Subscriber Line Plus Plus (ADSL++), technology developed by Centillium Communications for the Japanese market that extends downstream rates to 50 Mbit/s by using spectrum up to 3.75 MHz.
• Bonded DSL Rings (BDR), A shared ring topology at 400 Mbit/s
• Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and noise tolerance by sacrificing up stream speed
• Very High Speed Digital Subscriber Line (VDSL)
• Very High Speed Digital Subscriber Line 2 (VDSL2), an improved version of VDSL
• Etherloop Ethernet Local Loop
• Uni-DSL (Uni Digital Subscriber Line or UDSL), technology developed by Texas Instruments, backwards compatible with all DMT standards
• Gigabit Digital Subscriber Line (GDSL), based on binder MIMO technologies.^[6]
• Universal High bit rate Digital Subscriber Line (UHDSL) using fiber optics. Developed in 2005 by RLH Industries, Inc. Converts HDSL-1, 2 or 4 copper service into fiber optic HDSL service.

Transmission methods

Transmission methods vary by market, region, carrier, and equipment.

• 2B1Q: Two-binary, one-quaternary, used for IDSL and HDSL
• CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996 for ADSL, used for HDSL
• DMT: Discrete multitone modulation, the most numerous kind, also known as OFDM (Orthogonal frequency-division multiplexing)

See also

• Broadband Internet access
• Broadband Internet access worldwide
• Dynamic Spectrum Management (DSM)
• Electronic filter
• List of device bandwidths

References

1. ^ ece.wpi.edu
2. ^ Joseph W. Lechleider (August 1991). "High Bit Rate Digital Subscriber Lines: A Review of HDSL Progress" (fee required). IEEE Journal on Selected Areas in Communications 9 (6): 769–784. doi:10.1109/49.93088. http://ieeexplore.ieee.org/iel1/49/3051/00093088.pdf?tp=&isnumber=3051&arnumber=93088.
3. ^ SBC ATT Merger
4. ^ Verizon MCI merger
5. ^ Earthweb
6. ^ B. Lee, J. Cioffi, et al. (September 2007). "Gigabit DSL". IEEE Transactions on Communication. 55 (9): 1689–1692. doi:10.1109/TCOMM.2007.904374.

• Burstein, Dave (2002). DSL. John Wiley and Sons, New York. ISBN 0-471-08390-9. pp 53–86
• B. Lee, J.Cioffi, et al., Gigabit DSL, IEEE Transactions on Communications, Sep, 2007, pp 1689–1692

Categories: Digital Subscriber Line | Modems | Internet | Telecommunications

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