[Freeswitch-users] t38_gateway self - no re-Invite?

[Helmut Kuper]

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Hi Clemens,

Am 22.02.2011 13:31, schrieb Clemens Ebentheuer:
> Thx Helmut,
> 
> I´ve allready tried this:
>> switch_ivr_tone_detect_session(session, "t38", "2100", "rw", timeout,
>> 1,
>> data, NULL, t38_gateway_start)
>>
>> The recompile mod_spandsp and reload it into FS.
> 
> But no luck, I´ll try different frequencies.
> 
> Is it because FS doesn´t recognize it as a t38 fax? 

Yes, no ReINVITE from FS means no detection of CNG or CED.

AFAIK t38 works like this:
a) establish a call to FAX
b) exchange CNG and CED via G.711
c) Listen to CNG and/or CED and when present, switch to t.38 via ReINVITE

More about FAX process is here: http://telecom.tbi.net/fax-call.htm

It can be, that in your case the device is simply detecting the FAX
tones itself before FS does. When the device sends a ReINVITE to FS FS
has no chance to detect CNG anymore and hence never switch to t38.

Just trace the sending FAX-Device for SIP-Signalling and you should se a
ReINVITE from FAX-device to FS, if this is true.

You should check your t38-Device configuration. See if it is able to let
the t38 fax detection job only by FS.

Establish a call from the Fax device to a phone. Can your hear the CNG
signal when you pickup the phone? Does it suddenly stop? If so, the FAX
device or FS/mod_spandsp has sent a t38-ReINVITE.

Do you see something like "media Bug removed" in FS console (DEBUG
level) after 20 seconds of listening to 1100Hz? If so, then FS failed to
detect CNG, hence it never received it (or for too short to detect it).

On my Grandstream ATAs this wasn't possible, so I had to search for a hack.

Hope this will light up your problems a little bit.


> If so, I´m wondering why rxfax in t38 mode is working fine then - with re-invite.
Can't tell you with this, never tried that. sorry.

regards
helmut
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