‘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:


4    SDSL (symmetric digital subscriber line) and

4    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.



         upstream and downstream flows: the difference between ADSL and SDSL



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:


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

4    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)).

4    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.)