ADSL: Difference between revisions

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.

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.

There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close together) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telcos chose to make a virtue out of necessity, hence ADSL.

For conventional ADSL, downstream rates start at 256 kbit/s and typically reach 8 Mbit/s within 1.5 km (5000 ft) of the DSLAM equipped central office or remote terminal. Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 1024 kbit/s. The name ADSL Lite is sometimes used for the slower versions.

Note that distances are only approximations. Signal attenuation and Signal to Noise Ratio are defining characteristics, and can vary completely independently of distance (eg. non-copper cabling, cable diameter).

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+, also referred to as ITU G.992.5, boosts these rates to up to 25 Mbit/s for spans of less than 1.5 kilometers (5000 feet). ADSL2+ also offers seamless bonding options, allowing lines with higher attenuation or lower signal to noise (SNR) ratios to be bonded together to achieve theoretically the sum total of the number of lines (i.e. up to 50Mbit/s for two lines, etc), as well as options in power management and seamless rate adaption - changing the data rate used without requiring to resynchronise.

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 host a Web server.

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ADSL standards

Standard name	Standard type	Downstream rate	Upstream rate
ANSI T1.413-1998 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+. phillip davis 4 mellissa o'connor

See also

• DSL around the world
• Digital Subscriber Line for further details and other varieties
• DSL access multiplexer

External links

• UK Broadband Usergroup - Comprehensive news of all DSL activities in the UK
• DSL Reports - Extensive site on broadband with user reports from around the USA
• 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
• Whirlpool - Australian ADSL news, information and forums