So if i lived twice as far from the exchange would my upstream snrm be exactly the same as it is now?
Not exactly. There are three factors which affect the upstream speed (talking about 21CN/ADSL2+ here):
- the upstream target noise margin (6 dB on BT systems) means that in most cases that will be the noise margin at the time the connection is established.
- the number of tones allocated to the upstream part of the connection puts a maximum limit on the upstream speed. On a short line, this maximum limit may be reached without bumping into the target noise margin, so in this case the noise margin will be higher than the target.
- the amount of interference will affect the speed achieved for the given target noise margin.
So in most cases, the noise margin at connection time will equal the target noise margin, but on short lines it may be higher. After the connection is established, the noise margin will change, depending on interference conditions.