The answer is 2 frequencies.
A Full-Duplex QAM 64 K Modem uses two frequencies. One frequency is used for upstream and the other for downstream. A variety of common QAM forms are available and include, 16 QAM, 32 QAM, 64 QAM, and 256 QAM. For example, for domestic broadcast use, 64 and 256 QAM are used for cable modem and digital cable TV. The modulation scheme for this modem uses both amplitude and phase.
