Digital Subscriber Line
Encyclopedia
Digital subscriber line (DSL) 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 DSL is widely understood to mean Asymmetric Digital Subscriber Line
(ADSL), the most commonly installed DSL technology. DSL service is delivered simultaneously with wired telephone service
on the same telephone line
. This is possible because DSL uses higher frequency bands for data separated by filtering.
On the customer premises, a DSL filter
on each outlet removes the high frequency interference, to enable simultaneous use of the telephone and data.
The data bit rate
of consumer DSL services typically ranges from 256 kbit/s to 40 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 upstream
direction, (the direction to the service provider) is lower, hence the designation of asymmetric service. In Symmetric Digital Subscriber Line
(SDSL) services, the downstream and upstream data rates are equal.
, can be traced back to Claude Shannon's seminal 1948 paper: A Mathematical Theory of Communication
.
An early patent was filed in 1987 for the use of wires for both voice phones and as a local area network
.
The motivation of digital subscriber line technology was the Integrated Services Digital Network
(ISDN) specification proposed in 1984 by the CCITT
(now ITU-T) as part of Recommendation I.120, later reused as ISDN Digital Subscriber Line (IDSL). Employees at Bellcore (now Telcordia Technologies) developed Asymmetric Digital Subscriber Line
(ADSL) and filed a patent in 1988 by placing wide-band digital signals above the existing baseband
analog voice signal carried between telephone company
telephone exchange
s and customers on conventional twisted pair
cabling facilities.
Consumer-oriented ADSL 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 carrier
s (ILECs) provision Rate-Adaptive Digital Subscriber Line
(RADSL) to work on virtually any available copper pair facility—whether conditioned for BRI or not.
Engineers developed higher-speed DSL facilities such as High bit rate Digital Subscriber Line (HDSL) and Symmetric Digital Subscriber Line
(SDSL) to provision traditional Digital Signal 1
(DS1) services over standard copper pair facilities.
A DSL circuit provides digital service. The underlying technology of transport across DSL facilities uses high-frequency sinusoidal carrier wave
modulation, which is an analog signal transmission. A DSL circuit terminates at each end in a modem
which modulates patterns of bits 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
(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
and digital modulation methods.
Early DSL service required a dedicated dry loop, but when the U.S. Federal Communications Commission
(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 co-located 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 single copper pair facility.
On the subscriber's end of the circuit, inline low-pass DSL filter
s (splitters) are installed on each telephone to filter the high-frequency "hiss" that would otherwise be heard, but pass voice (5 kHz and below) frequencies.
Conversely, high-pass filter
s already incorporated in the circuitry of DSL modems filter out voice frequencies. Although ADSL and RADSL modulations 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 delivered 8 Mbit/s to the customer over about 2 km (1.2 mi) of unshielded twisted-pair copper wire. Newer variants improved these rates. Distances greater than 2 km (1.2 mi) significantly reduce the bandwidth
usable on the wires, thus reducing the data rate. ADSL loop extender
s increase these distances substantially.
via a local loop
, which is a physical pair of wires. Prior to the digital age, the use of the local loop for anything other than the transmission of speech, encompassing an audio frequency range of 300 to 3400 Hertz
(voiceband
or commercial bandwidth
) was not considered. However, as long distance trunks were gradually converted from analog to digital operation, the idea of being able to pass data through the local loop (by utilizing frequencies above the voiceband) took hold, ultimately leading to DSL.
For a long time it was thought that it was not possible to operate a conventional phone-line beyond low-speed limits (typically under 9600 bit/s). In the 1950s, ordinary twisted-pair telephone-cable often carried four megahertz (MHz) television signals between studios, suggesting that such lines would allow transmitting many megabits per second. One such circuit in the UK ran some ten miles (16 km) between Pontop Pike transmitter and Newcastle on Tyne BBC Studios. It was able to give the studios a low quality cue feed but not one suitable for transmission. 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
communications that allowed the limit to be greatly extended.
