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Digital subscriber line (DSL; originallydigital subscriber loop) is a family of technologies that are used to transmitdigital data overtelephone lines.[1] In telecommunications marketing, the term DSL is widely understood to meanasymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, forInternet access.
In ADSL, the data throughput in theupstream direction (the direction to the service provider) is lower, hence the designation ofasymmetric service. Insymmetric digital subscriber line (SDSL) services, the downstream and upstream data rates are equal.
DSL service can be delivered simultaneously withwired telephone service on the same telephone line since DSL uses higherfrequency bands for data transmission. On the customer premises, aDSL filter is installed on each telephone to prevent undesirable interaction between DSL and telephone service.
Thebit rate of consumer ADSL services typically ranges from256 kbit/s up to25 Mbit/s, while the later VDSL+ technology delivers between16 Mbit/s and250 Mbit/s in the direction to the customer (downstream), with up to40 Mbit/s upstream. The exact performance is depending on technology, line conditions, and service-level implementation. Researchers atBell Labs have reached SDSL speeds over1 Gbit/s using traditional copper telephone lines, though such speeds have not been made available for the end customers yet.[as of?][2][3][4]
Initially, it was believed that ordinary phone lines could only be used at modest speeds, usually less than 9600 bits per second. In the 1950s, ordinary twisted-pair telephone cable often carried 4 MHz television signals between studios, suggesting that such lines would allow transmitting many megabits per second. One such circuit in the United Kingdom ran some 10 miles (16 km) between theBBC studios inNewcastle-upon-Tyne and thePontop Pike transmitting station. However, these cables had other impairments besidesGaussian noise, preventing such rates from becoming practical in the field. The 1980s saw the development of techniques forbroadband communications that allowed the limit to be greatly extended. A patent was filed in 1979 for the use of existing telephone wires for both telephones and data terminals that were connected to a remote computer via a digital data carrier system.[5]
The motivation for digital subscriber line technology was theIntegrated Services Digital Network (ISDN) specification proposed in 1984 by the CCITT (nowITU-T) as part of RecommendationI.120, later reused asISDN digital subscriber line (IDSL). Employees at Bellcore (nowTelcordia Technologies) developedasymmetric digital subscriber line (ADSL) by placing wide-band digital signals at frequencies above the existingbaseband analog voice signal carried on conventionaltwisted pair cabling betweentelephone exchanges and customers.[6] A patent was filed by AT&T Bell Labs on the basic DSL concept in 1988.[7]
Joseph W. Lechleider's contribution to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of symmetric DSL.[8] This allowed Internet service providers to offer efficient service to consumers, who benefited 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 andinterleaved channel. Fast channel is preferred forstreaming multimedia, where an occasionaldroppedbit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where the delivered data must be error-free but latency (time delay) incurred by the retransmission of error-containing packets is acceptable.
Consumer-oriented ADSL was designed to operate on existing lines already conditioned forBasic Rate Interface ISDN services. Engineers developed high-speed DSL facilities such ashigh bit rate digital subscriber line (HDSL) andsymmetric digital subscriber line (SDSL) to provision traditionalDigital Signal 1 (DS1) services over standard copper pair facilities.
Older ADSL standards delivered8 Mbit/s to the customer over about 2 km (1.2 mi) ofunshielded twisted-pair copper wire. Newer variants improved these rates. Distances greater than 2 km (1.2 mi) significantly reduce thebandwidth usable on the wires, thus reducing the data rate. ButADSL loop extenders increase these distances by repeating the signal, allowing thelocal exchange carrier (LEC) to deliver DSL speeds to any distance.[9]
Until the late 1990s, the cost ofdigital signal processors for DSL was prohibitive. All types of DSL employ highly complexdigital signal processing algorithms to overcome the inherent limitations of the existingtwisted pair wires. Due to the advancements ofvery-large-scale integration (VLSI) technology, the cost of the equipment associated with a DSL deployment lowered significantly. The two main pieces of equipment are adigital subscriber line access multiplexer (DSLAM) at one end and aDSL modem at the other end.
It is possible to set up a DSL connection over an existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidthfiber-optic cable over the same route and distance. This is true both for ADSL and SDSL variations. DSL's continued use reflects advancements in electronics that have improved performance and reduced costs, despite the high expense of laying new physical infrastructure.
