Back on the original topic, if there is interference on the line, why does only my downstream noise get impacted? The upstream figure remains stable...
Ah, that's quite easy to explain.
The frequencies involved.
First of all, recall that the modulation technique used is
DMT (discreet multi-tone). The total available bandwidth is split into separate tones or channels. For ADSL or ADSL2, there are 256 channels available. For ADSL2+, there are 512 channels available. Each of those channels is 4.3125 kHz wide. There are two blocks of channels defined for an ADSL service, the upstream band (
US) and the downstream band (
DS). The US band is lower in frequency to the DS band.
As an example, attached below, are "screen-scrapes" of the bit loading per channel for my line. I've split it into two, the lower frequency US band and the higher frequency DS band. (I have an ADSL2+ service but due to the line length, thus the attenuation and other factors, my modem/router selects ADSL2 mode. Hence 256 available channels.)
More channels are used by the DS band than the US band. (The asymmetric part of ADSL.) The DS band runs from approximately 142 kHz to 1100 kHz whereas the US band runs from approximately 26 kHz to 129 kHz (I've rounded the frequency to the nearest 1 kHz).
So, basically, the more (in terms of numbers) "higher frequencies" used the more chance to be "splatted". A higher frequency channel will tend to be attenuated more than a lower frequency one -- hence less "carrying power" (as in "staying power").
Finally, look at that notch at channel 46. That corresponds to the Droitwich transmitter, on 198 kHz in the LW broadcast band. A long wavelength signal (low frequency, LF) will propagate further than a medium wavelength or short wavelength (MF or HF) signal.
I've probably typed far too much and in a confusing fashion.
I'm sure if you look at the main
Kitz site you will find a far more eloquent description.