|Internet protocol suite|
Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit digital data over telephone lines. In telecommunications marketing, the term DSL is widely understood to mean asymmetric digital subscriber line (ADSL), the most commonly installed DSL technology, for Internet access.
Digital data, in information theory and information systems, is the discrete, discontinuous representation of information or works. Numbers and letters are commonly used representations.
A telephone line or telephone circuit is a single-user circuit on a telephone communication system. This is 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. Telephone lines are used to deliver landline telephone service and Digital subscriber line (DSL) phone cable service to the premises. Telephone overhead lines are connected to the public switched telephone network.
Asymmetric digital subscriber line (ADSL) is a type of digital subscriber line (DSL) technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. ADSL differs from the less common symmetric digital subscriber line (SDSL). In ADSL, bandwidth and bit rate are said to be asymmetric, meaning greater toward the customer premises (downstream) than the reverse (upstream). Providers usually market ADSL as a service for consumers for Internet access for primarily downloading content from the Internet, but not serving content accessed by others.
DSL service can be delivered simultaneously with wired telephone service on the same telephone line since DSL uses higher frequency bands for data. On the customer premises, a DSL filter on each non-DSL outlet blocks any high-frequency interference to enable simultaneous use of the voice and DSL services.
Plain old telephone service (POTS), or plain ordinary telephone service, is a retronym for voice-grade telephone service employing analog signal transmission over copper loops. POTS was the standard service offering from telephone companies from 1876 until 1988 in the United States when the Integrated Services Digital Network (ISDN) Basic Rate Interface (BRI) was introduced, followed by cellular telephone systems, and voice over IP (VoIP). POTS remains the basic form of residential and small business service connection to the telephone network in many parts of the world. The term reflects the technology that has been available since the introduction of the public telephone system in the late 19th century, in a form mostly unchanged despite the introduction of Touch-Tone dialing, electronic telephone exchanges and fiber-optic communication into the public switched telephone network (PSTN).
A frequency band is an interval in the frequency domain, delimited by a lower frequency and an upper frequency. The term may refer to a radio band or an interval of some other spectrum.
A DSL filter is an analog low-pass filter installed between analog devices and a plain old telephone service (POTS) line. The DSL filter prevents interference between such devices and a digital subscriber line (DSL) service connected to the same line. Without DSL filters, signals or echoes from analog devices at the top of their frequency range can reduce performance and create connection problems with DSL service, while those from the DSL service at the bottom of its range can cause line noise and other problems for analog devices.
The bit rate of consumer DSL services typically ranges from 256 kbit/s to over 100 Mbit/s in the direction to the customer (downstream), depending on DSL technology, line conditions, and service-level implementation. Bit rates of 1 Gbit/s have been reached.
In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time.
In a telecommunications network or computer network, downstream refers to data sent from a network service provider to a customer.
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. Researchers at Bell Labs have reached speeds over 1 Gbit/s for symmetrical broadband access services using traditional copper telephone lines, though such speeds have not yet been deployed elsewhere.
In computer networking, upstream refers to the direction in which data can be transferred from the client to the server (uploading). This differs greatly from downstream not only in theory and usage, but also in that upstream speeds are usually at a premium. Whereas downstream speed is important to the average home user for purposes of downloading content, uploads are used mainly for web server applications and similar processes where the sending of data is critical. Upstream speeds are also important to users of peer-to-peer software.
A symmetric digital subscriber line (SDSL) is a digital subscriber line (DSL) that transmits digital data over the copper wires of the telephone network, where the bandwidth in the downstream direction, from the network to the subscriber, is identical to the bandwidth in the upstream direction, from the subscriber to the network. This symmetric bandwidth can be considered to be the opposite of the asymmetric bandwidth offered by asymmetric digital subscriber line (ADSL) technologies, where the upstream bandwidth is lower than the downstream bandwidth. SDSL is generally marketed at business customers, while ADSL is marketed at private as well as business customers.
Nokia Bell Labs is an industrial research and scientific development company owned by Finnish company Nokia. Its headquarters are located in Murray Hill, New Jersey. Other laboratories are located around the world. Bell Labs has its origins in the complex past of the Bell System.
