Carrierless Amplitude/Phase Modulation
(CAP) A design of Asymmetric Digital Subscriber Line transceiver developed by Bell Labs. CAP was the first ADSL design to be commercially deployed and, as of August 1996, was installed on more lines than any other.
CAP is a variation of Quadrature Amplitude Modulation, the modulation used by most existing modems in 1997. With CAP, the three channels (POTS, downstream data and upstream data) are supported by splitting the frequency spectrum. Voice occupies the standard 0-4 Khz frequency band, followed by the upstream channel and the high-speed downstream channel.