[asterisk-users] T38 fax detection using g729

[Larry Moore]

On 14/04/2011 10:16 PM, Niccolò Belli wrote:
> Il 14/04/2011 14:34, Larry Moore ha scritto:
>> allow=alaw,g729 ; alaw required for T.30 facsimile if T.38 fails to
> I didn't understand the point. If you enable both alaw and g729 it will
> simply use the preferred one: if it's g729 tone based detection will
> still not work, if it's alaw I will still use that bandwidth expensive
> codec. Please explain me because I can't get the point.
>

The point is simple, fax tone detection and T.30 signalling will require 
a high bandwidth codec, g729 will not work, there are plenty of articles 
out there on this.

Having the g729 codec allows you to have it in your handset 
configuration as the only codec and thus your voice conversations will 
use the lower bandwidth codec.

In my simple observations I have observed T.38 bandwidth consumption at 
around 5Kbps and 10Kbps, the exact reason for 10Kbps I have not 
concluded upon however, a g711a voice conversation consumes around 
10Kbps bandwidth with a packetisation time of 20ms.

Another option to reduce the amount of data traversing the internet is 
to enable VAD, it appears as though this may now work in Asterisk 1.8. I 
had experimented some time back in 1.6 with this hence in sip.conf I have;

directmedia=yes
directrtpsetup=yes

in my general section so they are active in my extensions used by 
handsets and I explicitly disable them for an extension that has a 
fax-modem attached to an ATA.  for a Voice call the RTP stream gets 
established directly to my VoIP providers system and if I want to send a 
fax from a fax-modem, whether or not T.38 is negotiated the RTP stream 
goes through Asterisk.

Larry.