[cisco-voip] IOS version for t.38?

[Andre Beck]

Hi Christopher,

On Mon, Dec 08, 2008 at 01:22:16PM -0800, Christopher wrote:
> Hello...
> 
> Summary: I am trying to implement t.38 for receiving faxes.  It doesn't
> work and I suspect the IOS version I am using.  What is a "known good
> version" for t.38?
> 
> Details:
>    2821 router
>    Version 12.4(15)T5  #   T train
>      c2800nm-advipservicesk9-mz.124-15.T5.bin
>    PRI incoming (2)
>    GigE to local Call Manager servers
>    4 Voice FXS interfaces

I've done my testing on essentially the same hardware, but running the
latest IOS that was there at the time (12.4(22)T).
   
> One site, everything is local on a GigE network.  The phones all work.
> Normal Faxes via FXS to a Hylafax server work.
> 
> If I call the phone number I am sending to the t38 modem, I do get a fax
> tone.  However, when using a real fax machine, the error on the
> receiving side is:  "Failure to receive silence (synchronization
> failure)."  Tested (and every fax call failed) using t38modem on both
> FreeBSD 6.3 and Linux (ubuntu).  When I use two t38 modems to "call"
> each other I can connect fine.   

That's exactly my observation, too. I fear though that the real issue
might be t38modem. Given the base this software is built upon (the R.I.P.
OpenH323 mess), I don't really trust it to do things correctly. Then again,
the issue might well be on the IOS side. When going into deep debugging,
there is some state during the establishment of the fax connection when
IOS constantly sends nothing put empty transport packets, but never joins
the actual V.22something conversation.
 
> I have googled and searched the cisco site and everything I read points
> to me doing everything correctly.

Though there is very little to be found when searching. There's a handful
of postings, most of them talking about similar issues, and mostly they
end in silence. I've got the impression that it might indeed not work
that well except when talking to itself. What version of t38modem did you
test, the ones usually bundled with distros or the newer stuff that uses
new VoIP stacks and can deal with SysV PTYs? I could only test with the
one bundled in Debian stable (still beeing Etch today), which could be
considered rather old.
 
> dial-peer voice 489 voip
>  description To VoIP fax
>  destination-pattern 489
>  session target ipv4:xx.yy.zz.20
>  codec g711ulaw
>  fax-relay ecm disable  # tried with and without
>  fax rate 14400
>  fax protocol t38 ls-redundancy 0 hs-redundancy 0 fallback none
>  no vad

Looks like my dial-peer, I've done some testing with non-zero redundancy
settings on both sides, but to no avail.
 
> Any ideas?

I was planning to test IOS against the Exchange 2007 T.38 implementation,
but wasn't able to convince the Exchange admin yet. Googling some more,
it appears that Exchange has broken T.38 in SP1 so a negative test will
not be a final pointer. The next potential test target would be Cisco
Fax server, but there's no demo/test download apparently. Considering that
mess I dediced to do something else and come back to the holy war against
faxes later ;)

If you find anything that makes t38modem work, I'm all ears.

TIA,
Andre.
-- 
   Real men don't make backups of their mail. They just send it out
    on the Internet and let the secret services do the hard work.

-> Andre Beck    +++ ABP-RIPE +++      IBH IT-Service GmbH, Dresden <-