Re: flexfax: Receiving fax only

[David Woolley <david@djwhome.demon.co.uk>]

> of things. - When trying to call the fax number using a regular
> telephone one can hear a lot of noise before the normal fax interaction
> appears to start. In other words, the modem starts sounding like a fax
> only after tried to send some other kind of data for a while.

This is not a valid test.  You need to generate fax calling tone from
the calling end for a fax modem to autodetect fax mode; in fact, I'm
pretty sure that that is the only difference between a fax call and a
300 bps data call.  If you don't send calling tone followed by V.21
originate carrier, the modem will probably go into a V.34 initialisation
sequence, and when that fails, eventually fall back to V.21 data mode.

Roughly the protocol is something like:

caller sends fax calling tone
callee send 2100Hz echo suppressor/answer tone, followed by V.21 (300bps)
carrier
conversation at 300 bps
caller sends high speed training sequence (e.g. V.29, 9600 bps half duplex)
somehow callee confirms successful train (may be by secondary channel)
caller sends fax data using V.29 etc.
caller and callee switch back to V.21 for end of page
cycle repeats

The first V.21 session is the infamous Phase B and the second is Phase D.
The V.29 portion is Phase C.

As a result, you will get rather a lot of different tone types in a valid
fax call.