PSK
Phase-shift keying: data sent as changes of phase of a single audio tone, packing more bits into each symbol than AFSK does. The packet modems follow the ITU modem standards: 2400 bit/s QPSK as V.26 and 4800 bit/s 8PSK as V.27. V.26 comes in alternatives A and B, and both ends need to agree on which.