V.29
V.29 is an ITU standard for fax transmissions in half- duplex mode. It operates with quadrature amplitude modulation( QAM) with 45-degree and 90-degree phase positions.
At a step rate of 2,400 baud and a transmission of four bits per step, V.29 achieves a transmission rate of 9.6 kbit/s with 8-phase phase shift keying( PSK).
The method is used for telefax and switches down the transmission rate to lower values in case of bad transmission conditions by means of Automatic Fallback( AFB).