Source : Free On-Line Dictionary of Computing

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.
     
        (1997-10-08)