These advantages made ADSL a better proposition for customers requiringInternet access than metered dial up, while also allowing voice calls to be received at the same time as a data connection. Telephone companies were also under pressure to move to ADSL owing to competition from cable companies, which useDOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to the popularity of ADSL technology. Some of the first field trials for DSL were carried out in 1996.[10]
Early DSL service required a dedicateddry loop, but when the U.S.Federal Communications Commission (FCC) requiredincumbent local exchange carriers (ILECs) to lease their lines to competing DSL service providers, shared-line DSL became available. Also known as DSL overunbundled 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 co-located in the sametelephone exchange as that of the ILEC supplying the customer's pre-existing voice service. The subscriber's circuit is rewired to interface with hardware supplied by the ILEC which combines a DSL frequency andplain old telephone service (POTS) signals on a single copper pair.
Since 1999, certain ISPs have been offering microfilters. These devices are installed indoors and serve the same purpose as DSL splitters, which are deployed outdoors: they divide the frequencies needed for ADSL and POTS phone calls.[11][12] These filters originated out of a desire to make self-installation of DSL service possible and eliminate early outdoor DSL splitters which were installed at or near the demarcation point between the customer and the ISP.[13]
By 2012, some carriers in the United States reported that DSL remote terminals with fiberbackhaul were replacing older ADSL systems.[14]
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Telephones are connected to thetelephone exchange via alocal loop, which is a physical pair of wires. The local loop was originally intended mostly for the transmission of speech, encompassing an audio frequency range of 300 to 3400hertz (commercial bandwidth). However, as long-distancetrunks were gradually converted from analog to digital operation, the idea of being able to pass data through the local loop (by using frequencies above the voiceband) took hold, ultimately leading to DSL.
Thelocal loop connecting the telephone exchange to most subscribers has the capability of carrying frequencies well beyond the 3400 Hz upper limit ofPOTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unusedbandwidth 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 to 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 ananalog modem would on a POTS connection. More usable channels equate 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 forupstream anddownstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individualchannels arebonded into a pair of virtual circuits, one in each direction. Like analog modems, DSLtransceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable. Once upstream and downstream circuits are established, asubscriber can connect to a service such as anInternet service provider or other network services, like a corporateMPLS network.
The underlying technology of transport across DSL facilities usesmodulation of high-frequencycarrier waves, an analog signal transmission. A DSL circuit terminates at each end in amodem which modulates patterns ofbits 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 passes on, 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 audio baseband, DSL modems modulate frequencies from 4000 Hz to as high as 4 MHz. This frequency band separation enables DSL service andplain old telephone service (POTS) to coexist on the same cables, known as voice-grade cables.[15] On the subscriber's end of the circuit, inlineDSL filters are installed on each telephone to pass voice frequencies but filter the high-frequency signals that would otherwise be heard as hiss. Also, nonlinear elements in the phone could otherwise generate audibleintermodulation and may impair the operation of the data modem in the absence of theselow-pass filters. DSL and RADSL modulations do not use the voice-frequency band sohigh-pass filters are incorporated in the circuitry of DSL modems filter out voice frequencies.
Because DSL operates above the 3.4 kHz voice limit, it cannot pass through aloading coil, which is an inductive coil that is designed to counteract loss caused by shunt capacitance (capacitance between the two wires of the twisted pair). Loading coils are commonly set at regular intervals in POTS lines. 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 loading coil placement. Because of this, phone companies endeavor to remove loading coils on copper loops that can operate without them. Longer lines that require them can be replaced with fiber to the neighborhood or node (FTTN).
Most residential and small-office DSL implementations reserve low frequencies for POTS, so that (with suitable filters and/or splitters) the existing voice service continues to operate independently of the DSL service. Thus POTS-based communications, includingfax machines anddial-up modems, can share the wires with DSL. Only one DSL modem can use thesubscriber line at a time. The standard way to let multiple computers share a DSL connection uses arouter that establishes a connection between the DSL modem and a localEthernet,powerline, orWi-Fi network on the customer's premises.
The theoretical foundations of DSL, like much ofcommunication technology, can be traced back toClaude Shannon's seminal 1948 paper, "A Mathematical Theory of Communication". Generally, higher bit rate transmissions require a wider frequency band, though the ratio ofbit rate tosymbol rate and thus to bandwidth are not linear due to significant innovations indigital signal processing anddigital modulation methods.
