ADSL Modulation
Today there are two competing Asymmetric Digital Subscriber Line (ADSL) modulation schemes which are variants on Quadrature Amplitude-phase Modulation (QAM). Conventional QAM on a carrier uses changes in amplitude and phase to create symbols which can be transmitted at a lower baud rate than the actual data rate. This is an efficient use of available bandwidth and an excellent way to transmit high-speed data over copper wires. ADSL is leveraging the QAM signaling techniques that have been used for many years in wireless communication.
(i) Carrierless Amplitude Pase (CAP) modulation
At start-up, CAP tests the quality of the the line and implements the most efficient QAM. CAP is low-cost due to its simplicity. It is not a standard.
(ii) Discrete Multi-Tone (DMT) Modulation
DMT divides the available frequency bandwidth into 256 subchannels or tones. At start-up the line is tested to determine the best distribution of subchannels to carry data. Typically, to rise above noise, more data resides in the lower frequencies and less in the upper ones. DMT is much more complex and hence initially more expensive than CAP. DMT is faster (x4 downstream, x10 upstream) than CAP at all distances. DMT is an ANSI and ETSI standard.