It was originally 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 United Kingdom ran some 10 miles (16 km) between the BBC studios in Newcastle-upon-Tyne and the Pontop Pike transmitting station. It was able to give the studios a low quality cue feed but not one suitable for transmission.[ 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 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.
The British Broadcasting Corporation (BBC) is a British public service broadcaster. Its headquarters are at Broadcasting House in Westminster, London, and it is the world's oldest national broadcasting organisation and the largest broadcaster in the world by number of employees. It employs over 20,950 staff in total, 16,672 of whom are in public sector broadcasting. The total number of staff is 35,402 when part-time, flexible, and fixed-contract staff are included.
The Pontop Pike transmitting station is a facility for telecommunications and broadcasting situated on a 312-metre (1,024-ft) high hill of the same name between Stanley and Consett, County Durham, near the village of Dipton, England. The mast is 149 metres (489 ft) high, giving an average antenna height of 461 metres (1,512 ft) above sea level. It is owned and operated by Arqiva.
The motivation for 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) by placing wide-band digital signals at frequencies above the existing baseband analog voice signal carried on conventional twisted pair cabling between telephone exchanges and customers.A patent was filed in 1988.
Integrated Services Digital Network (ISDN) is a set of communication 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 1988 in the CCITT red book. Prior to ISDN, the telephone system was viewed as a way to transport voice, with some special services available for data. The key feature of ISDN is that it integrates speech and data on the same lines, adding features that were not available in the classic telephone system. The ISDN standards define several kinds of access interfaces, such as Basic Rate Interface (BRI), Primary Rate Interface (PRI), Narrowband ISDN (N-ISDN), and Broadband ISDN (B-ISDN).
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors of the International Telecommunication Union (ITU); it coordinates standards for telecommunications.
ISDN Digital Subscriber Line (IDSL) uses ISDN-based digital subscriber line technology to provide a data communication channel across existing copper telephone lines at a rate of 144 kbit/s, slightly higher than a bonded dual channel ISDN connection at 128kbit/s. The digital transmission bypasses the telephone company's central office equipment that handles analogue signals. IDSL uses the ISDN grade loop without Basic Rate Interface in ISDN transmission mode. The benefits of IDSL over ISDN are that IDSL provides always-on connections and transmits data via a data network rather than the carrier's voice network.
Joseph W. Lechleider's contribution 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 (time delay) incurred by the retransmission of error-containing packets is acceptable.
Consumer-oriented ADSL was designed to operate on existing lines already conditioned for Basic Rate Interface ISDN services, which itself is a digital circuit switching service (non-IP), though most incumbent local exchange carriers (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 high 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.
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. But ADSL loop extenders increase these distances by repeating the signal, allowing the LEC to deliver DSL speeds to any distance.
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 lowered significantly. The two main pieces of equipment are a digital subscriber line access multiplexer (DSLAM) at one end and a DSL modem 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. 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.
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.
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 (telephone 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 and POTS signals on a single copper pair facility.
By 2012, some carriers in the United States reported that DSL remote terminals with fiber backhaul are replacing older ADSL systems.
Telephones are connected to the telephone exchange via a local 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 3400 hertz (voiceband or commercial bandwidth). 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.
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 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. Once upstream and downstream circuits are established, a subscriber can connect to a service such as an Internet service provider or other network services, like a corporate MPLS network.
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. On the subscriber's end of the circuit, inline low-pass DSL filters (splitters) are installed on each telephone to filter the high-frequency signals that would otherwise be heard as hiss, but pass voice frequencies. Conversely, high-pass filters 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 high-pass filters.
Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a load 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). Load coils are commonly set at regular intervals in lines placed only for POTS. 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 by conditioning other lines to avoid them through the use of 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, 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.
The theoretical foundations of DSL, like much of communication technology, can be traced back to Claude Shannon's seminal 1948 paper: A Mathematical Theory of Communication . Generally, higher bit rate transmissions require a wider frequency band, though the ratio of bit rate to symbol rate and thus to bandwidth are not linear due to significant innovations in digital signal processing and digital modulation methods.
A naked DSL (also known as 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 VoIP) or through another network (mobile telephony).
