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[Alex Atkin UK]

That was something I was going to mention. Poor AC balance can result in significant interference ingress. Given that the pairs are aluminium, it will be very difficult to manipulate the "tails" in the joints (to maintain the twists) without the wires fracturing, especially with sufficiently aged cables. Metal fatigue would described the situation.

When they were doing routine maintenance on the E-side joints from the local cabinet, mine completely snapped and when they looked it seemed a miracle I had been running ADSL2 through what seemed to now be merely white powder.  Once repaired they showed me that its literally so short now if it broke again, they'd have to find another pair.

I consider myself very fortunate that my D-side is AFAIK 100% copper so no longer have to worry about that, but it does make you wonder how people still stuck on alu especially on longer lines are able to get any service at all.

[rmk]

I consider myself very fortunate that my D-side is AFAIK 100% copper so no longer have to worry about that, but it does make you wonder how people still stuck on alu especially on longer lines are able to get any service at all.

With luck, I think. 

As of 07:46 this morning, it's behaving. - higher bit loadings, higher SNR per tone, lower QLN per tone.  13822kbps down, but only 424kbps up.

Enta technical tried a port reset this morning, and allow two hours, and then restart the "router" (they meant modem).  So, I unplugged the modem for a little over 30 minutes this afternoon.  It's now doing 13870kbps down 424kbps up - not much different, so I think the DLM has taken action against my upstream rate.  It doesn't seem to have impacted my downstream rate at all - in fact, it's now running faster than it has done since March. So, what's going on... this doesn't seem to be "normal" DLM behaviour from my understanding.

[rmk]

The line has been stable since yesterday - no resyncs, hardly any error-seconds.  Am I baiting it?  Probably.

However, the interleave is much higher - 557, whereas it was around 400 before.  Even with that, it's clocking around 13Mbps down, which is faster than it's been for quite some time.  Upstream is very poor though, at only 424kbps.

Can anyone explain exactly what a "port reset" would be please?  I don't think it's a reset of the DLM, since the line hasn't gone to fast path as it has in the past when the DLM has been reset.  How that could have solved this problem which was showing up as a noise issue in the statistics from two different modems?

Another interesting point - the dip in the QLN around 1307kHz - Any guesses?  There's transmitter the other side of London on 1305kHz near Enfield: https://www.google.co.uk/maps/@51.6504011,-0.0083601,3a,69.4y,275.8h,89.84t/data=!3m6!1e1!3m4!1skVgXliuIoE9C0jOvjmFNtA!2e0!7i16384!8i8192?hl=en However, that should couple equally into each side of the line pair and be rejected as a common mode signal.

Thanks.

[Alex Atkin UK]

Another interesting point - the dip in the QLN around 1307kHz - Any guesses?  There's transmitter the other side of London on 1305kHz near Enfield: https://www.google.co.uk/maps/@51.6504011,-0.0083601,3a,69.4y,275.8h,89.84t/data=!3m6!1e1!3m4!1skVgXliuIoE9C0jOvjmFNtA!2e0!7i16384!8i8192?hl=en However, that should couple equally into each side of the line pair and be rejected as a common mode signal.

Thanks.

Doesn't a transmitters output fluctuate based on the signal its carrying?  If so, I'm not sure how that would be cancelled out consistently, presumably with constant bitloading adjustments.

Plus wouldn't it still technically show on QLN anyway as that transmission is a permanent increase of the noise floor on those tones?

[burakkucat]

Another interesting point - the dip in the QLN around 1307kHz - Any guesses?  There's transmitter the other side of London on 1305kHz near Enfield: https://www.google.co.uk/maps/@51.6504011,-0.0083601,3a,69.4y,275.8h,89.84t/data=!3m6!1e1!3m4!1skVgXliuIoE9C0jOvjmFNtA!2e0!7i16384!8i8192?hl=en However, that should couple equally into each side of the line pair and be rejected as a common mode signal.

Looking at the shape of the curve, it does have the broad base characteristic of a relatively high-powered broadcast transmitter. (Relative to the tiny power of the xTUs at either end of xDSL link.) I agree, one would expect such a signal to be rejected as common mode but we have already speculated that the AC balance of the circuit is poor. Hence poor rejection of common mode signals.

Looking at the data here, I see --

Freq    Station Name    Transmitter Site    Power (Watts)    Grid Reference

1305    Premier            North Looe            500                   TQ229610
1305    Premier            Chingford              250                   TQ378965

[rmk]

Another interesting point - the dip in the QLN around 1307kHz - Any guesses?  There's transmitter the other side of London on 1305kHz near Enfield: https://www.google.co.uk/maps/@51.6504011,-0.0083601,3a,69.4y,275.8h,89.84t/data=!3m6!1e1!3m4!1skVgXliuIoE9C0jOvjmFNtA!2e0!7i16384!8i8192?hl=en However, that should couple equally into each side of the line pair and be rejected as a common mode signal.

That's not daft at all, but seems to be what is going on.  Everywhere that the quiet line noise rises, if I tune a MW radio to that frequency, there is a radio station there.

