Losing an audio packet here or there wouldn't normally be so bad for fax. Normally I would expect the fax protocol, especially ECM protocol, to be able to recover from it. However, Asterisk seems to not work in an ideal fashion for this purpose. Whenever Asterisk encounters a lost audio packet something called "packet loss concealment" is performed by placing a "PLC" frame there as a placeholder. When the audio is retransmitted the PLC frame is supposed to be converted into synthesized audio. Between what I have been told and from what I have observed, this conversion of PLC frames into synthesized audio does not happen with uLaw, alaw, or slinear codecs (the only codecs suitable for fax). Consequently the PLC frame is converted into zero-data... or 20 ms of silence... which is probably the worst-possible thing that could happen.
Turning ECM seems to cause most of my issues with FAX. Most newer machines have this on by default. However if there is any packet loss, then when ECM tries to resend and there is additional loss, then it gets in a loop and everything just fails. Whereas with ECM off, you may have an occasional extra or missing pixel, but most users never notice, and the speed is way faster. Most complaints are solved by jsut turning ECM off. Of course this does not necesarily help mortgage companies who seem to enjoy faxing 50page legal docs....
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