T.38 troubles

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Asterisk Users 3 Comments

Hi list,

I’m having difficulties when receiving faxes from the PSTN with this
relatively simple installation:
PSTN < --PRI--> GW < --T.38--> Asterisk

The gateway is a Mediatrix 3301 (firmware Dgw It’s
configured to transmit faxes as T.38. I may have missed something in its
configuration, but it does switch to T.38 when a fax is detected. On the
Asterisk side, I’m using 10.2.1 with spandsp-0.0.6-pre20 and ReceiveFax
from res_fax_spandsp. ${FAXSTATUS} returns FAILED and ${FAXERROR}
“Disconnected after permitted retries”.

I did a network capture, attached to this mail: from my understanding,
T.38 is accepted by Asterisk, then there seems to be some UDPTL traffic,
which I don’t understand…

Why does it fail, and what is wrong? I’d appreciate if someone could
send me advice / suggestions.

– – —
Jean-Denis Girard

SysNux Systèmes Linux en Polynésie française
http://www.sysnux.pf/ Tél: +689 50 10 40 / GSM: +689 79 75 27
default iconfax-ast10.pcap.bz2

3 thoughts on - T.38 troubles

  • Hi Jean-Denis,

    Your log shows the Mediatrix GW has problems. It sends a DCS signal to
    the Asterisk box, but doesn’t following it with TCF as it should. The
    asterisk box times out waiting for TCF and tries to take recovery action
    which fails.

    Spandsp has some workarounds for bugs in Mediatrix boxes. They usually
    work OK.


  • Hi Steve,

    Le 26/03/2012 14:50, Steve Underwood a écrit :

    Thanks for your analysis. Could it be a configuration problem on the

    What would you suggest then, is there anything to do to enable the
    workarounds? spandsp-0.0.6pre20 is the latest available, or should I try
    spandsp-20120324? As far as I know, there is no firmware update for the


  • Hi Steve,

    Le 26/03/2012 14:50, Steve Underwood a écrit :

    I just made a couple of tests with spandsp snapshot 20120328: I now get
    “The call dropped prematurely”. Network capture is attached, hope that

    default iconfax-ast10-2.pcap.bz2