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List:       asterisk-dev
Subject:    Re: [asterisk-dev] Proposal for T.38 transparent gateway design in
From:       Steve Underwood <steveu () coppice ! org>
Date:       2010-04-30 1:39:09
Message-ID: 4BDA34BD.90909 () coppice ! org
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On 04/30/2010 04:57 AM, Andreas Sikkema wrote:
> > > Also
> > > SIP-ATA 1<---SIP+T.38--->Asterisk<---SIP+G711---->  SIP-ATA 2
> > > is a valid scenario and should be supported. In this case Asterisk
> > > should reINVITE to G711 if the T.38 reINVITE was rejected by SIP-ATA 2.
> > > 
> > I would say this should *not* be supported; if someone has a FAX machine
> > attached to a SIP ATA, and that ATA does not support T.38, then they
> > should get one that does. Even if we did support this scenario (assuming
> > SIP-ATA1's FAX machine is the caller), the call is likely to not switch
> > to T.38 at all since there is no gateway that will detect the FAX
> > preamble on a TDM channel. If you configured Asterisk to do that, it
> > would detect the preamble from SIP-ATA2 and send a T.38 re-INVITE to
> > SIP-ATA2, not SIP-ATA2.
> > 
> There's a sizable amount of SIP ATA's in the wild that offer the ability to \
> configure an analogue port to "fax" port and therefore start each call from that \
> port on the PSTN side straight with a T.38 offer in the SDP, no re-invite's needed. \
> 
Try setting those ATAs to the "always a FAX" mode and you will find most 
still re-invite to T.38. If they didn't they would be unable to try the 
fallback position of FAXing by G.711 - an unreliable option, but still 
worth a try when T.38 is not an available option. It is rare for 
something to start in T.38 mode. It just doesn't work for a lot of 
devices supporting T.38. They *require* a re-invite to T.38. Even 
terminals incapable of anything but T.38 operation, like t38modem, start 
in audio mode for this compatibility reason.

Steve


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