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"It depends." At the really low data rates, like 110 or 300 bps (which =baud for those rates), there's no negotiation and it's all about consistent latency, which means low jitter. (Consistent high latency is better than inconsistent low latency, for this, so crank those jitterbuffer sizes up - jitter is really just small fast fluctuations in latency, and the line between jitter issues and latency issues is a fuzzy one.) At higher rates, it matters more. The usual 1200bps, for example, is 600 baud with two bits per baud and is like 110 and 300 in that there is no negotiation; this means that high but constant latency doesn't disturb it and turning the jitterbuffer up to what would normally be ridiculous levels can fix a lot. But it takes less to disturb 1200 than it does to disturb 300 or 110; bits are smaller, after all. A half-second of jitterbuffer delay is enough to pretty much wreck a voice call but all it will do to a 300 or 1200 modem call is introduce data latency (which may or may not matter, of course). At the rates involving ECC and negoation between the ends, it really depends on the codec. Most traditional modem codecs are designed to deal with the kinds of errors typically introduced by POTS. Errors typically introduced by VoI are very different and traditional modem codecs generally fall flat on their metaphorical faces when faced with them. Recently, modem codecs designed to deal with VoI-style errors have been designed, though I haven't seen them make it into the field except for the special case of faxes. I once did some experiments with faxes using traditional codecs and I found that, as long as you have absolutely zero packet loss and enough jitterbuffer to deal with latency variations, they work great, but I found even a single lost packet can take out several dozen lines of fax data. I don't know how sensitive traditional modem codecs are to high (but consistent) latency. My tests didn't involve that. So, if your QoS or traffic shaping operates to improve latency consistency without generating ploss, it is likely to help. If it introduces ploss, it is likely to hurt. (If it does neither, I find myself wondering what it _is_ doing. :-) /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B