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.

Modulation

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.