T38 Fax Relay AS5850 Debug Question

Unanswered Question
Mar 4th, 2010


I am troubleshooting an issue with failed faxes to a RightFax server in my environment.  This is for occasional failures from some outside analog fax machines from 1FB connections at their location.  Call flow is as follows:

Inbound PRI --> AS5850 H323 Gateway --> VoIP to RightFax server. (session target on VOIP peer is the RightFax server IP)

The RightFax vendor is requesting a "debug fax relay t30 all-level-1" for a failed call.  I have tested repeatedly but I am not getting any output from the debug.  My only thought is that this debug is related to Cisco Fax Relay specifically which the 5850 does not support.  Or is this debug not producing output because the leg to the RightFax server is VoIP only.  Are either of these correct assumptions?

What other debugs would provide the T30 packet information for the vendor to review?

Thank you for your time.


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William Bell Thu, 03/04/2010 - 14:30

That is a valid command for T.38 fax relay.  It displays messages from the DSP perspective.  You could also look at debug h245 asn1 since you are using a H.323 gateway.  It is a very "robust" output stream.  You would be looking for something like this:

*Mar 4 04:04:04.040: H245MSC OUTGOINGPDU ::=
value MultimediaSystemControlMessage ::= request : requestMode:

type dataMode :
application t38fax:
t38FaxProtocol udp : NULL
fillBitRemoval FALSE
transcodingJBIG FALSE
transcodingMMR FALSE
version 0
t38FaxRateManagement transferredTCF : NULL
t38FaxMaxBuffer 200
t38FaxMaxDatagram 72
t38FaxUdpEC t38UDPRedundancy : NULL



melissajwright Thu, 03/04/2010 - 14:36

Thank you for the repsonse, Bill.  Any thoughts why the debug with terminal monitoring and logging on would not produce any output for inbound call?

William Bell Thu, 03/04/2010 - 14:44

Well, I believe that debug comes into play after the call has switched over from a standard voice call to a fax call.  If the gateway or fax server fails to detect or negotiate the switchover then there wouldn't be any modulation/demodulation occurring.

This is an assumption on my part.  If my assumption is correct then you should see a failure to negotiating t.38 in the asn debug.




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patscheeler Tue, 03/23/2010 - 15:09

To help elaborate on Bill last post, that is correct.

During the call setup it will signal to switch from Voice to T.38, at which point the router needs to be properly configured for t.38 Fax.
Is the dial peer to handle the call being configured on your AS5850 or is it on another router (or in CUCM?)

I would highly suggest (and if possible) installing a tool called Wireshark on the RightFax server to capture the call and see what requests are being passed. This will let Me or any other RightFax technician better diagnose what's going on during the call.


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