Science Fair Project Encyclopedia
Asymmetric Digital Subscriber Line
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 analog modulation schemes, so the 'D' in ADSL is a misnomer -- ADSL is simply a very fast analog dial-up connection (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 within 1000 feet (300 m) of the central office. Rates can go as high as 52 Mbit/s within 100 meters (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 of up to 12 Mbit/s for spans of less than 2.5 kilometers (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 1.5 kilometers (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.
|Standard name||Standard type||Downstream rate||Upstream rate|
|ANSI T1.413 Issue 2||ADSL||8 Mbit/s||1.0 Mbit/s|
|ITU G.992.1||ADSL (G.DMT)||8 Mbit/s||1.0 Mbit/s|
|ITU G.992.2||ADSL Lite (G.Lite)||1.5 Mbit/s||0.5 Mbit/s|
|ITU G.992.3/4||ADSL2||12 Mbit/s||1.0 Mbit/s|
|ITU G.992.3/4 Annex J||ADSL2||12 Mbit/s||3.5 Mbit/s|
|ITU G.992.5||ADSL2+||24 Mbit/s||1.0 Mbit/s|
|ITU G.992.5 Annex L||ADSL2+||24 Mbit/s||3.5 Mbit/s|
Additionally, the non-Annex ADSL2 and ADSL2+ support an extra 256 kbit/s of upstream if the bandwidth normally used for POTS voice calls is allocated for ADSL usage.
The downstream and upstream rates displayed are theoretical maximums. Note also that because DSLAM and ADSL modems may have been implemented based on differing or incomplete standards some manufacturers may advertise different speeds. For example, Ericsson has several devices that support non-standard upstream speeds of up to 2 Mbit/s in ADSL2 and ADSL2+.
- ADSL around the world
- Digital Subscriber Line for further details and other varieties
- DSL access multiplexer
- ADSL - a short article on the basics of ADSL
- DSL Forum - Promotional trade organization for the ADSL industry
- adslguide.org.uk - A beginners guide to ADSL and an independent guide to UK ADSL providers
- SpeedTouch - ADSL modems
- Video: Connecting an ADSL modem to a Wi-Fi modem
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