[asterisk-users] Clarification on DAHDI Fax Detection

[Kevin P. Fleming]

On 01/03/2011 11:26 AM, Tom Rymes wrote:
> Hi folks,
>
> I was hoping that someone might be able to help clarify some confusion I
> have on DAHDI Fax detection after spending some time searching. My
> understanding is this:

I'll try.

>
> 1.) Echo cancellation is automatically disabled upon recognition of a
> CNG tone, regardless of the faxdetect setting. This can only be disabled
> at compile time.

No. CNG tone is never used to affect the state of an echo canceller. All 
G.168 compliant echo cancellers will respond to the CED tone (generated 
by the answering endpoint) and will reconfigure the echo canceller 
appropriately. Most modern ECs will *not* be disabled, but will enter a 
'linear' mode where they can do some echo suppression but not complete 
cancellation. DAHDI will detect CED when most software echo cancellers 
are in use and will disable them (since none of the available software 
ECs can go into linear mode). The Digium HPEC software EC will detect 
CED on its own and enter linear mode.

> 2.) faxdetect=incoming will, upon detection of a CNG tone, send the call
> to the fax extension.

If the CNG tone arrives from the network side of the DAHDI channel (the 
far endpoint), then yes.

> 3.) faxdetect=outgoing will ??????

The same thing, but if the CNG tone is being sent towards the DAHDI 
channel (from the near endpoint). This is rarely used.

> Also, do Digium cards with HW Echo Cancellation detect the CNG tones in
> hardware? If so, how does the faxdetect setting in DAHDI affect that
> behavior?

No, none of the Digium HW ECs detect and report CNG tones via the DSP; 
CNG tone detection is still done on the host CPU. 'faxdetect' is not set 
in DAHDI, it's set in chan_dahdi.conf.

-- 
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