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<h2>Digital subscriber line</h2><div class="wp-caption alignright" style="width: 310px"><a href="http://static.ddmcdn.com/gif/dsl-new.jpg" target="_blank"><img title="Digital subscriber line" alt="Digital subscriber line" src="http://static.ddmcdn.com/gif/dsl-new.jpg" width="300" height="270" /></a> </div><div class="wp-caption alignright" style="width: 310px"><a href="http://www.tvdictionary.com/sample_diagrams/ag_Signal_Ingress_low_res.jpg" target="_blank"><img title="Digital subscriber line" alt="Digital subscriber line" src="http://www.tvdictionary.com/sample_diagrams/ag_Signal_Ingress_low_res.jpg" width="150" height="135" /></a> </div><div class="wp-caption alignright" style="width: 310px"><a href="http://www.btcpl.net/Assets/ContentGraphics/adsl.gif" target="_blank"><img title="Digital subscriber line" alt="Digital subscriber line" src="http://www.btcpl.net/Assets/ContentGraphics/adsl.gif" width="150" height="135" /></a> </div>
Digital subscriber line is a family of technologies that provide Internet access by transmitting digital data over the wires of a local telephone network. 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. 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.
The bit rate of consumer DSL services typically ranges from 256 kbit/s to 40 Mbit/s in the direction to the customer, 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.
The theoretical foundations of DSL, like many other forms of communication technology, can be traced back to Claude Shannon's seminal 1948 paper: A Mathematical Theory of Communication. An early 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 motivation for digital subscriber line technology was the Integrated Services Digital Network 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 exchanges and customers on conventional twisted pair cabling facilities. 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 facilityâ€"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.
Early DSL service required a dedicated dry loop, but when the U.S. Federal Communications Commission 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 then rewired to interface with hardware supplied by the ILEC which combines a DSL frequency and POTS frequency on a single copper pair facility.
By 2012 some carriers reported steadily declining numbers of DSL users.
Telephones are connected to the telephone exchange 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 wasn't 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.
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 error containing packets is acceptable.
Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that 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 naked 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 and known as a ULL service in Australia. 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 landlines in the US dropped from 188 million in 2000 to 115 million in 2010, while the number of cellular subscribers has grown to 277 million. 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.
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.
Modern DSL gateways have more functionality and usually go through an initialization procedure very similar to a PC boot up. The system image is loaded from the flash memory; the system boots, synchronizes the DSL connection and establishes the IP connection between the local network and the service provider, using protocols such as DHCP or PPPoE. The system image can usually be updated to correct bugs, or to add new functionality.
The accompanying figure is a schematic of a simple DSL connection. The right side 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.
At the exchange, a digital subscriber line access multiplexer 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 adaptor or "DSL modem". This converts data between the digital signals used by computers and the voltage signal of a suitable frequency range which is then applied to the phone line.
In some DSL variations, 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.
Many DSL technologies implement an Asynchronous Transfer Mode 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 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 mechanisms to provide network details..
Transmission methods vary by market, region, carrier, and equipment.
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