In December, sync speeds were 29430/5704 Kbps, where they had more or less been for the 7 months since my connection was finally repaired near the end of May 2012.
What was the original estimate for your line?
DS Attainable rates have gone from 34Mb to 21.5Mb
As Chrysalis points out, the attainable figures when interleaving/FEC is turned on turn out to be artificially high.
So the likely realistic drop is from around 31Mbps to 21Mbps - so you've lost about a third of the line's throughput.
I think this is still within the bounds of "expected behaviour", even if it isn't very nice to experience.
My line, over the last 17 months, has gone from around 88Mbps attainable to 72Mbps, but most is in the last 6 months. It has survived that reduction since December, and is still synchronised at 80Mbps but with around 3dB SNRM.
The best graph I've seen that gives a hint about the random nature of the effect of crosstalk can be seen in a description of vectoring. Look at the graph on page 12 of this document
The blue crosses all represent the speed seen on various lines at different distances, and the variation between the best and worst is remarkable. For example, at 600 metres the best can get 65Mbps while the worst gets 45Mbps.
Your line is longer than 1km isn't it, so I guess you need to extrapolate to get comparable results.
Here's a before & after set of snapshot graphs:-
How did you make those? I'd like to try the same!
The really obvious differences are Bitloading, QLN & SNR.
However, I can also see slight deterioration in the Hlog graph, along with slightly increased attenuation as reported via the pbParams data.
The Hlog has changed slightly. As this says something about the underlying copper, it'd be interesting to see if this change has anything to do with time-of-day, weather or seasonal effects. Unfortunately, I guess it is only produced at each resync.
QLN seems to show more noise throughout, which would cause reduced SNRM, and reduced bit-loading. These seem to be consistent, without any notable singularities.
I also notice that TX Power has reduced slightly.
D1 has the same power, but D2 has reduced. Perhaps this is because the modem is using fewer frequencies in D2.
I presume such low speeds no longer require Interleaving, INP & delay setting.
If the lower speeds are caused by increased noise (as they seem to be), then you'd expect wideband interference to carry on in the same way.
If, however, DLM intervened to restrict your speed by banding, then interference can be reduced. However, you'd also see a higher SNRM than 6dB - which you don't.
What might have happened is that all your interference was being caused in tones that are no longer used. Without adverse effects there, DLM can be turned off. But I'm not sure that is the order in which you saw speed reductions happen - DLM turned off before you stopped using the tones.
Now, my question is, can this really all be down to increased crosstalk or is it possible that either something else has gradually caused QLN to deteriorate or is some sort of a line 'fault' brewing?
Wouldn't this be more visible in the Hlog graph? Or by QLN changes at specific frequencies?
I have no idea how many users between the cabinet & my house have broadband connections or even which flavour(s) of DSL they are using.
I also have no idea how to discover if there has been an increase in the number of broadband users, coinciding with the dates I noticed a deterioration.
The interferers can live further away too - their lines just need to run parallel and close to yours before yours branches out.
They're probably not ADSL users (of either flavour) - the QLN graph is being affected way above 2MHz.
Does anyone have any ideas/suggestions?
I don't believe requesting DLM to be reset would have any mileage (even in the unlikely event that such a request would be granted), particularly as Attainable rates are so low & there doesn't really appear to be much in the way of spare SNRM.
I don't think it would make much difference.
Are you still using the new firmware? Perhaps swapping back would say something (or onto it again)