Strange Downstream SNR Margin

Hi guys,

I can only think this is caused by some interference, but does anyone who has better understanding of this than myself know what could be causing this? Should I be concerned?

https://i.gyazo.com/71f0472065c8f24c21e64b8714a7f8bb...

Cheers!

William

Not this again.

It's dropped from what, 4.2 to 3.8 a few times? If it didn't cause any issues at the time, then no it's nothing to be concerned about.

I think you need to switch off all your monitoring software and stop looking for faults.

I've turned on the monitoring software for diagnosis. There's a fault there, but it's very low scale. I should probably investigate it but it's not affecting service yet.

Edited by deleted (Sun 19-Mar-17 17:00:24)

Particularly on ADSLx, less so on FTTC, downstream noise margin is very likely to fall by anything up to 3dB overnight. The interference hitting the copper between you and the DSLAM/MSAN is greater at night.

A simple fact. It's to do with reflections off one of the atmospheric belts when the sun is down. Which is why it is always recommended to connect well inside daylight hours for highest speed, and at night for greatest stability.

Think about it. For decades the default sync-time noise margin was 6dB. SpeedTouch modems would lose connection if it went below 3dM. Netgear would hold it as low as 0dB or even sometimes -1dB.

Why did BT Wholesale Sky and TalkTalk all set it at 6dB? Because it was deemed to be the lowest safe level. Nowadays with improved modem technology at both ends, BT Wholesale allow DLM to set connection time SNRM at 3dB. Openreach are trialling 3-5dB as you know for VDSL2.

It's a safety margin. It is expected to fall. It is rarely a fault, and if it is there will be much more serious and noticeable symptoms that a slight overnight decrease. Remember this page?

I understand the normal fluctuations in SNR margin, but this is different, the downstream SNR margin is regularly fluctuating in short intervals.

Edited by deleted (Sun 19-Mar-17 17:34:08)

Is it service affecting?

Yes, in terms of throughput.

Edited by deleted (Sun 19-Mar-17 21:26:30)

Sudden drops of snrm always makes me think of close by equipment interfering with the modem/router when switched on

DECT phones are particularly suspect in my case. Cause exactly what we see on the graph linked in the OP.

Dodgy microwave seals perhaps. Who knows?

In reply to a post by WilliamGrimsley:

Yes, in terms of throughput.

Have you got evidence if that? At the specific times?

I can confirm it's nothing in the house and the phone was not used during these blips. Anyway, it's been dropped to 4 dB now so obviously it's not causing a problem.

A reduction in the SNR margin will only cause throughput issues if there is an increase in uncorrectable errors requiring a retransmit. It would be best to monitor error rates to see if there is a correlation.

As for what's causing fluctuations, both long and short term, then the former is normally down to things like atmospherics whilst short term fluctuation cans be caused by any number of sources of EMF.

I understand that, there were no errors but the amount G.INP retransmits going on would suggest the reduction in throughput.

In reply to a post by WilliamGrimsley:

I understand that, there were no errors but the amount G.INP retransmits going on would suggest the reduction in throughput.

You really need to stop obsessing about your connection.

It may come over that way, but I'm not.

g.inp retransmissions are errors, just corrected ones, so if that is getting very high it could affect throughput to some extent, but high is the operative word. It would be transparent to the IP layer, so would not cause the sort of problems you see with packet loss, so it would never be that marked.

I already know that (that's meant in a positive not targeting way).

Just looking into your history you mentioned that you use ethernet power line adapters they are not the cleanest way to connect to a modem it has drawbacks increased FEC errors which is extra noise and can only guess that is what is causing your upstream to bounce from g.inp to fastpath and downstrean to sometimes enter the interleaving profile.

Would get rid of any ethernet powerline adapters and see how your line performs over 2 weeks.

I wish I didn't have to use them, but unfortunately we have to for our BT TV service.

NTE shift?

Too costly and I need the router next to the PC anyway. I have disconnected the powerline adapters several times and it makes no difference to line performance.

Edited by deleted (Tue 21-Mar-17 09:16:53)

Care to explain how use of something on the Ethernet side of a modem will generate increased FEC errors on the VDSL side of the modem? A small potential for radiated and conducted RFI but that is all.

powerline adapters/ networking use similar frequencies to vdsl, so could cause problems

In reply to a post by WilliamGrimsley:

Too costly and I need the router next to the PC anyway. I have disconnected the powerline adapters several times and it makes no difference to line performance.

Was a fault developing on your line back in Feb - no.

Do you have a fault now - no.

Are you obsessing over this - yes

Disable the stats it aint good for you, set an admin password you'll forget, grab a beer, forget about this nonsense.

+1

I'm not monitoring my line stats anymore, I just occasionally view them through the web GUI which I think is fine. If you have a problem with this then tough, because it's my decision and right to ask for help. I am only showing concern and interest.

Edited by deleted (Fri 24-Mar-17 08:11:15)

In reply to a post by WilliamGrimsley:

I'm not monitoring my line stats anymore, I just occasionally view them through the web GUI which I think is fine. If you have a problem with this then tough, because it's my decision and right to ask for help. I am only showing concern and interest.

Happy to help wherever I can, as are the other guys here. I noted in a previous thread you struggled to accept nothing was wrong... Saw this going down the same path.

I didn't struggle to accept, something was wrong and there are occasional spikes of severe interference on my line so something isn't right somewhere.

Now down to 3 dB, pretty happy we're back where we were.