[asterisk-users] T38 fax detection using g729

[Kevin P. Fleming]

On 04/14/2011 05:57 AM, Niccolò Belli wrote:
> Il 14/04/2011 12:25, Larry Moore ha scritto:
>> I made a suggestion on how you could check this i.e. have your incoming
>> call go directly to the fax extension, my 1.8.3.2 installation
>> immediately negotiates a T.38 connection in this sceanrio, of course I
>> enabled the fallback option so it will use g711a if T.38 is not accepted
>> by the peer.
>
> Yeah, I know, I just didn't have time to check it. Now I've just
> finished checking and Eutelia DOESN'T send any T.38 re-invite :(
> I hoped it was an asterisk bug because there was a chance it may be
> solved, now that I know it's an Eutelia problem I know it will never be
> fixed.

If you are the receiver of the call (and thus they are the sender of the 
call), it is *your* system's responsibility to initiate the switch to 
T.38, not theirs.

Note that switching to T.38 and 'faxdetect' are two different, but 
related, things. You are correct in that if you are using G.729 for the 
voice path for an incoming call, then detecting the CNG tone from the 
calling FAX machine will probably not be detected 100% reliably. If you 
are going to have a DID be able to accept both voice and FAX calls, and 
discriminate between them based on detection of the CNG tone, then 
you'll probably have to switch to G.711 a-Law in order to have the most 
reliable detection.

If you are getting free phone numbers, though, you'd be far better off 
to just assign a dedicated number for incoming FAX calls and not rely on 
'faxdetect' at all; this would allow you to use G.729 for your voice calls.

-- 
Kevin P. Fleming
Digium, Inc. | Director of Software Technologies
Jabber: kfleming at digium.com | SIP: kpfleming at digium.com | Skype: kpfleming
445 Jan Davis Drive NW - Huntsville, AL 35806 - USA
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