Editing Symmetric digital subscriber line

{{Short description|Digital subscriber line that transmits digital data from the network to the subscriber}}
{{Refimprove|date=December 2008}}

A '''symmetric digital subscriber line''' ('''SDSL''') is a [[digital subscriber line]] (DSL) that transmits digital data over the copper wires of the telephone network, where the [[bandwidth (computing)|bandwidth]] in the [[downstream (computer science)|downstream]] direction, from the network to the subscriber, is identical to the bandwidth in the [[upstream (networking)|upstream]] direction, from the subscriber to the network. This symmetric bandwidth can be considered to be the opposite of the asymmetric bandwidth offered by [[Asymmetric digital subscriber line 2|asymmetric digital subscriber line]] (ADSL) technologies, where the upstream bandwidth is lower than the downstream bandwidth. SDSL is generally marketed at business customers, while ADSL is marketed at private as well as business customers.

More specifically, SDSL can be understood as:
* ''In the wider sense'', an umbrella term for all DSL variant which offer symmetric bandwidth, including [[ISDN digital subscriber line | IDSL]], which offers 144&nbsp;kbit/s, [[HDSL]], [[HDSL2]], [[G.SHDSL]], which offers up to 22.784&nbsp;Mbit/s over four pairs of copper wires, as well as the SDSL variant below
* ''In the narrow sense'', a particular proprietary and non-standardized DSL variant for operation at 1.544&nbsp;Mbit/s or 2.048&nbsp;Mbit/s over a single pair of copper wires, without support for analog calls on the same line
* A term used by [[ETSI]] to refer to [[G.SHDSL]]

== Proprietary SDSL technology ==
SDSL is a rate-adaptive digital subscriber line (DSL) variant with [[T-carrier|T1]]/[[E-carrier|E1]]-like data rates (T1: 1.544&nbsp;[[Mbit/s]], E1: 2.048&nbsp;Mbit/s). It runs over one pair of copper wires, with a maximum range of {{convert|10000|ft|m}}. It cannot co-exist with a conventional voice service on the same pair as it takes over the entire  bandwidth.<ref>
  {{cite web
    | url        = http://www.cisco.com/en/US/docs/internetworking/technology/handbook/DSL_Dig_Subscr_Ln.html#wp1020696
    | title      = Internetworking Technology Handbook
    | publisher  = Cisco Systems, Inc.
    | accessdate = 2008-12-03
  }}</ref>

===Standardization efforts===
SDSL is a [[Proprietary protocol|proprietary]] technology that was never standardized. As such it usually only interoperates with devices from the same vendor. It is the predecessor of [[single-pair high-speed digital subscriber line|G.SHDSL]] which was standardized in February 2001 by [[International Telecommunication Union|ITU-T]] with recommendation G.991.2.<ref>
  {{cite web
    | url        = http://www.itu.int/rec/T-REC-G.991.2/en
    | title      = G.991.2 : Single-pair high-speed digital subscriber line (SHDSL) transceivers
    | publisher  = ITU-T
    | accessdate = 2008-12-04
  }}
</ref> SDSL is often confused with G.SHDSL and [[High bit rate Digital Subscriber Line|HDSL]];<ref>
  {{cite web
    | url        = http://www.blackbox.co.uk/technical/techoverviews/dsl.asp?id=2&tx=Technology%20Overviews
    | title      = Technology Overviews – DSL
    | publisher  = Black Box Network Services
    | accessdate = 2008-12-04
  }}
</ref> in [[Europe]], 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 system|legacy]]. Most new installations use G.SHDSL equipment instead of SDSL.{{fact|date=December 2008}}

===Target audience===
SDSL typically falls between ADSL and T1/E1 in price and was mainly targeted at small and medium businesses which don't need the service guarantees of [[Frame Relay]] or the higher performance of a [[leased line]].<ref>{{cite web | url = http://www.cisco.com/en/US/docs/internetworking/technology/handbook/DSL_Dig_Subscr_Ln.html#wp1020696
| title = Internetworking Technology Handbook
| publisher = Cisco Systems, Inc.
| accessdate = 2008-12-03
}}</ref>

==See also==
*[[ISDN]]
*[[List of interface bit rates]]

== References ==
{{Reflist}}

{{DSL technologies}}

{{DEFAULTSORT:Symmetric Digital Subscriber Line}}
[[Category:Digital subscriber line]]
[[Category:ITU-T recommendations]]