The Free On-line Dictionary of Computing (30 December 2018):

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)