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Asymmetric Digital Subscriber Line

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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.

Table of contents
1 Introduction
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 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 (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/2.5 kilometers). 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/1.5 kilometers.

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

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. In 2004, iiNet announced a very large rollout of DSLAM equipment nationally - the company hopes to have half of its ADSL customer base on its own DSLAMs by the end of 2005.

See Whirlpool for more information about Australian broadband.

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:

France

As of 2004, the French ADSL market is evolving in two directions: cheaper access for moderate speeds (512 kbit/s download), as well as access to higher speeds (2 Mbits/s) and associated services (such as television, or free phone communications through VoIP).

The largest operator is Wanadoo. Free has gained ground with its aggressive offer of 30€ a month for 2 Mbits/s (often, 4 Mbits/s in practice) broadband access, television, and free phone calls to French land lines.

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

Iran

ADSL in Iran appeared in January 2004. Still long way to go...

Latvia

ADSL services in Latvia are provided by Lattelekom. Majas DSL, service for home users, is very popular. It have common speed 256/64 and costs about 30 euros per month.

Mexico

ISPs that provide ADSL:

New Zealand

ADSL was launched in 1999 by Telecom New Zealand under the name Jetstream. There was a progressive roll out into local exchanges.

Poland

The most popular ADSL service for home users in Poland is Neostrada provided by TPSA.

Spain

ADSL arrived to Spain in 2000. The main companies are:

The common speed is 256/128, and it costs 39 euros.

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:

The infrastructure is owned by Swisscom Ltd. - http://www.swisscom.com, the hard- and software (DSLAM's) are produced and maintained by Alcatel (Schweiz) Ltd. - http://www.alcatel.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 6.0 kilometers thanks to RADSL (Rate Adaptive Digital Subscriber Line), although users with RADSL may have a reduced upstream rate. BT has promised an increase in the range of ADSL from the Autumn of 2004.

Typical consumer-orientated packages include a 256kbit/s, 512kbit/s, 1Mbit/s or 2Mbit/s downstream paired with a 256kbit/s upstream. The majority of home users choose 512/256 upstream/downstream packages with a 50:1 contention ratio, typically costing between 20 and 30 pounds per month. Packages designed for offices or businesses generally have a 20:1 contention ratio.

Most exchanges, local loops and backhauls are owned and managed by BT, with a choice of companies providing Internet peering, telephone support, billing and other added features. Several companies are also operating their own services using unbundled local loops, notably Bulldog Communications in the London area. A major resource for UK broadband information is the ADSLGuide, located at http://www.adslguide.org.uk/.

Since 2003 BT has been introducing SDSL capabilities to exchanges in many of the major cities. SDSL is Symmetric, and services are currently offered at download speeds of 256kbits/s, 512kbits/s, 1Mbit/s or 2Mbit/s. Unlike ADSL, which is typically 256kbits/s upload, SDSL upload speeds are the same as the download speed.

USA

In the USA, many different kinds of DSL services are offered by different companies:

See also

ADSL standards

External links