Asymmetric Digital Subscriber Line
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 modem can provide.
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2 ADSL in various countries 3 See Also 4 ADSL standards 5 External links |
Introduction
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.
ADSL in various countries
USA
In the USA, many different kinds of DSL services are offered by different companies:
- VDSL is offered by Qwest and is currently used to deliver video services as well as 1.5M/256K internet.
- ADSL is offered by a variety of providers, usually the same providers who offer voice services. The most popular variant is RADSL. Many providers, such as SBC, offer 3.0M/256K service for home use for around $49, including the ISP. Many have reported that SBC's service actually connects at 6.0M/256K. Others, such as Qwest, offer 1.5M/896K service for around $33.
- ADSL pricing is almost universially flat-rate. However, some ISPs charge for the bandwidth used.
- In many newer locations in the US, the DSLAM is actually installed in the local interface box because many neighborhoods have a fiber uplink to the CO (no copper exists between the CO and the house).
- 256K/256K DSL is often priced as a "dial-up replacement" service. Typical cost is around $25, including ISP, making the service particularly competitive with higher-priced dial-up ISPs. In many cases, this service is not "always on" and a user must initiate a PPPoE connection to connect to te internet.
- 7M/1M DSL for business use is available from most providers. Cost is typically around $250 a month, plus ISP fees.
- Cable Internet is far more popular than DSL in the US. DSL has faced implementation problems due to line length and quality. Cable internet generally offers far higher (albeit shared) bandwidth and is easier to implement.
Switzerland
In Switzerland there are different types of ADSL and line speeds. There is ADSL for Home use with speeds from 768 kbit/s downstream and 128 kbit/s upstream up to 3072 kbit/s downstream and 256 kbit/s upstream. The Business offer consists of three types, ranging from 768 kbit/s downstream and 640 kbit/s upstream, to the biggest version with 3072 kbit/s downstream and 640 kbit/s upstream. The ADSL pricing in Switzerland is flat.ADSL Providers in Switzerland:
- Ticinocom Ltd. - http://www.ticinocom.com
- Bluewin Ltd. - http://www.bluewin.ch
Since there are many providers you can visit: http://www.enter-adsl.ch
UK
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.
Greece
ADSL in Greece appeared commercially in the summer of 2003.
ADSL Line Providers in Greece:
ADSL Internet Providers are many.A specialised Greek website exists for ADSL technology in Greece: http://www.adslgr.com
Finland
First ADSL line providers started in 2000. A typical ADSL connection in Finland is 512/512 or 1024/512 and it costs 50-60 euros.
Some ADSL Line Providers:
- Sonera
- Saunalahti
- Finnet
Iran
ADSL in Iran appeared in January 2004.
Spain
ADSL arrived to Spain in 2000. The main companies are:
- TelefÃÂónica
- Terra
- Ya.com
- Wanadoo
- Tiscali
Australia
ADSL became available in Australia in 2000. The infrastructure is owned by Telstra, whose retail branch Bigpond was the only reseller until early 2002. Since then, other companies are:Speeds available are 256/64, 512/128 and 1.5/256. 512/512 is available but at quite a premium cost. Most ISPs have data caps or some sort of traffic shaping after a certain data limit.
In 2003, Internode began experimenting installing their own DSLAM equipment into Telstra exchanges.
See Whirlpool for more information about Australian broadband.
See Also
ADSL standards
External links