[Freeswitch-users] PROTOCOL_WARNING when sending a fax via T.38

[Markus Bönke]

Hello,

I want to send a fax between two freeswitches (1.4.20) via T.38. On the SENDER freeswitch, I have Sangoma A104 TDM card and use the following dialplan:  

        <action application="export" data="fax_enable_t38=true"/>
        <action application="export" data="fax_enable_t38_request=true"/>
        <action application="export" data="fax_verbose=true"/>
        <action application="export" data="execute_on_answer=t38_gateway self"/>
        <action application="bridge" data="sofia/external/${destination_number}@${destination_ip}"/>

On the RECEIVER freeswitch I use the this dialplan:

         <action application="answer" />
         <action application="playback" data="silence_stream://2000"/>
         <action application="set" data="fax_enable_t38_request=false"/>
         <action application="export" data="fax_enable_t38=true"/>
         <action application="rxfax" data="/tmp/FAX-${uuid}.tiff"/>

When I try to send a fax, on the RECEIVER freeswitch, I can see in the (second) INVITE the T.38 data is present: 

...
  a=T38FaxVersion:0
   a=T38MaxBitRate:14400
   a=T38FaxFillBitRemoval
   a=T38FaxRateManagement:transferredTCF
   a=T38FaxMaxBuffer:2000
   a=T38FaxMaxDatagram:400
   a=T38FaxUdpEC:t38UDPRedundancy
…

On the SENDER side I can see that it tries to start the t38_gateway

…
2015-09-17 23:36:51.092799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38 Rx     2: IFP 00
2015-09-17 23:36:51.092799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38 Rx     2: indicator no-signal
2015-09-17 23:36:51.092799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38G Changing - (0) ced -> no-signal
2015-09-17 23:36:51.092799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38G Changing to no-signal
2015-09-17 23:36:51.092799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38G Non-ECM mode
2015-09-17 23:36:51.172799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38 Rx     3: IFP 06
2015-09-17 23:36:51.172799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38 Rx     3: indicator v21-preamble
2015-09-17 23:36:51.172799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38G Changing - (0) no-signal -> v21-preamble
2015-09-17 23:36:51.172799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38G Changing to v21-preamble
2015-09-17 23:36:51.172799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38G Non-ECM mode
2015-09-17 23:36:52.072799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38G HDLC underflow at 4
2015-09-17 23:36:52.172799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38 Rx     4: IFP c0 01 80 00 00 ff
2015-09-17 23:36:52.172799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38 Rx     4: (0) data v21/hdlc-data + 1 byte(s)
…

But then fails with a protocol error

2015-09-17 23:34:07.652799 [DEBUG] mod_spandsp_fax.c:286 FLOW T.38 Tx     1: indicator v21-preamble
2015-09-17 23:34:07.652799 [ERR] mod_spandsp_fax.c:659 INVALID WRITE: 8:3
2015-09-17 23:34:07.652799 [ERR] mod_spandsp_fax.c:671 TERMINATING T30 STATE
…
2015-09-17 23:34:07.652799 [WARNING] mod_spandsp_fax.c:286 PROTOCOL_WARNING T.38 Tx packet handler failure

On the RECEIVER side the fax fails with „result (49) The call dropped prematurely“. Can this be because of a wrong configuration (dialplans/mod_spandsp), or is this a bug?

Thanks and Regards

Markus 

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