The local loop
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 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
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 traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channel
s are bonded
into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceiver
s 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
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 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 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 endeavor 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
over the decades that have increased performance and reduced costs even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. Several factors contributed to the popularity of DSL 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
.
(a.k.a. standalone or dry loop DSL) is a way of providing DSL services without a PSTN (analogue
telephony
) service. It is useful when the customer does not need the traditional telephony
voice service because voice service is received either on top of the DSL services (usually Voice over IP
) or through another network (mobile telephony
).
It is also commonly called a "UNE" for Unbundled Network Element, in the USA. It has 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, and Verizon's merger with MCI
, 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 landline
s 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). This lack of demand for landline voice service has resulted in the expansion of naked DSL availability.
Naked DSL products are also marketed in some other countries e.g. Australia, New Zealand and Canada.
, 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:
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 (in blue). The right side the shows a DSLAM residing in the telephone company'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.
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.
Most DSL technologies 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.
(DSLAM) terminate
s 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.
(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 bridge
d 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.
" 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:
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
data transmission over the wires of a local telephone network
Access network
An access network is that part of a telecommunications network which connects subscribers to their immediate service provider. It is contrasted with the core network, which connects local providers to each other...
. DSL originally stood for digital subscriber loop. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line
Asymmetric Digital Subscriber Line
Asymmetric digital subscriber line is a type of 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...
(ADSL), the most commonly installed DSL technology. DSL service is delivered simultaneously with wired telephone service
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 many parts of the world....
on the same telephone line
Telephone line
A telephone line or telephone circuit is a single-user circuit on a telephone communication system...
. This is possible because DSL uses higher frequency bands for data separated by filtering.
On the customer premises, a DSL filter
DSL filter
A DSL filter is an analog low-pass filter installed between analog devices and a plain old telephone service telephone line, in order to prevent interference between such devices and a digital subscriber line service operating on the same line...
on each outlet removes the high frequency interference, to enable simultaneous use of the telephone and data.
The data bit rate
Bit rate
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time....
of consumer DSL services typically ranges from 256 kbit/s to 40 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 upstream
Upstream (networking)
In computer networking, upstream refers to the direction in which data can be transferred from the client to the server . This differs greatly from downstream not only in theory and usage, but also in that upstream speeds are usually at a premium...
direction, (the direction to the service provider) is lower, hence the designation of asymmetric service. In Symmetric Digital Subscriber Line
Symmetric Digital Subscriber Line
Symmetric digital subscriber line can have two meanings:* In the wider sense it is a collection of Internet access technologies based on DSL that offer symmetric bandwidth upstream and downstream...
(SDSL) services, the downstream and upstream data rates are equal.
History
Theory behind DSL, like many other forms of communicationCommunication
Communication is the activity of conveying meaningful information. Communication requires a sender, a message, and an intended recipient, although the receiver need not be present or aware of the sender's intent to communicate at the time of communication; thus communication can occur across vast...
, can be traced back to Claude Shannon's seminal 1948 paper: A Mathematical Theory of Communication
A Mathematical Theory of Communication
"A Mathematical Theory of Communication" is an influential 1948 article by mathematician Claude E. Shannon. As of November 2011, Google Scholar has listed more than 48,000 unique citations of the article and the later-published book version...
.
An early patent was filed in 1987 for the use of wires for both voice phones and as a local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
.
The motivation of digital subscriber line technology was the Integrated Services Digital Network
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...
(ISDN) specification proposed in 1984 by the CCITT
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
(now ITU-T) as part of Recommendation I.120, later reused as ISDN Digital Subscriber Line (IDSL). Employees at Bellcore (now Telcordia Technologies) developed Asymmetric Digital Subscriber Line
Asymmetric Digital Subscriber Line
Asymmetric digital subscriber line is a type of 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...
