[Freeswitch-dev] mod_fax

Stéphane Alnet stephane at shimaore.net
Wed Jan 7 17:32:37 PST 2009

> In RFC2833, support for relaying modem connect tones (CNG, [/]ANS[am] and
> co) is optional, and relatively few implementations support them.
> Also CNG and ANS are not generated or generated incorrectly by many fax
> machines, you need to look for V.21H HDLC flags, or even T.30 data in the
> HDLC frames, to know for sure that it's a fax.

Alright, I guess that from an application perspective, developers
generally are not really trying to do fax detection. They are only
trying to differentiate between a "human" and a fax machine (or
So I guess "fax detection" in that relaxed sense can be summarized as:
- on an attached audio channel (AUDIO_MODE), did we detect CNG, ANS*,
or anything that would make the software switch to fax-relay mode -- I
would assume there's some kind of event that spandsp generates in that
- on a SIP/H.323 channel supporting T.38, did a switch-over to T.38 occur.
- (any other channel mode?)

(Not that I want to drag the discussion any further if nobody cares about it.)

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