Guess it could be crosstalk. I would guess the SNR spiking/jumping ECI oddities some are talking about are what I'm seeing.
Crosstalk doesn't cause SNRm spiking. FEXT is a constant source of noise that will remain static as long as the disturbers line is synchronised.
Whilst it does affect SNRm - it reduces the Signal to Noise ratio - you only really see the affects when the other party connects/disconnects.
When they connect, the effects of FEXT are that it reduces your signal strength. Think of it like someone turning the volume down on your TV.
Sharp spiking is most usually some sort of EMI/RFI or occasionally a high open type line fault.
The very first case of SNRM oscillations was brought to my attention a couple of years ago.
SNRm oscillations are sometimes a 'feature' when a line has suddenly lost sync, most times it will occur after a power cut/power surge.
Its not unique to VDSL and can also occur on ADSL. Its been something I've been aware of for many years dating right back to the very early days of routerstats when I had a BT Voyager2100 so that has got to be well over 10yrs ago.
It would always occur after a powercut my end and it could randomly occur if something caused a remote sync. 99 times out of 100 a router reboot would fix the issue.
With the advent of VDSL I saw it also happening on some lines so even before I myself had fttc, I started suggesting to do a full power down of the modem and a new fresh resync and again this usually fixed the problem. I've many times over the years suggested a reboot to see if it fixes it.
I think because now we have MDWS we are more aware of it, back in 2005 there were very few people who ran routerstats 24/7 to see this happen. We also seemed to have far less remote syncs with ADSL than VDSL. BT would install a MSAN and it would remain untouched for many years unless there was a line card problem, but the MSAN itself would keep going.
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My current issue though is different. Although it very first started after an exchange type outage, it does not always respond to a modem power down. Nor does it always take a resync to trigger it. There was no valid reason why the error rate should have shot sky high at exactly 3pm yesterday. During those periods, using broadband is difficult. Such high rates of errors mean that even just browsing is difficult.
I am now interleaved and have INP but thats just really masking things. I can see that the SNRm spiking is still there and I can see I am getting high peaks of FECs - far more than would be considered normal - see below for a bad one at 10.45 yesterday.
My QLN shows differences too. Theres been a change of floor level and its more spikey. The U2 band has changed shape.
The first graph shows signs of crosstalk in D2, the 2nd graph its masked by the spikeyness. D3 appears worst affected, although not confined soley to it.
Hlog is fine - possibly something going on at around tones 3300-3800 in D3 but its not anything that would cause undue concern.
HLog Floor level remains unchanged which is why I dont believe this is a physical line issue and instead its noise being introduced onto the line.
Since it started Ive gone from a stable 84Mbps attainable to now 70Mbps interleaved without any of the normal signs that would indicate crosstalk.
Also I think its highly unlikely that an engineer is going to be connecting a new user at 04.45 in the morning and causing me 15 mins downtime.
As I type this, according to DSLstats, my upstream SNRm is pulsating between 12.5dB and 8.2 dB.
btw, spoke to my neighbour last night, she says her problems started the same week as mine. She doesnt monitor but had noticed a reduction in speed, which continued to drop until she had no sync at all. Being she's with Sky they first sent out one of their own engineers to test her internal equipment, then decided to send Openreach.
She doesnt know what they did to fix it other than it was an external fault not in or on her premises.
VDSL/ADSL is such a flaky technology.
You aint wrong there.
