Fax machines may be capable of a few different modulations, and within each of these modulations there are different speeds. Here are the common modulations that are used in G3 fax calls.
Common Modulations Used in G3 Fax Calls
FSK (Frequency Shift Keying)
DPSK (Differential Phase Shift Keying)
4800, 7200, 9600
QAM (Quadrature Amplitude Modulation)
7200, 9600, 12200, 14400
TCM (Trellis Coded Modulation)
V.21 is simplest modulation method and operates at 300 bps. All of the T.30 messaging (signaling protocol used by fax machines) uses V.21 modulation. When the actual fax page is transmitted, a higher-speed modulation is used rather than V.21. The V.21 speed of 300 bps is much too slow for the large amount of data that makes up a fax page, so a faster speed is needed. Fax machines always attempt to transmit their pages at the highest possible speeds, and the V.17 standard, with a top speed of 14400 bps, provides the fastest page-transmission speed for a G3 fax call. If both fax machines support V.17, this is the modulation that will be attempted. If the faxes fail to train using V.17 at 14.4 Kbps, the fax devices try the next fastest speed within that same modulation. Training is a process that occurs when fax machines attempt to agree on the modulation that will be used for page transmission.
If V.17 is not supported by one of the fax devices, the sending device tries the next modulation with highest possible speed. Similarly, if all the modulation speeds within V.17 failed to train, the sending device tries another slower modulation type, such as V.29.