20-10-2012, 03:30 PM
Symmetric Digital Subscriber Line (SDSL)
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(SDSL) is a Digital Subscriber Line
Symmetric Digital Subscriber Line (SDSL) is a Digital Subscriber Line
(DSL) variant with E1-like data rates (72 to 2320 Kbit/s). It runs over one pair of copper wires, with a maximum range of about 3 kilometers or 1.86 miles. The main difference between ADSL and SDSL is that SDSL has the same upstream data rate as downstream (symmetrical), whereas ADSL always has smaller upstream bandwidth (asymmetrical). However, unlike ADSL, it can't co-exist with a conventional voice service on the same pair as it takes over the entire bandwidth. It typically falls between ADSL and T-1/E-1 in price, and it is mainly targeted at small and medium businesses who may host a server on site, (e.g. a Terminal Server or Virtual Private Network) and want to use DSL, but don't need the higher performance of a leased line.
SDSL was never properly standardized until Recommendation G.991.2 (ex-G.shdsl) was approved by ITU-T. SDSL is often confused with G.SHDSL; 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.
SDSL equipment usually only interoperates with devices from the same vendor, though devices from other vendors using the same DSL chipset may be compatible. Most new installations use G.SHDSL equipment instead of SDSL
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Asymmetric Digital Subscriber Line (ADSL)
Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voice band modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. A splitter - or micro filter - allows a single telephone connection to be used for both ADSL service and voice calls at the same time. Because phone lines vary in quality and were not originally engineered with DSL in mind, it can generally only be used over short distances, typically less than 3mi (5.5 km) [William Stallings' book].
At the telephone exchange the line generally terminates at a DSLAM where another frequency splitter separates the voice band signal for the conventional phone network. Data carried by the ADSL is typically routed over the telephone company's data network and eventually reaches a conventional internet network. In the UK under British Telecom the data network in question is its ATM network which in turn sends it to its IP network IP Colossus.
Explanation
A gateway is commonly used to make an ADSL connection. The modem in the picture is also a wireless access point, hence the antenna. The distinguishing characteristic of ADSL over other forms of DSL is that the volume of data flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market ADSL as a service for consumers to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require high speed in the other direction.
Updating ADSL and DSL entries
CarlosRibeiro 17:23, 21 October 2006 (UTC). I've dropped a note at the DSL history. I moved some notes on DSL installation that applied specifically to ADSL here. It refers to some history on the usage of splitters, that is not longer absolutely required but is still of interest, both historically and also to understand some practical aspects of the technology.