That step change up and down can be indicative of a wide band noise source being switch off and then back on, it may not be yours but could be a neighbour or on te street. If it is then it may well be teh answer to your loss of speed issue.

The Openreach line to my house is around 300 metres in length and supported 80/20 synch rates for several years.
As local FTTC take up has increased the crosstalk has resulted in the line now synching at 67/18 with the help of G.INP and a 3 dB downstream margin.
Dslstats reports simillar downstream Signal to Noise step changes when neighbours switch off their modem/routers.

It's also Christmas lights going up all over the place of course. Often causing havoc especially if faulty.

Given that it was first noticed around 7th December ... that could be a possible explanation.

Thanks all, interesting points. Fingers crossed for the Christmas lights, given they are seasonal, but I fear skandia2's explanation might be the reason - someone with a new FTTC install and maybe a dodgy internal wiring setup introducing a load of noise.

I will try and isolate other possible sources within my house. Pity I didn't have DSL stats setup a while ago such that I know precisely when the sync rate dropped.

I would suspect the USG given I first noticed this when the PSU on the USG failed, but removing the USG doesn't resolve the issue (see results above using BT HH6).

So, have you considered it could be one of your PSUs? What casue the USG one to fail? Did it damage the HG612 supply
too?

They are just basic switch mode 12v supplies so do generate a lot of noise but is mostly filtered out.

In reply to a post by MHC:

So, have you considered it could be one of your PSUs? What casue the USG one to fail? Did it damage the HG612 supply
too?

They are just basic switch mode 12v supplies so do generate a lot of noise but is mostly filtered out.

I have considered it may be one of my PSUs. I have tested removing a variety of those in the same area as the modem but will need to make my way around the rest of the devices in the house. As to the modem itself, I don't have a spare PSU to try, however, the SNR margin remains the same when syncing using the BTHH6, i.e. when the HG612 and its PSU is not connected, so that would seem to rule out the HG612.

I am not sure what caused the USG PSU to fail but from reading online they are not particularly reliable.

Edited by oracleredux (Wed 15-Dec-21 16:30:46)

I've mananaged to dig out my DSLStats historical data (all 115,000 entries) so now have an exact history of when the line degraded.

Historically, the downstream SNR Margin was around 7.3dB (upstream around 15.3db). This was reflected in line sync which always at the max allowed by BT: 79,999 / 19,999Kbps (with max obtainable being 85,492 /29,289).

On 16 November at 14:48 the downstream SNR margin dramatically reduced to 2dB (the upstream SNRM reduced slightly to 14/12dB or so). DS SNR had been holding steady at around 7.3dB for a long period and suddenly collapsed to 2dB.

This did not immediately impact on sync which held at 79,999 / 19,999Kbps on the narrower SNRMs of 2dB/12dB. Also there doesn't appear to have been any change in the stats when the USG PSU failed (03:00 on 27 Nov).

The HG612 held sync until I switched it off (to exchange for BT HH6) on 6 Dec. When I switched the HG612 back on the next day (7 Dec at 23:43), the sync rate dropped to 67,774Kbps in order to achieve a DS SNRM of 6.3db. There has been no subsequent change for the last 10 days.

Extract of data referred to above

In case it helps, I have the full logs (115,000 entires across 148 data points) but don't know what many of the stats relate to.

Charted data:
DS US Sync chart
SNR Margins

Edited by oracleredux (Sat 18-Dec-21 11:14:27)

If you go back to the recent post where the SNR improved for a short while - that was a 5.3 dB improvement.

Your historical data shows a drop on 16 Nov from 7.3 to 2 dB which is 5.3 dB. Coincidence? Or not? I would suggest there is something casung it - maybe not your house but a neighbour. Could be an electrical device, or as suggested, Xmas lights, or ...

Have you tried turning off EVERYTHING in your house and garage except the modem or hub. And I mean everything, lights, fridges, chargers. That will eliminate your property and point towards external.

edit to add:

Andl looking at your long term stats there are six points where the DS Max has returned to the previous level similaerly the Up Max - the six spikes. Which reinforces a possible single noise source. Can you pinpoint the time of day those occured.

INterested to hear Pluralist's thoughts too.

Edited by MHC (Sat 18-Dec-21 11:33:03)

The description given and the timing sounds very much like a bad crosstalker.

Basically another line on the cabinet that runs in close proximity to your line within the ducts/multi pair bundle has signed up to an FTTC service.

The interference this line causes when synced causes a large drop in downstream SNRM, with a smaller drop in the upstream SNRM.
At the next resync this means your line loses quite a bit of sync speed.

You posted a snapshot of the downstream and upstream SNRM briefly increasing and then reverting back to their normal values.
That matches what it looks like when a bad crosstalker turns off their modem, then turns it back on again.

A single bad crosstalker can knock over 20Mb from a lines sync rate/attainable rate. Other crosstalkers may have little to no impact at all. It's very much a lottery.

It's either a crosstalker as described above or it's some kind of electrical interference (from a pump or generator for example).

Imo is more likely a crosstalker though as I can't think of any equipment/machinery/appliance that would suddenly have that much impact on a line and be running 24/7.
That kind of interference tends to go on and off at regular intervals.