asterisk users - Jan 2011 - Too Few Fax Detections

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?

Assuming the answer to that question is yes, what might be causing the system to
not detect faxes on the other 14 calls?

Many thanks,

Tom

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.)

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
skype: kpfleming | jabber: kfleming at digium.com
Check us out at www.digium.com & www.asterisk.org