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

[Victor Chukalovskiy]

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