[Freeswitch-users] What's wrong with Fax ECM and T.38?

[David Witham]

Hi Victor,

I haven't observed that myself (although I hadn't looked for it either) but it makes some sense. ECM is used in T.30 to force retransmission where checksum errors are detected. This is effectively the same thing that is trying to be achieved with the redundancy packet mechanism in t.38 so I guess those two mechanisms could conceivably conflict, effectively re-transmitting re-transmitted packets.

regards,
David
________________________________________
From: freeswitch-users-bounces at lists.freeswitch.org on behalf of Victor Chukalovskiy
Sent: Tuesday, 21 April 2015 04:10
To: FreeSWITCH Users Help
Subject: [Freeswitch-users] What's wrong with Fax ECM and T.38?

Good day,

My appologies as this is not specific to FS per say. I'm looking if
someone has feedback on a broader topic.

I noticed that when T.30 DIS / DCS signals between fax machines
negotiate error correction mode (ECM) it often causes issues to various
T.38 implementations.
I also noticed that certain soft-switches or media gateway would
intercept DIS signal and overwrite ECM bit to "false". Thus making
far-end fax machine believe call should proceed as non-ECM.
In FS there is a channel variable for it. Some Cisco, Metaswitch gear do
that as well.

Would someone know what is it about ECM Fax mode that makes it less
"t.38 friendly"? What forces vendors to implement tricks to prevent Fax
machines from negotiating ECM?

Many thanks,
-Victor






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