(ADSL) and filed a patent in 1988 by placing wide-band digital signals above the existing baseband
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...
analog voice signal carried between telephone company
Telephone company
A telephone company is a service provider of telecommunications services such as telephony and data communications access. Many were at one time nationalized or state-regulated monopolies...
telephone exchange
Telephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
s and customers on conventional twisted pair
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 from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
cabling facilities.
Consumer-oriented ADSL 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 carrier
Incumbent local exchange carrier
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 breakup 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...
s (ILECs) provision Rate-Adaptive Digital Subscriber Line
Rate-Adaptive Digital Subscriber Line
'Rate-Adaptive Digital Subscriber Line is a variation of Asymmetric Digital Subscriber Line technology. In RADSL the DSL modem adjusts the upstream bandwidth to create a wider frequency band for the downstream traffic...
(RADSL) to work on virtually any available copper pair facility—whether conditioned for BRI or not.
Engineers developed higher-speed DSL facilities such as High bit rate Digital Subscriber Line (HDSL) and Symmetric Digital Subscriber Line
Symmetric Digital Subscriber Line
Symmetric digital subscriber line can have two meanings:* In the wider sense it is a collection of Internet access technologies based on DSL that offer symmetric bandwidth upstream and downstream...
(SDSL) to provision traditional Digital Signal 1
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 North America, Japan, and South Korea...
(DS1) services over standard copper pair facilities.
A DSL circuit provides digital service. The underlying technology of transport across DSL facilities uses high-frequency sinusoidal carrier wave
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
modulation, which is an analog signal transmission. A DSL circuit terminates at each end in a modem
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...
which modulates patterns of bits 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
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 many parts of the world....
(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
Digital signal processing is concerned with the representation of discrete time 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...
and digital modulation methods.
Federal Communications Commission
The Federal Communications Commission is an independent agency of the United States government, created, Congressional statute , and with the majority of its commissioners appointed by the current President. The FCC works towards six goals in the areas of broadband, competition, the spectrum, the...
(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
Unbundled Network Element
Unbundled Network Elements are a requirement mandated by the United States Telecommunications Act of 1996. They are the parts of the telecommunications network that the incumbent local exchange carriers are required to offer on an unbundled basis...
, 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 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 single copper pair facility.
On the subscriber's end of the circuit, inline low-pass DSL filter
DSL filter
A DSL filter is an analog low-pass filter installed between analog devices and a plain old telephone service telephone line, in order to prevent interference between such devices and a digital subscriber line service operating on the same line...
s (splitters) are installed on each telephone to filter the high-frequency "hiss" that would otherwise be heard, but pass voice (5 kHz and below) frequencies.
Conversely, high-pass filter
High-pass filter
A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...
s already incorporated in the circuitry of DSL modems filter out voice frequencies. Although ADSL and RADSL modulations 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 delivered 8 Mbit/s to the customer over about 2 km (1.2 mi) of unshielded twisted-pair copper wire. Newer variants improved these rates. Distances greater than 2 km (1.2 mi) significantly reduce the bandwidth
Bandwidth (computing)
In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
usable on the wires, thus reducing the data rate. ADSL loop extender
ADSL loop extender
An ADSL loop extender or ADSL repeater is a device that a telephone company can place midway between the subscriber and central office to extend the distance and increase the channel capacity of their DSL connection...
s increase these distances substantially.
Basic technology
Telephones are connected to the telephone exchangeTelephone exchange
In the field of telecommunications, a telephone exchange or telephone switch is a system of electronic components that connects telephone calls...
via a local loop
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...
, which is a physical pair of wires. Prior to the digital age, the use of the local loop for anything other than the transmission of speech, encompassing an audio frequency range of 300 to 3400 Hertz
Hertz
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 the sine wave, particularly those used in radio and audio applications....
(voiceband
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...
or commercial bandwidth
Commercial bandwidth
Commercial bandwidth is a term for the regular capacity of the telephone network required for intelligible speech. It was defined as 300 hertz to 3,400 hertz, although the modern PSTN is theoretically capable of transmitting from 0 Hz to 7,000 Hz using ISDN....