It is also commonly called a UNE (for unbundled network element) in the United States; in Australia it is known as a ULL (unconditioned local loop);in Belgium it is known as "raw copper" and in the UK it is known as Single Order GEA (SoGEA). It started making a comeback in the United States 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 offered naked DSL to consumers. The number of telephone landlines in the United States dropped from 188 million in 2000 to 115 million in 2010, while the number of cellular subscribers has grown to 277 million (as of 2010).This lack of demand for landline voice services has resulted in the expansion of naked DSL availability.
Naked DSL products are also marketed in some other countries e.g., Israel, Australia, New Zealand, and Canada.[ citation needed ]
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 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 between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.
The accompanying figure is a schematic of a simple DSL connection (in blue). The right side shows a DSLAM residing in the telephone company's central office. The left side shows the customer premises equipment with an optional router. The router manages a local area network (LAN) which connects PCs and other local devices. With many service providers, the customer may opt for a modem which contains both a router and wireless access. This option (within the dashed bubble) often simplifies the connection.
At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off to 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.
The customer end of the connection consists of a terminal adapter or "DSL modem". This converts data between the digital signals used by computers and the analog 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.
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. Each way has its practical and economic limitations.
When the DSL modem powers up it goes through a series of steps to establish connections. The actual process varies from modem to modem but generally involves the following steps:
Modern DSL gateways often integrate routing and other functionality. Their initialization is very similar to a PC boot up. The system image is loaded from the flash storage; 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 as DHCP or PPPoE. According to Implementation and Applications of DSL Technology (2007), the PPPoE method far outweighed DHCP in terms of deployment on DSLs, and PAP was the predominant form of subscriber authentication used in such circumstances.The system image can usually be updated to correct bugs, or to add new functionality.
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) to authenticate with a user ID and password, and to provide network details (Point-to-Point Protocol over Ethernet (PPPoE) or Point-to-Point Protocol over ATM (PPPoA)).
Transmission methods vary by market, region, carrier, and equipment.
DSL technologies (sometimes collectively summarized as xDSL) include:
The line-length limitations from telephone exchange to subscriber impose severe limits on data transmission rates. Technologies such as VDSL provide very high-speed but short-range links. VDSL is used as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures).
In telecommunications, broadband is wide bandwidth data transmission which transports multiple signals and traffic types. The medium can be coaxial cable, optical fiber, radio or twisted pair.
Very-high-bit-rate digital subscriber line (VDSL) and very-high-bit-rate digital subscriber line 2 (VDSL2) are digital subscriber line (DSL) technologies providing data transmission faster than asymmetric digital subscriber line (ADSL).
A digital subscriber line access multiplexer is a network device, often located in telephone exchanges, that connects multiple customer digital subscriber line (DSL) interfaces to a high-speed digital communications channel using multiplexing techniques.
Internet access is the ability of individuals and organizations to connect to the Internet using computer terminals, computers, and other devices; and to access services such as email and the World Wide Web. Internet access is sold by Internet service providers (ISPs) delivering connectivity at a wide range of data transfer rates via various networking technologies. Many organizations, including a growing number of municipal entities, also provide cost-free wireless access.
Single-pair high-speed digital subscriber line (SHDSL) is a form of symmetric digital subscriber line (SDSL), a data communications technology for equal transmit and receive data rate over copper telephone lines, faster than a conventional voiceband modem can provide. As opposed to other DSL technologies, SHDSL employs trellis-coded pulse-amplitude modulation (TC-PAM). As a baseband transmission scheme, TC-PAM operates at frequencies that include those used by the analog voice plain old telephone service (POTS). As such, a frequency splitter, or DSL filter, cannot be used to allow a telephone line to be shared by both an SHDSL service and a POTS service at the same time. Support of symmetric data rates made SHDSL a popular choice by businesses for private branch exchange (PBX), virtual private network (VPN), web hosting and other data services.
In telephony, pair gain is the transmitting of multiple POTS signals over the twisted pairs traditionally used for a single traditional subscriber line in telephone systems. Pair gain has the effect of creating additional subscriber lines. This is typically used as an expedient way to solve subscriber line shortage problems by using existing wiring, instead of installing new wires from the central office to the customer premises. The term was invented in the middle 20th century by analogy with earlier use of gain to extend telephone local loops far from the telephone exchange.
