[asterisk-users] Clarification on DAHDI Fax Detection

[Kevin P. Fleming]

On 01/03/2011 07:08 PM, Steve Underwood wrote:
> 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.

This doesn't match up with what the manufacturers of the two G.168 ECs 
that Digium distributes have told me personally about their products. 
Their ECs behave differently for FAX and 'regular' modems, but they do 
that based on the detection of the V.21 preamble, ANSam and other 
signals in addition to CED, which seemed to be much more detail than was 
warranted in my response to the OP :-)

>>
>>> 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
>
>
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-- 
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
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