If you imagine a normal columnal plot (or line graph plot) of SNR, color each point with the magnitude (black being 0, red being 1, green being 60), rotate it on the X-axis, as if you're looking down on it, then rotate it on the Z-axis, so it appears vertical, then the right-most column is the current graph and everything to the left is history. Tone 1 is at the top of the graph.
I think I got this part
In the first image posted, the lower 66% of D1 takes up about 40% of the top of the graph - ignoring U0 which was disabled in an attempt to minimise any possible harmonic cross-talk from the highest amplitude signal. Reading from left to right, it's evident that D1 becomes brighter about 400 seconds (pixels) in.
This is the hard part to interpret. The first problem was just figuring (easily) where the different bands were. Some horizontal line in a different colour (white?) would help identify the bands, and make it easy to see the affected spectrum.
The second problem is harder ... the colour represents 60 SNR values, so it can be hard to judge whether an apparent change from "light green" to a "lightish green" signifies a big change in SNR. Or, perhaps the change in colour is an apparent one because one or two rows of pixels change to "light green", but were already surrounded by "light green" - making the colour more solid. Each pixel represents 10 tones, so the aggregation technique becomes important too.
The third problem is consistency. In the original post, you mention U1. In the title and this post you mention D1. I guess you mean D1?
IMO, your graph is really good at telling me there was a change (and in this case, just the one change), and how widespread it was across the spectrum. But it doesn't give me a very good feel for how big the change was (in dB) nor how individual tones were affected.
Having seen that, I would then move on to look at the "normal" graphs - one that plots the summary SNR over time, over the same time period. And one that plots the SNR/tone - and see one of those from just before the change to one just after.
Without knowing any of those, though, I will take a guess:
- Your problem is one of crosstalk, caused by other FTTC subscribers. In this case, it is likely to be one single subscriber.
- If this was a one-off occurrence, then the subscriber was just activated.
- If this happens regularly, then they have a habit of turning their modem off when not using the internet
- The scale of difference is of the order of 1-5dB across a whole chunk of spectrum
- The impacted spectrum is likely to be the overlap between the spectrum your modem can use and the spectrum their modem can use.
That is an uninformed guess, mind!
I've attached one of the logs of SNR over time from my line, from just a few days ago. That was likely caused by crosstalk from a new subscriber. Unfortunately, I wasn't logging the SNR/tone graphs in a way that lets me check them.