) was not considered. However, as long distance trunks were gradually converted from analog to digital operation, the idea of being able to pass data through the local loop (by utilizing frequencies above the voiceband) took hold, ultimately leading to DSL.
For a long time it was thought that it was not possible to operate a conventional phone-line beyond low-speed limits (typically under 9600 bit/s). In the 1950s, ordinary twisted-pair telephone-cable often carried four megahertz (MHz) television signals between studios, suggesting that such lines would allow transmitting many megabits per second. One such circuit in the UK ran some ten miles (16 km) between Pontop Pike transmitter and Newcastle on Tyne BBC Studios. It was able to give the studios a low quality cue feed but not one suitable for transmission. However, these cables had other impairments besides Gaussian noise
Gaussian noise
Gaussian noise is statistical noise that has its probability density function equal to that of the normal distribution, which is also known as the Gaussian distribution. In other words, the values that the noise can take on are Gaussian-distributed. A special case is white Gaussian noise, in which...
, preventing such rates from becoming practical in the field. The 1980s saw the development of techniques for broadband
Broadband
The term broadband refers to a telecommunications signal or device of greater bandwidth, in some sense, than another standard or usual signal or device . 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
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...
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 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
Analog signal
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...
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
Upstream (networking)
In computer networking, upstream refers to the direction in which data can be transferred from the client to the server . This differs greatly from downstream not only in theory and usage, but also in that upstream speeds are usually at a premium...
and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
s are bonded
Channel bonding
Channel bonding is a computer networking arrangement in which two or more network interfaces on a host computer are combined for redundancy or increased throughput....
into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceiver
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When no circuitry is common between transmit and receive functions, the device is a transmitter-receiver. The term originated in the early 1920s...
s 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
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 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 and interleaved channel
Interleaving
In computer science and telecommunication, interleaving is a way to arrange data in a non-contiguous way to increase performance.It is typically used:* In error-correction coding, particularly within data transmission, disk storage, and computer memory....
. Fast channel is preferred for streaming multimedia, where an occasional dropped bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...
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
Loading coil
In electronics, a loading coil or load coil is a coil that does not provide coupling to any other circuit, but is inserted in a circuit to increase its inductance. The need was discovered by Oliver Heaviside in studying the disappointing slow speed of the Transatlantic telegraph cable...
. 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 endeavor 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
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
over the decades that have increased performance and reduced costs even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive. Several factors contributed to the popularity of DSL technology:
- Until the late 1990s, the cost of digital signal processorDigital signal processorA digital signal processor is a specialized microprocessor with an architecture optimized for the fast operational needs of digital signal processing.-Typical characteristics:...
s for DSL was prohibitive. All types of DSL employ highly complex digital signal processingDigital signal processingDigital signal processing is concerned with the representation of discrete time 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...
algorithms to overcome the inherent limitations of the existing twisted pairTwisted pairTwisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...
wires. Due to the advancements of Very-large-scale integrationVery-large-scale integrationVery-large-scale integration is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device.The first semiconductor...
(VLSI) technology, the cost of the equipment associated with a DSL deployment lowered significantly. The two main pieces of equipment are a Digital subscriber line access multiplexerDigital subscriber line access multiplexerA digital subscriber line access multiplexer is a network device, located in the telephone exchanges of the telecommunications operators. It connects multiple customer digital subscriber line interfaces to a high-speed digital communications channel using multiplexing techniques...
(DSLAM) at one end and a DSL modemModemA 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...
at the other end. - 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
Fax
Fax , sometimes called telecopying, is the telephonic transmission of scanned printed material , normally to a telephone number connected to a printer or other output device...
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
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
, Powerline
Power line communication
Power line communication or power line carrier , also known as power line digital subscriber line , mains communication, power line telecom , power line networking , or broadband over power lines are systems for carrying data on a conductor also used for electric power transmission.A wide range...
