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5 See Also
6 ADSL standards
7 External links
As compared to other forms of DSL, ADSL has the distinguishing characteristic that the data can flow faster in one direction than the other, i.e., asymmetrically. Providers usually market ADSL as a service for people 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 bandwidth in the other direction.
ADSL can use any of a variety of modulation techniques, but the ANSI and ETSI standards use DMT modulation schemes. It is worth noting that in contrast to the modulation schemes that baseband technologies like Gigabit Ethernet use, ADSL uses primarily analogue modulation schemes, so the 'D' in ADSL is a misnomer -- ADSL is simply a very fast analogue dial-up connexion (using PPPoE) with much higher symbol rates and much faster handshaking between modems.
For conventional ADSL, downstream rates start at 256 kbit/s and typically reach 9 Mbit/s (if one is less than 1000 feet from the central office) but can go as high as 52 Mbit/s over short ranges of within 100 metres (so-called VDSL). Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 768 kbit/s. The name ADSL Lite is sometimes used for the slower versions.
A newer variant called ADSL2 provides higher downstream rates (up to 12 Mbit/s for spans of less than 8000 feet). Higher symbol rates and more advanced noise-shaping are responsible for these increased speeds. ADSL2+ boosts these rates to up to 25 Mbit/s for spans of less than 5000 feet.
Because of the relatively low data-rate (compared to optical backbone networks) ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as Web traffic; ATM runs widely over ADSL technology to ensure that this remains a possibility.
ADSL service providers may offer either static or dynamic IP addressing. Static addressing is preferable for people who may wish to connect to their office via a virtual private network, for some Internet gaming, and for those wishing to use ADSL to connect a Web server.
In the United Kingdom, users had to live within 3.5 kilometers of the local telephone exchange to receive ADSL, but the range has grown to 5.5 kilometers thanks to RADSL (Rate Adaptive Digital Subscriber Line), although users with RADSL will have a lower upstream rate.
The typical home ADSL connection in the UK has 512 kbit/s downstream, and 256 kbit/s upstream (it may run slower if the user has RADSL), with a 50:1 contention ratio. Packages designed for offices or businesses have a 20:1 contention ratio and range from 512 kbit/s to 2 Mbit/s in downstream speed.
ADSL Line Providers in Greece: