DSL: Exploiting Unused Capacity on Telephone Lines
Standard telephone services are carried on a pair of copper wires coming into a house or office. DSL [=Digital Subscriber Line] technology uses the spare capacity on these phone lines to move data at much higher rates than that offered by older-style analogue modems.
It works because voice (and fax) traffic only occupies a small segment of the lower-frequency spectrum available on copper wires, from 0 to 3,400 hertz [= cycles per second]. DSL uses sophisticated algorithms to package and transmit data across the same wires at much higher frequencies, typically using frequencies above 4Khz. This allows the same wire to carry much more data than it was originally intended for, while allowing the co-existence of voice, fax and data services on the same line, provided filters are used to separate the signals.
The method of sending multiple frequencies down the one line is called 'multiplexing' and is the defining characteristic of 'broadband'. Multiplexing is represented in the figure below, where the blue line represents voice and fax data, and the red line higher-frequency digital data:
What does 'xDSL' stand for?
'xDSL' means 'any kind of DSL' and is used because there are various kinds of DSL service available.
Most of the news is about 'ADSL' which is the type of DSL that connects through an ordinary household telephone service. A is for 'Asymmetric' - faster TO the home than FROM the home. This suits most users, who generally receive more data than they send.
ADSL plans range from 256/64Kbps up to 1500/256Kbps. with the faster plans being generally more expensive. There's intense competition in the market, so prices are constantly shifting. ADSL availability is dependent on the readiness of the local exchange, and your proximity to it, among other factors. Bigpond's availability indicator is here.
Next we'll look at other varieties of DSL either on the market or being planned.