On Jan 6, 2011, at 8:56 AM, Kevin P. Fleming wrote:

> 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.)

That does make sense: the incoming calls are directed to a FreePBX ring group 
consisting of three IAXModems handled by HylaFAX, and I am fairly certain that 
they answer the call nearly instantaneously. 

I do realize that the detection is really intended for non-dedicated lines; I'm 
just trying to ensure it works before I start using it. 

Thanks for the response.

Tom
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