Naked DSL is a way of providing only DSL services over alocal loop. It is useful when the customer does not need the traditionaltelephony voice service because voice service is received either on top of the DSL services (usuallyVoIP) or through another network (E.g.,mobile telephony). It is also commonly called anunbundled network element (UNE) in the United States; in Australia it is known as an unconditioned local loop (ULL);[16] in Belgium it is known as "raw copper" and in the UK it is known as Single Order GEA (SoGEA).[17]
It started making a comeback in the United States in 2004 whenQwest started offering it, closely followed bySpeakeasy. As a result ofAT&T's merger withSBC,[18] andVerizon's merger withMCI,[19] those telephone companies have an obligation to offer naked DSL to consumers.
On the customer side, a DSL modem is hooked up to a phone line. The telephone company connects the other end of the line to aDSLAM (digital subscriber line access multiplexer), which concentrates a large number of individual DSL connections into a single box. The DSLAM cannot be located too far from the customer because ofattenuation between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.
The above figure is a schematic of a simple DSL connection (in blue). The right side shows a DSLAM residing in the telephone company's telephone exchange. The left side shows the customer premises equipment with an optional router. The router manages alocal area network which connects PCs and other local devices. The customer may opt for a modem that contains both a router and wireless access. This option (within the dashed bubble) often simplifies the connection.
At the exchange, a DSLAMterminates the DSL circuits and aggregates them, where they arehanded off to other networking transports such as a PON network or Ethernet. The DSLAM terminates all connections and recovers the original digital information. In the case of ADSL, the voice component is also separated at this step, either by a filter or splitter integrated in the DSLAM or by specialized filtering equipment installed before it.[20] Load coils in phone lines, used for extending their range in rural areas, must be removed to allow DSL to operate as they only allow frequencies of up to 4000 Hz to pass through phone cables.[21][22][23]
The customer end of the connection consists of aDSL modem. This converts data between the digital signals used by computers and the analogvoltage signal of a suitable frequency range which is then applied to the phone line.
In some DSL variations (for example,HDSL), the modem connects directly to the computer via a serial interface, using protocols such asEthernet orV.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.
Most DSL technologies require the installation of appropriateDSL filters at the customer's premises to separate the DSL signal from the low-frequency voice signal. The separation can take place either at thedemarcation point, or with filters installed at thetelephone outlets inside the customer premises. It is possible for a DSL gateway to integrate the filter, and allow telephones to connect through the gateway.
Modern DSLgateways often integrate routing and other functionality. The system boots, synchronizes the DSL connection and finally establishes the internet IP services and connection between the local network and the service provider, using protocols such asDHCP orPPPoE.
Many DSL technologies implement anAsynchronous Transfer Mode (ATM)layer over the low-levelbitstream layer to enable the adaptation of a number of different technologies over the same link.
DSL implementations may createbridged orrouted networks. In a bridged configuration, the group of subscriber computers effectively connect into a singlesubnetwork. The earliest implementations usedDHCP to provide theIP address to the subscriber equipment, withauthentication viaMAC address or an assignedhostname. Later implementations often usePoint-to-Point Protocol (PPP) to authenticate with a user ID and password.
Transmission methods vary by market, region, carrier, and equipment.
| Full name | Abbreviation | ITU-T standard | Date |
|---|---|---|---|
| Asymmetric digital subscriber line | ADSL | G.992.1 (G.dmt) | 1999 |
| ADSL2 | G.992.3 (G.dmt.bis) | 2002 | |
| ADSL2plus | G.992.5 | 2003 | |
| Asymmetric digital subscriber line-Reach Extended | ADSL2-RE | G.992.3 | 2003 |
| Single-pair high-speed digital subscriber line | SHDSL | G.991.2 | 2003 |
| Very-high-bit-rate digital subscriber line | VDSL | G.993.1 | 2004 |
| VDSL2 -12 MHz long reach | G.993.2 | 2005 | |
| VDSL2 -30 MHz short reach | G.993.2 | 2005 |
DSL technologies (sometimes collectively summarized asxDSL) include:
The line-length limitations from telephone exchange to subscriber impose severe limits on data transmission rates. Technologies such asVDSL provide very high-speed but short-range links. VDSL is used as a method of deliveringtriple play services (typically implemented infiber to the curb network architectures).
Terabit DSL, is a technology that proposes the use of the space between the dielectrics (insulators) on copper twisted pair lines in telephone cables, as waveguides for 300 GHz signals that can offer speeds of up to 1 terabit per second at distances of up to 100 meters, 100 gigabits per second for 300 meters, and 10 gigabits per second for 500 meters.[35][36] The first experiment for this was carried out with copper lines that were parallel to each other, and not twisted, inside a metal pipe meant to simulate the metal armoring in largetelephone cables.[37][38]
