... One seems to be experiencing nothing more nor less than the speeds and connection issues I am (which is to say, rarely if ever obtaining sufficient capacity to watch YouTube clips or use the BBC's iPlayer function). He maintains that his DL speed is continuously less than 1mbs ...
Well that would likely point at a cable issue somewhere downstream that is affecting multiple lines, or an external source of inteference.
The noise margin is how far above the required SNR for a given line speed the current line is.
Therefore if for a 6 Mbs line speed you need an SNR of 30dB and your line has an SNR of 42dB then your noise margin will be 12dB (The higher the better as you have a lot more leeway).
If your SNR went down to 29 the speed of 6Mbs would not be supported and you would lose your connection. Therefore a preset leeway (in this case 6dB) is required. You will find that a higher speed like 7Mbs requires an SNR of say 32dB therefore the 42dB would still support this with a 10dB noise margin. The line could resync to this higher speed without hitting the preset 6dB minimum. It needs this leeway as the noise will change during the day by about 2~4dB so it makes sure you don't hit the required SNR for that speed.
If you do you'll drop the connection or get high errors and the line will drop down to a lower speed.
Therefore the SNR is a natural product of the line and the higher the better it gives you a higher noise margin and the line speed can ramp up higher.
ANything higher than 6dB will give you the best speed and the higher the better. A high margin but lower than maximum connection speed means the line hasn't reprofiled to the correct speed. A maximum speed connection with a high noise margin is the very best as it means the technology iss the limiting factor and upgrades to the technology will give you even better results (assuming still on copper).
It is directly SNR related. Noise margin is the total SNR minus the required SNR for that line. That is why it is called the margin.
It is a little confusing being called "noise margin" but that is just shorthand.
Connection just cut out. Some stats: