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@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 _________________________________________________________________________ Professional FreeSWITCH Consulting Services: consulting@freeswitch.org http://www.freeswitchsolutions.com Official FreeSWITCH Sites http://www.freeswitch.org http://confluence.freeswitch.org http://www.cluecon.com FreeSWITCH-users mailing list FreeSWITCH-users@lists.freeswitch.org http://lists.freeswitch.org/mailman/listinfo/freeswitch-users UNSUBSCRIBE:http://lists.freeswitch.org/mailman/options/freeswitch-users http://www.freeswitch.org