Science Fair Project Encyclopedia
Digital Subscriber Line
Digital Subscriber Line, or DSL, is 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 a digital signal over the unused frequency spectrum. This allows an 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 1990s 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.
How it works
The Public Switched Telephone Network was initally designed to carry POTS calls, as the concept of data communications as we know it today did not exist. For reasons of economy, the system passes audio between 300 and 3,500 Hz, which is regardrd as the range required for human speech to be clearly intelligible. Dial-up services using modems are constrained by the POTS channel's Shannon capacity, which indicates the maximum data rate which can be supported by a given amount of bandwidth.
The local loop connecting the central office to most subscribers is capable of carrying frequencies well beyond the 3.5 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be as high as the tens of Megahertz. DSL takes advantage of this unused part of the circuit by creating 4.3 kHz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor. The pool of usable channels is then split into two groups for upstream and downstream traffic based on a preconfigured ratio. 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 or not they are usable.
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 bit-rate Digital Subscriber Line)
- G.SHDSL (ITU-T Standardised replacement for early proprietary SDSL)
- ANSI Working Group T1E1.4, a standards group for DSL
- DSL Forum, a promotional trade organization for the ADSL industry
- DSL HOWTO for Linux
- Howstuffworks.com; "How DSL Works"
- DSL vs. Cable Modem Service Compares the advantages and disadvantages of DSL and cable modem Internet access.
- Broadbandreports - Broadband information, forums, help, and ISP ratings
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