Initial observations..
- Bit Loading is a consequence rather than a cause, so best to leave it until last.
- QLN (quiet line noise) has, overall, actually gone down between 29/07 and 05/08.. so it's not a crosstalk issue.
- Hlog (line attenuation) is virtually unchanged between the two dates.
- SNR (signal to noise ratio) .. that's the interesting one... SNR across the DSL spectrum has dropped, yet not in response to changes in QLN or Hlog.
Doesn't the QLN graph from 5th August actually show lower attenuation across all 3 bands i.e. increased noise?
(-140 being "quieter" than -120)
That could possibly explain why SNR has dropped.
You lost me. HLog measures attenuation. QLN measures noise.
Hlog (attenuation) is essentially unchanged between 29/07 and 05/08.
There was more line noise measured on the 29/07 than on 05/08.
Yet, counter-intuitively, with attenuation unchanged and less noise, SNR nevertheless was reduced across the xDSL spectrum.
If we study the drop in SNR between 29/07 and 05/08 it indicates a managed cut-back in transmitter power at the DSLAM. A different PSD mask applied by Openreach, perhaps?
Sorry, I should have stated lower "sound" attenuation measured in dBm/Hz.
The way I have understood QLN is that as the QLN graph is "inverted", the higher the visual appearance the less the noise is attenuated i.e. the noisier the connection.
We quite often discuss noise "spikes" that have a value of say -120 dBm/Hz compared to the underlying value of say -140dbM/Hz.
The examples from July & August appear (to me) to overall show a general increase in "noise".
As you state, Hlog (signal attenuation) is virtually identical, so the signal to noise ratio has taken a hit as depicted in the SNR graph.
Overall DS SNRM levels had also deteriorated from around 10.6dB to 5.3dB, again suggesting either that the signal had weakened or that noise had increased.
This reduced SNRM had the effect of lowering attainable rates from 59168 Kbps to 40292 Kbps.
Over time, QLN appears to have deteriorated on my own connection (quite gradually), suggesting increased crosstalk as more customers are connected.
I believe I was one of the first to be connected to my cabinet as BT checks were stating that FTTC wasn't yet available at the time Plusnet arranged my connection.
(See the attached typical QLN graphs from my connection - October 2011 & July 2012)
Have I misunderstood the interpretation of QLN values (genuine question)?