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Broadband Internet

DSL : Digital Subscriber Line...

DSL or xDSL, is a family of technologies that provide digital data transmission over the wires of a local telephone network. DSL originally stood for Digital Subscriber Loop, although in recent years, many have adopted Digital Subscriber Line as a more marketing-friendly term for the most popular version of DSL, ADSL over UNE.

Typically, the download speed of DSL ranges from 128 kilobits per second (kbit/s) to 24,000 kbit/s depending on DSL technology and service level implemented. Upload speed is lower than download speed for Asymmetric Digital Subscriber Line (ADSL) and equal to download speed for Symmetric Digital Subscriber Line (SDSL).


Digital Subscriber Line technology was originally implemented as part of the ISDN specification.

ADSL technology dates back to 1988, when Joe Lechleider at Bellcore (now Telcordia Technologies) adapted DSL to carry a digital signal over the unused frequency spectrum available on the twisted pair cables running between the telephone company's central office and the customer premises.[1]

United States telephone companies started promoting DSL when cable television companies began marketing broadband Internet access and VoIP telephony. Until this point, DSL service had primarily been provided over a dedicated "dry loop". When the FCC required ILECs to lease lines to DSL providers such as Earthlink, however, a move to shared-line DSL (also known as DSL over UNE) was made to avoid the need for custom installations. With DSL over UNE, an ordinary twisted-pair telephone line is connected to both a POTS-splitter, to provide voice services, and a DSLAM, to provide data services. An inline low-pass filter (sometimes known as a microfilter) keeps the data signal from being audible when the voice circuit is used. When the low-pass filter is not used, an audible humming noise can be heard on the phone. In addition to that, internet speed will appear to slow down a lot.

DSL is the principal competition of cable modems for providing high speed Internet access to home consumers in Europe and North America. Older ADSL standards can deliver 8 Mbit/s over about 2 km (1.25 miles) of unshielded twisted pair copper wire. The latest standard, ADSL2+, can deliver up to 24 Mbit/s, depending on the distance from the DSLAM. [2] Some customers, however, are located farther than 2 km (1.25 miles) from the central office, which significantly reduces the amount of bandwidth available (thereby reducing the data rate) on the wires.

Technical Information

The local loop of the Public Switched Telephone Network was initially designed to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as commercial bandwidth.

At the local telephone exchange (UK terminology) or central office (US terminology) the speech is generally digitized into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore – according to the Nyquist theorem – any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).

The laws of physics - specifically, the Shannon limit - caps the speed of data transmission. By definition, the difference between narrowband data transmission, such as through a dial-up modem connection, and broadband, such as DSL, is that narrowband transmits only one signal at a time. Broadband doesn't change the speed limit of the information highway; it simply adds more lanes.

The local loop connecting the telephone exchange to most subscribers is capable 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 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.

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. Interleafed channel works better for file transfers, where transmission errors are impermissible, even though resending packets increases latency.

The commercial success of DSL and similar technologies largely reflects the fact that in recent decades, while electronics have been getting faster and cheaper, the cost of digging trenches in the ground for new cables (copper or fiber optic) remains expensive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Not long ago, the cost of such signal processing would have been prohibitive but because of VLSI technology, the cost of installing DSL on an existing local loop, with a DSLAM at one end and a DSL "modem" at the other end is orders of magnitude less than would be the cost of installing a new, high-bandwidth fiber-optic cable over the same route and distance.

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 is to use a router that establishes a connection between the DSL modem and a local Ethernet, HomePlug, or Wi-Fi network on the customer's premises.

Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as an Internet service provider.

Dry-loop DSL or "naked DSL," which does not require the subscriber to have traditional land-line telephone service, started making a comeback in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC,[1] and Verizon's merger with MCI,[2] those telephone companies are required to offer naked DSL to consumers.[3]

Even without the regulatory mandate, however, many ILECs would want to offer naked DSL to consumers. The number of telephone landlines in the US has dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has grown to 195 million. [3] Given a choice of customers buying naked DSL versus no sale at all, most telephone companies are offering naked DSL.


The customer end of the connection consists of a DSL modem. This converts data from the digital signals used by computers into a voltage signal of a suitable frequency range which is then applied to the phone line.

In the early days of DSL, installation required a technician to visit the premises. A "splitter" was installed near the demarcation point, from which a dedicated data line was installed. Today, many DSL vendors offer a self-install option, in which they ship equipment and instructions to the customer. In this case, since no changes are made to the cable plant on the customer premises, all the phone wires are carrying both POTS and DSL signal frequencies; therefore the customer generally needs to plug a DSL filter into each telephone outlet. However, this can sometimes cause degradation of the DSL signal (especially if more than 5 analogue devices are connected to the line) because the DSL signal is present on all telephone wiring in the building. A way to circumvent this is to install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer and may not work on some (poorly designed) household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use.

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.


September 14, 2006, 14:27 BST


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Price Comparison of Major UK Broadband Providers

Service Speed Price Limit
BT Broadband 8MB** £9.95* 2GB
BT Broadband 8MB** £14.99* 6GB
BT Broadband 8MB** £22.99* 40GB
AOL Broadband 1MB** £14.99 None
AOL Broadband 2MB** £24.99 None
AOL Broadband 8MB** £29.99 None
NTL Broadband 1MB** £8.99* None
NTL Broadband 4MB** £12.49* None
NTL Broadband 10MB** £17.49* None
Telewest Broadband 2MB** £8.99* None
Telewest Broadband 4MB** £12.50* None
Telewest Broadband 10MB** £17.50* None
Click for more broadband comparisons...
Last Updated: 30 August 2006
*Introductory Rate
**Theoretical connection speed

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Service Speed Price Limit
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