, or Wi-Fi
Wi-Fi
Wi-Fi or Wifi, is a mechanism for wirelessly connecting electronic devices. A device enabled with Wi-Fi, such as a personal computer, video game console, smartphone, or digital audio player, can connect to the Internet via a wireless network access point. An access point has a range of about 20...
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
Internet service provider
An Internet service provider is a company that provides access to the Internet. Access ISPs directly connect customers to the Internet using copper wires, wireless or fiber-optic connections. Hosting ISPs lease server space for smaller businesses and host other people servers...
.
Naked DSL
A naked DSLNaked DSL
A naked DSL is a digital subscriber line without a PSTN service — or the associated dial tone...
(a.k.a. standalone or dry loop DSL) is a way of providing DSL services without a PSTN (analogue
Analog signal
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...
telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
) service. It is useful when the customer does not need the traditional telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
voice service because voice service is received either on top of the DSL services (usually Voice over IP
Voice over IP
Voice over Internet Protocol is a family of technologies, methodologies, communication protocols, and transmission techniques for the delivery of voice communications and multimedia sessions over Internet Protocol networks, such as the Internet...
) or through another network (mobile telephony
Mobile telephony
Mobile telephony is the provision of telephone services to phones which may move around freely rather than stay fixed in one location. Mobile phones connect to a terrestrial cellular network of base stations , whereas satellite phones connect to orbiting satellites...
).
It is also commonly called a "UNE" for Unbundled Network Element, in the USA. It has started making a comeback in the US in 2004 when Qwest
Qwest
Qwest Communications International, Inc. was a large United States telecommunications carrier. Qwest provided local service in 14 western U.S. states: Arizona, Colorado, Idaho, Iowa, Minnesota, Montana, Nebraska, New Mexico, North Dakota, Oregon, South Dakota, Utah, Washington, and Wyoming.On April...
started offering it, closely followed by Speakeasy
Speakeasy (ISP)
Speakeasy, Inc. is a broadband internet service provider and Voice over IP carrier based in Seattle, Washington, United States. They are known for their liberal usage policies for home users whereby subscribers are explicitly permitted to run any number of serversand are permitted to resell their...
. As a result of AT&T
AT&T
AT&T Inc. is an American multinational telecommunications corporation headquartered in Whitacre Tower, Dallas, Texas, United States. It is the largest provider of mobile telephony and fixed telephony in the United States, and is also a provider of broadband and subscription television services...
's merger with SBC, and Verizon's merger with MCI
MCI Inc.
MCI, Inc. is an American telecommunications subsidiary of Verizon Communications that is headquartered in Ashburn, Virginia...
, those telephone companies have an obligation to offer naked DSL to consumers.
Even without the regulatory mandate, however, many ILECs
Incumbent local exchange carrier
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 breakup 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...
offer naked DSL to consumers. The number of telephone landline
Landline
A landline was originally an overland telegraph wire, as opposed to an undersea cable. Currently, landline refers to a telephone line which travels through a solid medium, either metal wire or optical fibre, as distinguished from a mobile cellular line, where transmission is via radio waves...
s 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). This lack of demand for landline voice service has resulted in the expansion of naked DSL availability.
Naked DSL products are also marketed in some other countries e.g. Australia, New Zealand and Canada.
Typical setup
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 attenuationAttenuation
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 by water.In electrical engineering and telecommunications, attenuation affects the...
, 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:
- The DSL transceiver performs a self-test.
- The DSL transceiver checks the connection between the DSL transceiver and the computer. For residential variations of DSL, this is usually the EthernetEthernetEthernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
(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/s serial interface). Also, some variations of DSL (such as SDSL) use synchronous serial connections. - 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
Gateway (telecommunications)
In telecommunications, the term gateway has the following meaning:*In a communications network, a network node equipped for interfacing with another network that uses different protocols....
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
Flash memory
Flash memory is a non-volatile computer storage chip that can be electrically erased and reprogrammed. It was developed from EEPROM and must be erased in fairly large blocks before these can be rewritten with new data...
