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.

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 connect a Web server.

How ADSL works

On the wire

ADSL uses two separate frequency bands. With standard ADSL, the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication. It is, however, possible to alter this frequency division, but it will cause issues with crosstalk.

Frequency plan for ADSL

Each of these is further divided into smaller chunks of 4.3125 kHz. During initial training, the ADSL modem tests which of the available chunks have an acceptable signal-to-noise ratio. The distance from the telephone exchange, or noise on the copper wire, may introduce errors on some frequencies. By keeping the chunks small, an error on one frequency thus need not render the line unusable: the chunk will not be used, resulting in reduced throughput on the ADSL connection.There is a direct relationship between the number of chunks available and the throughput capacity of the ADSL connection. The exact data capacity per chunk depends on the modulation method used.

ADSL can use any of a variety of modulation techniques, CAP was the de facto standard for xDSL deployments up until 1996, deployed in 90 percent of xDSL installs. Now it is deprecated in favour of DMT/OFDM modulation schemes.OFDM is used in ADSL connections that follow the G.DMT (ITU G.922.1) standard. (Annex A refers to ADSL-over-POTS).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 connection (using PPPoE or PPPoA) with much higher symbol rates and much faster handshaking between modems.

ADSL standards

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+


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