I have done several dozens of FAX test between fax machines connected to analog ports on IP enabled devices using G.711ulaw and no fax relay or T.38.
I have a Storm unit from Shunra that I use to cause delay, loss, congestion, bandwidth shaping, etc.
I used 8 paged of medium complexity pages as my base for all tests.
My tests have indicated that low packet loss has a profound impact (as it should). Low levels of packet loss (.5%) were found to cause problems.
Delay caused transfer time problems. A network delay of 10 ms increased the transfer time from 4:20 to 6:10. A 20ms delay increased the transfer time from 4:20 to 7:35.
Jitter caused both quality and transfer time problems. A jitter of 20 ms caused a 30% quality degradation and increased the transfer time from 3:26 to 4:33.
Joe