[asterisk-users] Intermittent T.38 pass through

[JR Richardson]

Hi All,

I've been testing reliability with t.38 faxing pass through with * 1.4.21.1,
Linksys ATA's 2102 x 2, Sharp UX-B800SE and Cannon ImageClass D880.

cannon> <2102 #1> <SIP> <*> <SIP> <2102 #2> <sharp

Started out with default settings on all devices, configured Asterisk to
handle T.38 pass through, the configuration I believe is solid.  I get
relaiable results faxing from the sharp to the cannon.  I get intermittent
results, 50% success or failure, faxing from cannon to sharp.

The first thing I did was to switch the faxes, so the ATA's and Asterisk
remain the same, just switched the faxes to the oposite end of the path.
Doing this comfirmed the original results, sharp to cannon is reliable,
cannon to sharp is unreliable.

What I observe faxing from sharp to cannon:
path sets up as ulaw RTP, cannon answers, RTP switched to UDPTL, fax
completes

Faxing from cannon to sharp:
path sets up as ulaw RTP, sharp answers, RTP switches to UDPTL only half the
time
when UDPTL is active, fax completes, when the path stays with RTP, fax
always fails

If I understand correctly, Asterisk switches the media stream to UDPTL when
it hears valid fax tones on each side of the path, if it only detects fax
tones on 1 path leg, then it keeps the media path through RTP.  Or is the
mechanism switching to UDPTL in the SIP headers?

So, I adjusted db levels on the FXS ports, higher and lower, no effect.  I
increased jitter, reduced jitter, disabled jitter, no effect.  Ensured echo
can's were off, no effect. Manually set faxes to 14.4bps, ecm off, no
effect.  Even switched telephone cord, no effect.

On these Linksys 2102's, you can predial #99 to force the ATA to enable fax
t.38, this works and is reliable, no RTP is setup, just UDPTL.

So my question is this:  Can I setup Asterisk to only allow t.38 pass
through from these ATA's, without the need to use the #99 in every dial
string from the fax machine?

Thanks.

JR
-- 
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JR Richardson
Engineering for the Masses
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