; 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 (in blue). The right side the shows a DSLAM residing in the telephone company'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.
Customer Equipment
The customer end of the connection consists of a terminal adaptor or "DSL modem". This converts data between the digital signals used by computers and the voltageVoltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
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
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
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.
Most DSL technologies 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
Demarcation point
In telephony, the demarcation point is the point at which the public switched telephone network ends and connects with the customer's on-premises wiring. It is the dividing line which determines who is responsible for installation and maintenance of wiring and equipment -- customer/subscriber, or...
, or with filters installed at the telephone outlets inside the customer premises. Either way has its practical and economical limitations.
Exchange equipment
At the exchange, a digital subscriber line access multiplexerDigital subscriber line access multiplexer
A digital subscriber line access multiplexer is a network device, located in the telephone exchanges of the telecommunications operators. It connects multiple customer digital subscriber line interfaces to a high-speed digital communications channel using multiplexing techniques...
(DSLAM) terminate
Electrical termination
Electrical termination of a signal involves providing a terminator at the end of a wire or cable to prevent an RF signal from being reflected back from the end, causing interference...
s 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 ModeAsynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(ATM) layer
Abstraction layer
An abstraction layer is a way of hiding the implementation details of a particular set of functionality...
over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.
DSL implementations may create bridge
Bridging (networking)
Bridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown...
d or routed
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP
Dynamic Host Configuration Protocol
The Dynamic Host Configuration Protocol is a network configuration protocol for hosts on Internet Protocol networks. Computers that are connected to IP networks must be configured before they can communicate with other hosts. The most essential information needed is an IP address, and a default...
to provide network details such as the IP address
IP address
An Internet Protocol address is a numerical label assigned to each device participating in a computer network that uses the Internet Protocol for communication. An IP address serves two principal functions: host or network interface identification and location addressing...
to the subscriber equipment, with authentication
Authentication
Authentication is the act of confirming the truth of an attribute of a datum or entity...
via MAC address
MAC address
A Media Access Control address is a unique identifier assigned to network interfaces for communications on the physical network segment. MAC addresses are used for numerous network technologies and most IEEE 802 network technologies, including Ethernet...
or an assigned host name. Later implementations often use Point-to-Point Protocol
Point-to-Point Protocol
In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
(PPP) or Asynchronous Transfer Mode
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
(ATM) (Point-to-Point Protocol over Ethernet
Point-to-Point Protocol over Ethernet
The Point-to-Point Protocol over Ethernet is a network protocol for encapsulating Point-to-Point Protocol frames inside Ethernet frames. It is used mainly with DSL services where individual users connect to the DSL modem over Ethernet and in plain Metro Ethernet networks...
(PPPoE) or Point-to-Point Protocol over ATM
Point-to-Point Protocol over ATM
The Point-to-Point Protocol over ATM is a network protocol for encapsulating PPP frames in AAL5. It is used mainly with DOCSIS and DSL carriers....
(PPPoA)), while authenticating with a userid and password and using Point-to-Point Protocol
Point-to-Point Protocol
In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
(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 playTriple play (telecommunications)
In telecommunications, triple play service is a marketing term for the provisioning of two bandwidth-intensive services, high-speed Internet access and television, and a less bandwidth-demanding service, telephone, over a single broadband connection. Triple play focuses on a combined business...
" 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 LineHigh data rate Digital Subscriber LineHigh-bit-rate digital subscriber line was the first DSL technology to use a higher frequency spectrum of copper, twisted pair cables. HDSL was developed in the US, as a better technology for high-speed, synchronous circuits typically used to interconnect local exchange carrier systems, and also to...
(HDSL / HDSL2), was the first DSL technology that used a higher frequency spectrum of copper, twisted pair cables. - Symmetric Digital Subscriber LineSymmetric Digital Subscriber LineSymmetric digital subscriber line can have two meanings:* In the wider sense it is a collection of Internet access technologies based on DSL that offer symmetric bandwidth upstream and downstream...
