[asterisk-users] T.38 passthru on 1.8.5

[Karsten Wemheuer]

Hi,

Am Dienstag, den 30.08.2011, 09:44 -0400 schrieb Fabian Borot:
> 
>  Hello
> We want to implement T.38 passthru with asterisk 1.8.5 [Asterisk
> 1.8.5.0 built by root @ asterisk1-8.labdomain.com on a x86_64 running
> Linux on 2011-08-26 21:31:22 UTC]
> 
> The call flow is:
> quintum gateway --> asterisk --> Dialogic IMG 1010
> 
> the call starts as a voice call, the remote fax picks up and we hear
> the fax tone, the we see the re-invite from the IMG asking for t.38,
> the RE-Invite is passed back to the user side [quintum gateway] whcih
> reply with 200 OK with t.38 and the nothing else happens. After 20
> secs of "inactivity" the quintum sends another Invite with voice only
> and then a BYE.
> 
> We do see that the quintum sends a lot of messages like this from the
> quintum's IP [192.168.1.18] but we do not see that asterisk sends the
> packages to the destination
> 
> UDPTL (SIP/2345850624337933): packet to 192.30.189.146:12020 (type 0,
> seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
>  UDPTL (SIP/): packet from 192.168.1.18:10260 (type 0, seq 0, len 6)
> 
> we have this settings on sip.conf
> faxdetect = yes
> t38pt_udptl = yes,maxdatagram=400 [I have tested with several
> combinations t38pt_udptl = yes;t38pt_udptl = yes,fec etc]
> 
> When we send the fax from the quintum to the Dialogic IMG the fax
> works 100% of the times.
> I enabled fax set debug on and udptl set debug on but the console does
> not show almost anything but the udptl packets shown above.
> What else should I do?Any ideas/help is greatly appreciated

I assume there is some NAT/firewall on the way? (The 192.30.189.146 is
public internet and 192.168.1.18 is RFC1918). Be sure to let udptl pass
through your firewall/NAT. Udptl is using a port range of its own
(different from the range used for audio). 

Audio is always symmetric. Audio in one directions implies audio in the
opposite direction. A NAT Gateway open for one direction is normally no
problem for audio. 
T.38 is not symmetric. One side sends and waits for some kind of ACK.
The other side waits for data. If the sending side gets through the NAT
Gateway all is fine. If the sending side is not getting through, the
transmission is aborted (after timeout).

HTH,

Karsten