That means the line is behaving as an antenna, which probably means my DSL signals are also being broadcast over the mediumwave spectrum!

There are radio stations receivable on: 558, 720, 909, 1053, 1089, 1107, 1152, 1215, 1305, 1458, 1548kHz.  The one on 1305 is by far the strongest, receivable on the neighbouring frequency channels. These correspond to VDSL2 tones: 129, 167, 211, 244, 253, 257, 267, 282, 303, 338, 359. On each of those tones and neighbouring tones (dependent on the frequency) there is an uplift in the QLN figures, a drop in the SNR figures, and as expected a drop in the bit loadings.

Looking at the ADSL line (which only goes up to tone 256) I see a similar correlation there - so both lines seem to be affected by AM radio stations.

Looking at the shape of the curve, it does have the broad base characteristic of a relatively high-powered broadcast transmitter. (Relative to the tiny power of the xTUs at either end of xDSL link.) I agree, one would expect such a signal to be rejected as common mode but we have already speculated that the AC balance of the circuit is poor. Hence poor rejection of common mode signals.

Thanks, and yes, that is where I found the grid reference to then locate exactly where the transmitter is - almost the diametrically opposite side of London to me. Given that both of our lines seem to have very similar QLN profiles matching the AM stations that are available, either both lines are similarly unbalanced, or it's entirely normal for the quality of cabling around here.

I'm not surprised given that the various underground joints in the copper cables that I've seen engineers working on have been a mass of untwisted wires - I saw a number while an Openreach engineer was investigating audible noise issues on this line last year. He gave up and switched the line onto a different pair back to, I presume, the PCP next to the FTTC cabinet.

[j0hn]

Can anyone explain exactly what a "port reset" would be please?  I don't think it's a reset of the DLM, since the line hasn't gone to fast path as it has in the past when the DLM has been reset.  How that could have solved this problem which was showing up as a noise issue in the statistics from two different modems?

A DLM reset would indeed have put the line to fastpath in the past. That's not the case now though.

A DLM reset will default a line to Interleaving.
So if you already had Interleaving it would only cause a resync with no change.
If you were on fastpath at the time of a DLM reset then it would resync with Interleaving applied.

Enta technical tried a port reset this morning, and allow two hours, and then restart the "router" (they meant modem).  So, I unplugged the modem for a little over 30 minutes this afternoon.  It's now doing 13870kbps down 424kbps up - not much different, so I think the DLM has taken action against my upstream rate.  It doesn't seem to have impacted my downstream rate at all - in fact, it's now running faster than it has done since March. So, what's going on... this doesn't seem to be "normal" DLM behaviour from my understanding.

I don't think ISP's can remotely perform a port reset on FTTC.
Sounds like the rep was going through the ADSL script.

ISP's can't do any tests/diagnostics on FTTC that result in a DLM reset.
They need to put in a request to OpenReach to get a DLM reset.
Unless that has recently changed that is.

[rmk]

ISP's can't do any tests/diagnostics on FTTC that result in a DLM reset.
They need to put in a request to OpenReach to get a DLM reset.

Thanks J0hn.

Yes, they also said "wait two hours and then force the modem to resync" probably to give OR time to process the request?

[rmk]

Now that the line seems to have stabilised, and I have this detailed logging setup, I have a few more observations:

1. Since my line seems to be a good antenna, the daily pattern of noise looks to match what happens with AM radio - with the daily cycle of Ionosphere changes affecting the propagation of MW and SW radio signals.

2. I identified a sudden drop of 0.5dB on the SNR each evening at various times, and the occasional additional 0.5dB drop but only when the former drop is already there. The former turned out to be a LED bulb in our kitchen, the other side of the house; probably interference coupled into the mains finding its way to the DSL modem.  The latter turns out to be the bathroom lights/fan (one 2ft fluorescent tube and one LED) interacting with this LED bulb.  Replacing the LED bulb has made both effects completely vanish.

I also replaced the "flat" RJ11 with a ADSL nation (aka Tandy) twisted pair RJ11 cable - I wasn't expecting much difference, and the best I can see is it made 0.2dB improvement on the SNR and about 200kbps on the attainable rate.  The QLN is probably the most noticeable difference, with sightly less variation.

[Alex Atkin UK]

I also replaced the "flat" RJ11 with a ADSL nation (aka Tandy) twisted pair RJ11 cable - I wasn't expecting much difference, and the best I can see is it made 0.2dB improvement on the SNR and about 200kbps on the attainable rate.  The QLN is probably the most noticeable difference, with sightly less variation.

Yeah an improvement that low can basically be chalked up to margin of error, unless it consistently performs better every single resync.  Still, it doesn't hurt to have a better cable especially with you finding issues from household electronics already.

I've always been surprised I don't have more problems as I literally have an air conditioner plugged into the same outlet as the modems (although they are on a surge protected strip) and a fridge the other side of the room on the same circuit.  The lawn mower is even on the same circuit when in use, although I do have a line conditioner in the adjacent socket, just in case, and so many surge protectors dotted around the house. (though no idea which still work as they overdrive the darn surge good LEDs so they fail, probably to make you buy a new one)