Here is some information about DSL, but as a user you don't need to master these knowledge to choose the right DSL / Internet service. Sometimes it only brings confusion. The keys to choose right product are price, bandwidth, quality and service.

General Introduction (Wikipedia)

DSL or xDSL is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, but as of 2009[update] the term digital subscriber line has been widely adopted as a more marketing-friendly term for Asymmetric Digital Subscriber Line (ADSL), the most popular version of consumer-ready DSL. DSL can be used at the same time and on the same telephone line with regular telephone, as it uses high frequency bands, while regular telephone uses low frequency.

The download speed of consumer DSL services typically ranges from 384 kilobits per second (kbps) to 20 megabits per second (Mbps), depending on DSL technology, line conditions and service-level implementation. Typically, upload speed is lower than download speed for ADSL and equal to download speed for the rarer Symmetric Digital Subscriber Line (SDSL).

More Detailed Definition (clark-tele.com)

‘DSL’ stands for ‘digital subscriber line’. The term is a general term applied to a variety of different technologies used to achieve ‘broadband’ or high speed digital transmission over 2-wire or 4-wire ‘standard copper’ public telephone network access lines – usually for the purpose of high speed Internet connection. All DSL technology can be subdivided into one of two types:

   SDSL (symmetric digital subscriber line) and
   ADSL (asymmetric digital subsriber line)

The prime difference between ‘SDSL’ and ‘ADSL’ is the speed of transmission in the ‘downstream’ direction (the direction from the network towards the user) – relative to the speed of transmission in the ‘upstream’ direction (te direction from the user towards the network). In SDSL the transmission rate in downstream and upstream directions is the same (i.e. symmetric). In ADSL, the downstream rate of transmission is greater than the upstream bitrate (i.e. asymmetric). The commonest form of DSL is ADSL.

Other types of DSL

As well as SDSL and ADSL, a number of other DSL abbreviations and ‘types of DSL’ have been invented over time. These include: HDSL, XDSL, VDSL. In reality, these are all variants of the basic SDSL and ADSL types of DSL or simply alternative terminology:

HDSL (high speed digital subscriber line) is a particular type of SDSL – usually providing 2 Mbit/s transmission in both downstream and upstream directions

VDSL (very high speed digital subscriber line) is able to operate at very high speed (e.g. up to 50 Mbit/s) over copper cable – but only over short distances. Typically VDSL is used in ‘hybrid’ networks, comprising short copper cable connections from VDSL customer premises to locally placed street cabinets and then by means of glass fibre to the network operator’s exchange building site (this type of hybrid network is sometimes referred to as ‘fibre-to-the-curb’ (FTTC)).

XDSL is sometimes used as a generic term to mean ‘any type of DSL’. The ‘X’ stands in place of a letter making up a recognised DSL abbreviation. Thus XDSL may be used as a shortform to mean ‘any of: ADSL, HDSL, SDSL, VDSL etc.)

What is needed for DSL connection?

When DSL / ADSL is operated on a 2-wire analogue telephone (standard telephone) or ISDN line (e.g. BRI or ISDN2), the telephone service operates normally in the ‘baseband’ of the connection. The high speed data connection provided by ADSL uses only the high frequency signal transmission capabilities of the connection. To keep the the telephone/ISDN and ADSL services apart, high frequency filters or splitters are used at both ends of the user’s connection. At the customer’s premises, the splitting device is called simply a DSL splitter or DSL filter. In addition to the DSL splitter or DSL filter, the ADSL customer must provide a DSL modem (most devices sold as DSL modems are actually a combination of DSL modem, router and firewall) to plug into the DSL spliter or filter. The customer’s computer or LAN (local area network) is then connected to the DSL modem using either standard ethernet (10/100baseT with an 8-pin RJ-45 connector) or using WLAN (wireless LAN).

Want more information?

For those who want even more information on DSL technology, here are some links might be helpful.

• Wikipedia - Digital Subscriber Line
• clark-tele.com - Guide to Broadband and DSL
• HowStuffWorks - DSL

P.S. text partially from Wikipedia; clark-tele.com.

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