On 01/05/2011 08:12 PM, Thomas Rymes wrote:
OK, after my last message about fax detection, I feel a bit better informed and 
able to press forward. I started looking into this because I was getting lots 
of false positive fax detection errors in the logs with faxdetect=both set in 
chan_dahdi.conf.

Anyhow, I do not currently use fax detection, and we have a dedicated Fax DID 
on our PRI, so setting faxdetect=no works fine. Having said that, I would like 
to sort it out as I may want to use fax detection in the future. Unfortunately, 
I seem to be having odd results. I set faxdetect=incoming last night and 
restarted dahdi and asterisk. Since that time, we have received 17 faxes, but I 
only have three fax detections in my asterisk log, so far as I can tell:

# grep -i fax /var/log/asterisk/full
[Jan  5 05:53:39] NOTICE[6686] chan_dahdi.c: Fax detected, but no fax extension
[Jan  5 10:24:27] NOTICE[11834] chan_dahdi.c: Fax detected, but no fax extension
[Jan  5 11:48:52] NOTICE[13804] chan_dahdi.c: Fax detected, but no fax extension

All three calls listed are indeed fax calls, and since there is no fax 
extension in that context, the call just proceeds along as if nothing happened 
(which is appropriate).

My question is this: If I have received 17 faxes since enabling fax detection, 
shouldn't I see ~17 entries in the log?

How are you delivering the inbound FAX calls to your FAX machine? If you are sending them back out a DAHDI channel (to an FXS port on an analog card, for example), then as soon as the two channels are bridged the audio never comes up to Asterisk (under normal circumstances), it stays in DAHDI, so the Asterisk DSP can't detect the CNG tone. If the FAX machine answers the incoming call fairly quickly, there may not be any opportunity for the CNG to be detected. In addition, you may not be even receiving any audio from the calling FAX machine until you answer the incoming channel (depending on your PRI provider).

If you want to have the best chance to detect each incoming FAX using the Asterisk DSP, you'll have to answer the incoming channel as soon as it hits the dialplan, then wait 3 or 4 seconds, then send the call onwards to your actual FAX machine. FAX detection is really expected to be used on calls that would otherwise be answered by a non-FAX endpoint (IVR, voicemail, user with a phone, etc.)

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Kevin P. Fleming
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