ADSL: Standards

Discrete Multitone (DMT)
DSL has two main standards: Discrete Multitone (DMT) and
Carrier-less Amplitude/Phase (CAP).

John Cioffi developed DMT. This standard is the most modern
standard used in most DSL modems today. It utilizes the
telephone signal in a very efficient manner. DMT separates
a single telephone channel in to 247 channels, each with
4KHz. As Data propagates through the channel the DMT looks
ahead to ensure data propagate on stable channels. It is
common for data to change channels several times in DMT.
DSL is a mode of Internet connectivity that uses the same
telephone wires that carry voice and is setup to allow
voice conversation while surfing the internet. Though DMT
is a very efficient standard for signal propagation, it is
prone to cross channel interference. To fix this DSL access
providers install a Low-Pass filter at the wall jack of
every phone not connected to the DSL modem. The low-pass
filter blocks any frequency above 4KHz from entering phone
conversations.

Carrier Amplitude/Phase (CAP)

CAP is an older standard used in earlier DSL installations.
This standard worked by taking the single telephone signal
and dividing it into three bands. The first band was for
voice and had a range of 0KHz to 4KHz, upstream data is
carried in the band of 25KHz 160KHz, and downstream data is
carried in the band of 240KHz and up. This standard
provided protection against cross-band interference by
placing huge gaps in between the frequencies of the
different bands. However, the CAP standard did not provide
enough flexibility to ensure a clear, stable channel for
signal propagation. In other words, since the signal was
separated to there bands there were only three bands for
data propagation. If the upstream, or downstream channels
were degraded propagation would be impossible. The only
advantage CAP has over DMT is its ease of installation but
its lack of flexibility has given rise to the dominance in
use of DMT.