Fax calls Fail, SBC Unable to Transcode from T.38 (core) to G711fb/PCMA (peer)

Asset ID:	1-72-2208466.1
Update Date:	2017-05-17
Keywords:

Solution Type Problem Resolution Sure

Solution 2208466.1 : Fax calls Fail, SBC Unable to Transcode from T.38 (core) to G711fb/PCMA (peer)

Applies to:

Acme Packet 3820 - Version S-Cx6.4.0 and later
Information in this document applies to any platform.

Symptoms

Transcoded calls from T.38 to G711fb does not work well. In signaling everything looks ok.

T.38 INVITE is forwarded with m=0 port for t38 and adding a new m= audio line to the end of SDP, which is all good. Then SBC receives 'fcs-BAD-sig-end' from the endpoint and and 'no signal' thereafter.

Cause

If we see wireshark traces, beginning of the t.38 traffic from Cisco looks like this:

seq=0 no-signal

seq=1 ced

seq=2 no-signal

seq=0 no-signal <<===== the sequence numbers are restarted!!!!

seq=1 v21-preamble

seq=2 v21 DIS

The restart of the sequence numbers causes the next packets with sequences 0-2 to be discarded by the dsp (in SBC) because it thinks they are duplicates. Customer upgraded the Cisco device, still the same issue.

Note: Other devices like Dialogic, FreeSWITCH, Asterisk etc can handle these bad sequence numbers and transcoding works well there.

Solution could be either to address this T.38 protocol flaw (for bad sequence numbers) on the endpoint, but as this endpoint works well with all other devices (except SBC), there was a need to make SBC more immune to T.38 specs.

Solution

Issue fixed with respect to Bug 23306398 in SCZ720m6p8.

Details of Issue fixed:

Cisco Device was repeating the first 3 sequence numbers causing DSP in SBC to drop the session.

A workaround was added in code to drop all T.38 packets with indexes 0,1,2. Further if our DSP vendors make themselves more immune to T.38 spec in their newer firmware and fix this issue at their end , this workaround might not be delivered in next SBC releases.

References

<BUG:24614438> - TEMPERATURE SENSOR PHY TCM2 IS AT 76.000 DEGREES C, OVER MAJOR THRESHOLD OF 76.0
<BUG:23306398> - TRANSCODING NOT WORKING FROM T.38 TO G711FB

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