Noise Margin, by the way, is how much "noise" the modem has up it's sleeve before selecting a lower speed. So regardless of line conditions, it should be pretty much static - the terminating equipment (eg. DSLAM) at the exchange/cabinet will be set with a "target" Noise Margin. 6dB is about the lowest that this is generally set to, and would be considered an "aggressive" profile.
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