From Wikipedia, the free encyclopedia.
Digital Subscriber Line, or DSL, refers to a family of technologies that provide a digital connection over the copper wires of the local telephone network. Its origin dates back to 1988, when an engineer at Bell research lab devised a way to carry digital signal over the unused frequency spectrum. This allows ordinary phone line to provide digital communication without blocking access to voice services. Bell's management, however, were not enthusiastic about it, as it was not as profitable as renting out a second line for those consumers who preferred to still have access to the phone when dialing out. This changed in the late 90s when cable companies started marketing broadband Internet access. Realising that most consumers would prefer broadband Internet to a second dial out line, Bell companies rushed out the DSL technology that they had been sitting on for the past decade as an attempt to slow broadband Internet access uptake, to win market share against the cable companies.
As of 2004, DSL provides the principal competition to cable modems for providing high speed Internet access to home consumers in Europe and North America.
How it works
When the telephone system was designed, it was intended for voice calls and, for reasons of economy, it was designed to transmit over a frequency range from around 300 Hz to 3 to 4 kHz (although human speech can range up to around 15 kHz, it can easily be understood if limited to this range).
However, the local loop of copper telephone connections to individual subscribers can usually communicate using a much wider range of frequencies, ranging from these low frequencies up to 200 - 800 kHz, depending on the quality of the circuit and the sophistication of the equipment.
DSL services were devised as a means of overcoming the restriction on the amount of data that could be transmitted over the "voiceband" (see Shannon capacity), by using these additional frequency ranges.
While DSL services often reserve the 0.3 - 4 kHz band for voice calls over the "plain old telephone service" (POTS), the frequencies above and below this range can be used to transmit data.
A DSL connection takes place between equipment of the subscriber and the telephone exchange, with some other protocol used between the exchange and whoever the subscriber really wants to connect to, typically an Internet service provider. This differs from a normal telephone connection, where the public telephone network links subscribers to each other. The quality of service however falls when the distance between subscriber and telephone exchange exceed 5.5 kilometers.
The subscriber end of the connection consists of a DSL modem. This converts data from the digital electronic pulses used by computers into a digital audio stream of a suitable frequency range for the particular DSL variant in use.
In addition the subscriber may need to install a passive electronic filter (known variously as a "filter", "micro-filter" or a "splitter") if using the POTS service on the same line (and possibly also to improve the DSL termination and prevent echoes). This ensures that the DSL modem and the telephone only receive the frequencies they are designed to handle. Subscribers can plug a filter into an existing telephone socket when using a "wires-only" service; or alternatively the DSL provider may install it.
At the exchange a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. It also separates out the voice component.
Protocols and configurations
Many DSL technologies implement an 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 PPP over Ethernet or ATM (PPPoE or PPPoA, also known as PPPoATM), while authenticating with a userid and password and using PPP mechanisms to provide network details.
The reach-restraints (line length from Central Office to Subscriber) reduce as data rates increase, with technologies like VDSL providing short-range links (typically "fibre to the curb" network scenarios).
Example DSL technologies (sometimes called xDSL) include:
- ADSL (Asymmetric Digital Subscriber Line)
- HDSL (High Bit Rate Digital Subscriber Line)
- RADSL (Rate Adaptive Digital Subscriber Line)
- SDSL (Symmetric Digital Subscriber Line, a standardised version of HDSL)
- VDSL (Very high speed Digital Subscriber Line)
- G.SHDSL (ITU-T Standardised replacement for early proprietary SDSL)
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