After Openreach turned out to make repairs on my line 4, the downstream sync rate dropped by between 11-17% to 2.51 Mbps d/s sync from a former typical 2.8-3.0 Mbps sync rate. The upstream is unchanged. The downstream sync rate has been around 2.8-3.1Mbps consistently for the last four years, and was at 3.0-3.1Mbps this winter. Is there anything I can do to get things back to the way they were?
I complained to AA and got a very unhelpful reply which appeared as if they might be saying I didn’t know what I was talking about. Seeing as they have a published no "
bullsh*t" policy, I could have a go at them on that basis, but feeling charitable and working in “assume good faith”, a fairly rare thing for me, I am proceeding on the basis that this has been a misunderstanding based on definitions of rates; I quoted
synch rates to them, while they could have been thinking about their egress rates ie rate limiters’ rates which are = sync rate * 87.91% approx iirc. Or they could have been thinking about IP + TCP SDU (ie TCP payload) rates at 84.2 - 86.1% of synch rate. Those differences are large enough to cover the gaps between differently defined sets of rates and could make it just miscommunication. Despite feeling miffed at the possibility I had been brushed off I noticed that AA tech support have set up regular capture of the synch rate on a repeating timer, every hour a server remotely queries my modem via BT. That’s good because then they will be seeing synch rates not one of the other measures. And it means they are doing something.
Just recently I have watched the SNRM very carefully, waiting until such a time as the downstream synch rate may happen to go above 3.15 Mbps. Now 3Mbps is the downstream SNRM target, and a +0.15 Mbps additional gap seems to be sufficient such that if you choose a moment when the synch rate is at about that level and then force a resynch then you will then end up with a higher downstream synch rate. I’ve mange to push it up by small amounts three times by this feeble method, in total from about 2.51 to 2.667 Mbps d/s synch. It’s not much but it is a sizeable chunk of what I lost now regained (went down from a former 2.8-3.1 Mbps downstream sync rate to 2.51 Mbps at lowest). I managed to creep up in steps: from 2.51 Mbps to 2.564 to 2.61 to 2.667 Mbps. However I feel that there is no reason to assume that this method will get me very far long term though, perhaps I’m wrong. I assume that the further up I manage to crawl, the more infrequent the opportunities may become, also if I were to get things wrong one time then I think I could then easily lose the entire total of such feeble gains, and possibly even more, in one single large drop. Anyway, I don’t see that there’s anything sustainable about these tactics.
There’s also the question of the strange shape of my SNRM-vs-tones graph nowadays. As mentioned earlier, Line 4 has a weird dent or depression or ‘sag’, as Burakkucat said, in the top of its graph as mentioned earlier in the thread, for reasons unknown.
Does this odd-looking feature have anything to do with my
downstream sync rate loss problem?
Since it’s a depression in such a graph then this could either mean (i)
more noise at the middle frequencies or (ii)
less signal at those frequencies. Usually without additional information it’s impossible to tell which of the two it is because all you have in SNR is a
ratio derived from two quantities. In this case though the second option seems totally unbelievable; I can’t see how there could be reduced signal levels only in middle frequencies, lower than the higher signal levels at frequencies both below
and above. So we must be left with the first option: higher noise in a selective frequency band, which is fairly wide, from > tone 40 to < tone 80 on the SNRM-vs-tones graph, but on a bit loading graph the depression is from tone 48 - 78 inclusive and is 1 bit deep. So that’s (78 - 48+1)=31 bits per symbol period lost ie a rate reduction of 31 * 1 * 4k = 124 kbps. So if that were filled in it would mean 2.667 + 0.120 = 2.791, and if the top were rounded off into a more realistic-looking hump, following the general SNRM-vs-tones curve rather than being sliced flat, that might mean another ~ 60kbps, which would take us all the way back to roughly where we should be. So the SNRM curve depression does fit in with the theory of a noise source - it’s about the right size so that it fits. Another way in which this fits is that the dent/sag is in the downstream tones only and only the downstream synch rate is affected; the upstream rate is not.
Another question: looking at the stats regularly, I noticed my downstream ‘attainable rate’ has gone up too when the synch rate has gone up. Could someone remind me what this is all about as I’ve forgotten completely? Does it mean I have a chance of getting to that rate if I do <
something>? - no idea what though.
Right now the d/s stable rate is 2908 kbps @3.1 dB SNRM - and current sync rate 2667 kbps.