Symmetric digital subscriber line (SDSL) can have two meanings:

• In the wider sense it is a collection of Internet access technologies based on DSL
  Digital Subscriber Line
  Digital subscriber line 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. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line ,...
  
   that offer symmetric bandwidth
  Bandwidth (computing)
  In computer networking and computer science, bandwidth, network bandwidth, data bandwidth, or digital bandwidth is a measure of available or consumed data communication resources expressed in bits/second or multiples of it .Note that in textbooks on wireless communications, modem data transmission,...
  
   upstream
  Upstream (networking)
  In computer networking, upstream refers to the direction in which data can be transferred from the client to the server . This differs greatly from downstream not only in theory and usage, but also in that upstream speeds are usually at a premium...
  
   and downstream
  Downstream (computer science)
  In a telecommunications network or computer network, downstream refers to data sent from a network service provider to a customer.Although the best voiceband modems are called 56 kbit/s modems, downstream speeds can be limited to a few tens of kilobits per second with even lower upstream speeds...
  
  . It is considered the opposite of asymmetric digital subscriber line
  Asymmetric Digital Subscriber Line
  Asymmetric digital subscriber line is a type of digital subscriber line technology, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice...
  
   (ADSL) technologies where the upstream bandwidth is lower than the downstream bandwidth.
• In the narrow sense SDSL is a particular DSL variant that supports data only on a single line and does not support analog calls.

SDSL technology

SDSL is a rate-adaptive digital subscriber line (DSL) variant with T1

T-carrier

In telecommunications, T-carrier, sometimes abbreviated as T-CXR, is the generic designator for any of several digitally multiplexed telecommunications carrier systems originally developed by Bell Labs and used in North America, Japan, and South Korea....

/E1

E-carrier

In digital telecommunications, where a single physical wire pair can be used to carry many simultaneous voice conversations by time-division multiplexing, worldwide standards have been created and deployed...

-like data rates (T1: 1.544 Mbit/s, E1: 2.048 Mbit/s). It runs over one pair of copper wires, with a maximum range of 10000 feet (3,048 m). It cannot co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth.

Standardization efforts

S-DSL is a proprietary

Proprietary protocol

In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual.-Enforcement:Proprietors may enforce restrictions through patents and by keeping the protocol specification a trade secret...

 technology that was never standardized. As such it usually only interoperates with devices from the same vendor. It is the predecessor of G.SHDSL which was standardized in February 2001 by ITU-T

International Telecommunication Union

The International Telecommunication Union is the specialized agency of the United Nations which is responsible for information and communication technologies...

 with recommendation G.991.2. SDSL is often confused with G.SHDSL and HDSL; in Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, G.SHDSL was standardized by ETSI using the name 'SDSL'. This ETSI variant is compatible with the ITU-T G.SHDSL standardized regional variant for Europe.

As there is a standardised successor available, SDSL installations today are considered legacy

Legacy system

A legacy system is an old method, technology, computer system, or application program that continues to be used, typically because it still functions for the users' needs, even though newer technology or more efficient methods of performing a task are now available...

. Most new installations use G.SHDSL equipment instead of SDSL.

Target audience

SDSL typically falls between ADSL and T1/E1 in price and was mainly targeted at small and medium businesses who don't need the service guarantees of Frame Relay

Frame relay

Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology...

 or the higher performance of a leased line

Leased line

A leased line is a service contract between a provider and a customer, whereby the provider agrees to deliver a symmetric telecommunications line connecting two or more locations in exchange for a monthly rent . It is sometimes known as a 'Private Circuit' or 'Data Line' in the UK or as CDN in Italy...

.