(SDSL / SHDSL), the volume of data flow is equal in both directions. - Symmetric High-speed Digital Subscriber Line (G.SHDSL), a standardized replacement for early proprietary SDSL.
- Asymmetric Digital Subscriber LineAsymmetric Digital Subscriber LineAsymmetric digital subscriber line is a type of 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...
(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 (Centillium has been acquired by TranSwitch Corp.) for the Japanese market that extends downstream rates to 50 Mbit/s by using spectrum up to 3.75 MHz.
- Bonded DSL RingsBonded DSL RingsDSL Rings is a telecommunications technology developed by Canadian startup Genesis Technical Systems, based in Calgary. The DSL technology re-uses existing copper telephone network cabling to provide bandwidth of up to 400 Mb/s...
(DSL Rings), A shared ring topology at 400 Mbit/s - Rate-Adaptive Digital Subscriber LineRate-Adaptive Digital Subscriber Line'Rate-Adaptive Digital Subscriber Line is a variation of Asymmetric Digital Subscriber Line technology. In RADSL the DSL modem adjusts the upstream bandwidth to create a wider frequency band for the downstream traffic...
(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 2Very high speed digital subscriber line 2Very-high-speed digital subscriber line 2 is an access technology that exploits the existing infrastructure of copper wires that were originally deployed for traditional telephone service. It can be deployed from central offices, from fiber-optic connected cabinets located near the customer...
(VDSL2), an improved version of VDSL - EtherloopEtherloopEtherloop is a kind of DSL technology that combines the features of Ethernet and DSL. It allows the combination of voice and data transmission on standard phone lines...
Ethernet Local Loop - (Extended-) Reach Digital Subscriber LineHigh Speed Voice and Data LinkHigh Speed Voice and Data Link is a high speed voice and data provisioning method that allows telcos and ISPs to provide up to three voice channels and data on a copper pair over extremely long loops....
- 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.
- 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.
- Internet Protocol Subscriber Line (IPSL), developed by Rim Semiconductor in 2007, allowed for 40 Mbit/s using 26 AWGAWGAWG may refer to:In science and technology:* American wire gauge, a standardized sizing system for electrically conducting wire* Arbitrary waveform generator, a piece of electronic test equipment used to generate electrical waveforms...
copper telephone wire at a 5500 ft (1,676.4 m) radius, 26 Mbit/s at a 6000 ft (1,828.8 m) radius. The company operated until 2008.
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 ModulationCarrierless Amplitude Phase ModulationCarrierless amplitude phase modulation is a variant of quadrature amplitude modulation . 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.CAP...
- deprecated in 1996 for ADSL, used for HDSL - DMT: Discrete multitone modulation, the most numerous kind, also known as OFDM (Orthogonal frequency-division multiplexingOrthogonal frequency-division multiplexingOrthogonal frequency-division multiplexing is a method of encoding digital data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio...
)
See also
- Broadband Internet accessBroadband Internet accessBroadband Internet access, often shortened to just "broadband", is a high data rate, low-latency connection to the Internet— typically contrasted with dial-up access using a 56 kbit/s modem or satellite Internet with inherently high latency....
- List of countries by number of Internet subscriptions
- Dynamic Spectrum ManagementDynamic Spectrum ManagementDynamic spectrum management , also referred to as dynamic spectrum access , is a set of techniques based on theoretical concepts in network information theory and game theory that is being researched and developed to improve the performance of a communication network as a whole...
(DSM) - Electronic filterElectronic filterElectronic filters are electronic circuits which perform signal processing functions, specifically to remove unwanted frequency components from the signal, to enhance wanted ones, or both...
- List of device bandwidths
External links
- ADSL Theory — Information about the background & workings of ADSL, and the factors involved in achieving a good sync between your modem and the DSLAM.
- Aware, Inc. White Paper, “VDSL2: The ideal Access Technology for Delivering Video Services, Rev 2”