Rate-adaptive digital subscriber line (RADSL) is a pre-standard asymmetric digital subscriber line (ADSL) solution. RADSL was introduced as proprietary technology by AT&T Paradyne, later GlobeSpan Technologies Inc., in June 1996. In September 1999, RADSL technology was formally described by ANSI in T1.TR.59-1999. RADSL supports downstream data rates of up to approximately 8 Mbit/s, upstream data rates up to approximately 1 Mbit/s, and can coexist with POTS voice on the same line.
ITU G.992.5 is an International Telecommunication Union standard for asymmetric digital subscriber line (ADSL) broadband Internet access. The standard has a maximum theoretical download speed of 24 Mbit/s. Utilizing G.992.5 Annex M upload speeds of 3.3 Mbit/s can be achieved.
In telecommunications, ITU G.992.2 is an ITU standard for ADSL using discrete multitone modulation. G.lite does not strictly require the use of DSL filters, but like all variants of ADSL generally functions better with splitters.
ITU G.992.3 is an ITU standard, also referred to as ADSL2 or G.dmt.bis. It optionally extends the capability of basic ADSL in data rates to 12 Mbit/s downstream and, depending on Annex version, up to 3.5 Mbit/s upstream. ADSL2 uses the same bandwidth as ADSL but achieves higher throughput via improved modulation techniques. Actual speeds may decrease depending on line quality; usually the most significant factor in line quality is the distance from the DSLAM to the customer's equipment.
A digital subscriber line (DSL) modem is a device used to connect a computer or router to a telephone line which provides the digital subscriber line service for connectivity to the Internet, which is often called DSL broadband.
An Ethernet extender is any device used to extend an Ethernet or network segment beyond its inherent distance limitation which is approximately 100 metres (330 ft) for most common forms of twisted pair Ethernet. These devices employ a variety of transmission technologies and physical media.
High-bit-rate digital subscriber line (HDSL) is a telecommunications protocol standardized in 1994. It was the first digital subscriber line (DSL) technology to use a higher frequency spectrum over copper, twisted pair cables. HDSL was developed to transport DS1 services at 1.544 Mbit/s and 2.048 Mbit/s over telephone local loops without a need for repeaters. Successor technology to HDSL includes HDSL2 and HDSL4, proprietary SDSL, and G.SHDSL.
A DSL loop extender is a device that a telephone company can place between subscriber premises equipment and central office interfaces to extend the distance and increase the channel capacity of digital subscriber line (DSL) connections. ADSL repeaters are deployed by rural telephone companies trying to provide rural Internet service to farms and small towns where it is impractical to place the DSLAM closer to the subscriber. Typical distance improvements with a loop extender are shown in the diagram below, with rate in megabits per second and distance in thousands of feet.
The prevalent means of connecting to the Internet in Germany is DSL, introduced by Deutsche Telekom in 1999. Other technologies such as Cable, FTTH and FTTB (fiber), Satellite, UMTS/HSDPA (mobile) and LTE are available as alternatives.
The United Kingdom has been involved with the Internet since it was created. The telecommunications infrastructure provides Internet access to businesses and home users in various forms, including cable, DSL, and wireless. The Internet country code top-level domain (ccTLD) for the United Kingdom is .uk and is sponsored by Nominet.
A modem is a hardware device that converts data between transmission media so that it can be transmitted from computer to computer. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used with any means of transmitting analog signals from light-emitting diodes to radio. A common type of modem is one that turns the digital data of a computer into modulated electrical signal for transmission over telephone lines and demodulated by another modem at the receiver side to recover the digital data.
G.fast is a digital subscriber line (DSL) protocol standard for local loops shorter than 500 m, with performance targets between 0.1 and 1 Gbit/s, depending on loop length. High speeds are only achieved over very short loops. Although G.fast was initially designed for loops shorter than 250 meters, Sckipio in early 2015 demonstrated G.fast delivering speeds over 0.1 Gbit/s nearly 500 meters and the EU announced a research project.
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