Signal-to-noise ratio (often abbreviated SNR or S/N) is an electrical engineering measurement defined as the ratio of a signal power to the noise power corrupting the signal.
In less technical terms, signal-to-noise ratio compares the level of a desired signal (such as music) to the level of background noise. The higher the ratio, the less obtrusive the background noise is. The concept can also be understood as normalizing the noise level to 1 (0 dB) and measuring how far the signal 'stands out'. In some instances interleaving can help raise the noise margin to an acceptable level.
In general, higher signal to noise is better; the signal is 'cleaner'.
Attenuation is the gradual loss in intensity of any kind of flux through a medium(i.e. the reduction in signal strength due to length of your phone line). For instance, sunlight is attenuated by dark glasses, and X-rays are attenuated by lead.In ADSL the signal is attenuated by length of copper lines. Attenuation is normally directly linked to the length of your line. Copper is traditionally used in the local loop and the higher gauge of copper will give the best signal, however some lines may have some aluminium or aluminium joints on the line which will increase resistance... as will oxidization of joints. Attenuation is mesured in db or noise. The more noise the weaker the data signal
In general, lower Attenuation is better; the signal is 'stronger'.
Heres is my table of comparisons
6dB or below is very 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 little or no sync problems
20dB-28dB is excellent
29dB or above is outstanding
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
The following guide (distance vs. attenuation vs speed) gives you an guestimate what you can achieve:
<1km should be 23-24Mbit (nice speed, but doesn't it bug you that Telkom people walk through your bedroom?)
1.0km = 13.81dB = 23Mbit
1.5km = 20.7dB = 21Mbit
2.0km = 27.6dB = 18Mbit
2.5km = 34.5dB = 13Mbit
3.0km = 41.4dB = 8Mbit
3.5km = 48.3dB = 6Mbit
4.0km = 56dB = 4Mbit
4.5km = 62.1dB = 3Mbit
5.0km = 69dB = 2Mbit
>5.0km (you are pretty much poked --- sorry for you)
Really good tips can be found in this thread
Some neat tricks for adsl lines
by GedMarc- these tips help you improve the quality of your line.
more info thanks to MagicDude4Eva