ADSL and VDSL

ADSL, short for Asymmetric Digital Subscriber Line, is a technology that carries high-speed digital data over the ordinary copper telephone wires already installed in homes and businesses. It works by using frequencies above those needed for voice calls, so a single line can carry both a telephone conversation and a data connection at the same time. A small filter, or splitter, separates the voice and data signals so they do not interfere with each other.

The “asymmetric” part of the name refers to the fact that the downstream rate, data flowing toward the user, is faster than the upstream rate flowing back to the network. This suits common activities such as web browsing and downloading, where most data travels toward the user. Achievable speeds fall off as the distance from the telephone exchange increases, because the signal weakens over long copper runs, so subscribers closer to the exchange generally get higher rates.

VDSL, or Very-high-bit-rate Digital Subscriber Line, is a faster member of the same DSL family, and its later version VDSL2 raises speeds further. It achieves higher rates by using a wider range of frequencies, but it is more sensitive to distance, so it is typically deployed where fibre reaches a nearby street cabinet and only a short copper run remains. The term cable modem is sometimes used loosely for the box that terminates a broadband line, though true cable modems run over coaxial cable television networks rather than telephone wiring.

Frequently asked questions

Why is ADSL called asymmetric?

Because the download speed is higher than the upload speed, matching typical home use where more data is received than sent.

Can I use the phone and ADSL at the same time?

Yes. ADSL uses frequencies above the voice band, and a splitter separates the two so calls and data work simultaneously.

How does VDSL differ from ADSL?

VDSL uses a wider frequency range for higher speeds but works only over shorter distances, so it is usually paired with fibre reaching a nearby cabinet.