H323 T.38 Fax Relay Call flow

Unanswered Question
May 26th, 2009


After analizing a H323 T.38 call trace I noticed that the calling Fax send a Facility RM message to the called fax. Shouldn't always be the Called Fax the one that sends the Facility RM message??

Calling <------(Connect)------ Called

Calling -----(Facility RM)---> Called

Thanks in advance.

I have this problem too.
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gogasca Thu, 05/28/2009 - 19:56

what is facility rm ?

normally u will see t.38 caps in h.245 tcs...

and once call is established tone is sent inband, but during connection (call active fax tone should detected at the called side to understand is a fax call)

whats the problem? and topology?

guillermohurtado Fri, 05/29/2009 - 06:14


RM: Request Mode.


Cisco GW ---- SoftSwitch ---- Cisco GW

I do see T.38 capabilities in the H.245 TCS, but I'm not sure if the Fax tone is sent inband in a H.323 T.38 Fax Relay enviroment. I believe the terminating gateway sends a Facility Request Mode message to the originating gateway after receiving the fax tone sent by the called fax. But in my case the calling GW is the one that sends the Facility, and thats is what I don't understand.

Please visit the next link and go to H.323 T.38 Fax Relay:


Thaks a lot in advance.-


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