[VoiceOps] T38 delay management

Robert Johnson fasterfourier at gmail.com
Wed Jul 13 16:13:05 EDT 2011

I'm running into occasional problems with failed fax transmissions due
to what appears to be excessive delay between receipt of the DIS from
the receiving fax and sending of the DCS by the transmitting machine.
Here's the setup:

-SPA2102 ATA running 5.2.12 with the following fax-related configuration bits:
  <FAX_CED_Detect_Enable_1_ ua="na">Yes</FAX_CED_Detect_Enable_1_>
  <FAX_CNG_Detect_Enable_1_ ua="na">Yes</FAX_CNG_Detect_Enable_1_>
  <FAX_Passthru_Codec_1_ ua="na">G711u</FAX_Passthru_Codec_1_> <!--
options: G711u/G711a -->
  <FAX_Codec_Symmetric_1_ ua="na">Yes</FAX_Codec_Symmetric_1_>
  <FAX_Passthru_Method_1_ ua="na">ReINVITE</FAX_Passthru_Method_1_>
<!-- options: None/NSE/ReINVITE -->
  <FAX_Process_NSE_1_ ua="na">No</FAX_Process_NSE_1_>
  <FAX_Disable_ECAN_1_ ua="na">Yes</FAX_Disable_ECAN_1_>
  <FAX_Enable_T38_1_ ua="na">Yes</FAX_Enable_T38_1_>
  <FAX_T38_Redundancy_1_ ua="na">1</FAX_T38_Redundancy_1_> <!--
options: 0/1/2/3 -->
  <FAX_T38_V29_Only_1_ ua="na">No</FAX_T38_V29_Only_1_>
  <FAX_Tone_Detect_Mode_1_ ua="na">callee
only</FAX_Tone_Detect_Mode_1_> <!-- options: caller or callee/caller
only/callee only -->
-ATA registered to [commercial asterisk-based multitenant PBX system]
running asterisk 1.6 and kamamilio 1.5
-Calls terminated through Level3, Global Crossing, and T38faxing.com

Fax calls are set up properly, ReINVITE is issued, RTP stops and T38
session starts as usual. Once the T38 session starts, it appears that
it takes about 2.8 seconds for the ATA / fax combination to return a
DCS message once the DIS message hits from the terminating gateway.
This, when combined with other delays (a few tens of milliseconds
between the ATA and terminating gateway) seems to cause the T1 timer
(I think that's the one...) on some receiving faxes to expire. A few
more DIS messages are sent and then they give up.

What is a normal amount of time for a DCS to be returned by the ATA
when hit with a DIS? Is there anything that can be done to prune the
delay down?

Thanks in advance.

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