SN Margin (AKA Signal to Noise Margin or Signal to Noise Ratio)
Relative strength of the DSL signal to Noise ratio. 6dB is the lowest dB manufactures specify for modem to be able to synch. In some instances interleaving* can help raise the noise margin to an acceptable level. The higher the number the better for this measurement.
> 6dB or below is bad and will experience no synch or intermittent synch problems
> 7dB-10dB is fair but does not leave much room for variances in conditions
> 11dB-20dB is good with no synch problems
> 20dB-28dB is excellent
> 29dB or above is outstanding

Line Attenuation
Measure of how much the signal has degraded between the DSLAM and the modem. Maximum signal loss recommendation is usually about 60dB. The lower the dB the better for this measurement.
> 20dB and below is outstanding
> 20dB-30dB is excellent
> 30dB-40dB is very good
> 40dB-50dB is good
> 50dB-60dB is poor and may experience connectivity issues
> 60dB or above is bad and will experience connectivity issues

Took from http://forum.lowyat.net/topic/222851