Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

G3 Modulations

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

ITU Standard Speeds (bps) Modulation Type
V.21300FSK (Frequency Shift Keying)
V.27ter2400, 4800DPSK (Differential Phase Shift Keying)
V.294800, 7200, 9600QAM (Quadrature Amplitude Modulation)
V.177200, 9600, 12200, 14400TCM (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.