[asterisk-users] Clarification on DAHDI Fax Detection

[Steve Underwood]

On 01/04/2011 04:22 AM, Kevin P. Fleming wrote:
> 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.
That's not true. Modern echo cancellers normally disable completely. It 
is arguable whether they should disable completely for FAX, but they 
need to behave properly for all modems. For any duplex modem, disabling 
only the NLP is useless. They need to cancel end to end, so they don't 
get upset by a continuously adapting canceller, and so they can minimise 
the issues caused by the highly non-linear G.711 channel.
>
>> 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.
>
Steve