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SNR varibale over time - using SNMP to monitor

pcmanning
Grafter
Posts: 41
Registered: 01-08-2007

SNR varibale over time - using SNMP to monitor

Anyone else done something like this...

Use SNMP to monitor your SNR (or margin) over time? I've been doing mine (Cisco 877) to try and help with an open call I have with PN (PN doing OK so far - just taking time to track the issue - with BT at the mo).

Anyway results summary...

SNR is stable for hours - about 13.5/14db usually.
SNR varies at odd times, dropping done to zero (or below I think).
Usual times for bad SNR are after about 17:00 until mid evening. But other patches occur during the day, or even in the middle of the night.

I'm assuming that this is interference from some source (or multiple sources). Possibly from other lines.

Anyway the result of this is that at bad times the router drops the line, if it does it frequently, then the "Stable Rate" drops off, from 1500 (max for the line length), through 1000 to 500. However, if it later then drops the connection once a day or so for three days, it retrains back up to 1500! At full speed I can easily download Megs of stuff at close to 1500 - so clearly the line's capable of doing the job - on a good day!

Anyone else with issues of sync speed bothered to do this? Seems that a one off check of SNR (as BT do) is not going to reveal much

Paul
10 REPLIES
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

SNR varibale over time - using SNMP to monitor

Hi,

It's actually quite normal for the SNR to fluctuate. The SNR figure is just a measure of how much signal you are receiving compared to the noise. Over the course of the day the noise will increase and decrease in the background (you are more likely to see more noise in an evening from interference from street lights and atmospherics and that sort of thing).

That would likely explain the evening results you've seen. But as you say there's likely another cause of the interference as well.

The big problem though is the drop off to zero, that's quite a big change from 14dB. I'm sure you've probably checked everything here:

http://usertools.plus.net/tutorials/id/13

but it's worth a read in case there's anything that's been missed.
pcmanning
Grafter
Posts: 41
Registered: 01-08-2007

SNR varibale over time - using SNMP to monitor

Done all the checks as part of the open call.

The variation of SNR seems far to much as it makes the line unstable. BUT also, I'm rural, so the line goes underground passed at most 10 houses to the green cab, and then underground to the exchange down the main road (labeled as an A road, but actually of B road standard. In the entire 4.5kms to the exchange, there's not a single street light!!

SO I'm assuming interference from other lines - which IMHO shouldn't happen, or electric cables, or what?

cheers
Paul
Community Veteran
Posts: 3,364
Thanks: 15
Registered: 06-04-2007

SNR varibale over time - using SNMP to monitor

Hi Paul,

For sure everything stated by Mr T is correct about what can happen on long lines.

A long line means the signal from the exchange is weaker, so an eletrical noise source close to your house will have a large affect compared to if it were close to the exchange.

What is the attenuation on your line? Greater than 55dB?

The most annoying part of MaxDSL is the need for the 3 days of stability before speeds increase, BT are probably going to introduce something called Capped MaxDSL, so you could select a package that always tries to achieve a 2Mb speed and no more every time your modem connects, this is good, the bad is that uploads will be limited to 288kb/s again compared to 488kb/s. This could make sending photos by e-mail or uplaoding photos to a website very slow especially with the average camera now producing 1MB/picture.

SW.
--
3Mb FTTC
https://portal.plus.net/my.html?action=data_transfer_speed
JJ
Grafter
Posts: 229
Registered: 12-08-2007

SNR varibale over time - using SNMP to monitor

Paul,

Now this is a bit of a wild card but you don’t happen to live near Rampisham (Dorchester), Woofferton (Ludlow) or Skelton (Penrith).

John H
pcmanning
Grafter
Posts: 41
Registered: 01-08-2007

SNR varibale over time - using SNMP to monitor

No - Gower Penisular near Swansea. Nothing to do with putting DSLAM's in the green cabs - I'm guessing thats why you ask?

Capped rates would probably force mine down to 500kbs I guess - rubbish. I'd be happier for BT to tweek things so that I had a constantly variable rate dependent on the conditions - assuming that they can't sort the interference.

59db attenuation for the record.

Paul
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

SNR varibale over time - using SNMP to monitor

Quote
SO I'm assuming interference from other lines - which IMHO shouldn't happen, or electric cables, or what?


Shouldn't, but doesn't mean that it isn't the cause. An error or corrosion (or something daft like a bird/cat/dog attacking the line) somewhere can cause a crossed line or loop in the copper pair.

The electricity cables could cause interference as you say, or with the underground cabling it can easily get flooded (don't suppose there's any patter with the rain, e.g. it always drops when or just after it rains) or cold/hot weather. Both extremes of cold and heat can cause damage to the copper.

59dB is a very long line, it's just inside the limit for 1Mbps (under 60dB) but it's still a rather long copper run and a lot of places for something to be causing a problem.
pcmanning
Grafter
Posts: 41
Registered: 01-08-2007

SNR varibale over time - using SNMP to monitor

Did think it was rain perhaps, but that would change the attentuation rather than the SNR? But seen no connection with weather. Looks more like interference from other traffic - early week day evening would be people coming home from work/school and logging on.

Paul
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

SNR varibale over time - using SNMP to monitor

Attenuation doesn't change all that much, I remember back in the day where people had lines that were marginal for 2Mbps or even for 512kbps BT went entirely by the attenuation figure and it would rarely change on a line.

Could be that it's some kind of crossed line then, best thing is to raise it via the fault checker and get BT to take a look into it.
pcmanning
Grafter
Posts: 41
Registered: 01-08-2007

SNR varibale over time - using SNMP to monitor

Have a call open already, awaiting second BT visit on Friday!

Was just interested in the amount of SNR variation. i.e. what's acceptable and what's not.

Paul
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

SNR varibale over time - using SNMP to monitor

Hopefully the engineer will be able to find the cause, but it may not be an easy task.

Tough call to say what's normal, a 3-4dB variation would be completely normal on any line and probably up to about 10dB would be normal on a longer line. 13-14dB certainly isn't within the norms.