SNRM big change.

Hi guys,

I've had a couple of power cuts and some massive interference from some new power line adapters in the past few days.

My connection was capable of ~43Mbps with an SNRM of 6.5dB, however I used the --maxDataRate command on my unlocked modem to limit the connection to 38.7Mbps with an SNRM of 7.9dB (in an attempt to get interleaving taken off - bit of an experiment).

As a result of the power cuts and interference, DLM put me on a banded profile of 34.9Mbps, with an SNRM of 9.6dB. Two days later, it has taken me off the banded profile.

I am now connected at 37.8Mbps again, however I now only have an SNRM of 6.4dB, when I previously had 7.9dB. My max attainable has dropped by about 1.5Mbps.

Can anyone explain why that might be the case?

I've been logging my connection with DSLStats 24/7 if that will help, and I've ran the --pbParams command on my modem and cannot see any changes in attenuation.

Thanks for the help.

Edited by deleted (Thu 18-Sep-14 17:01:22)

Possibly crosstalk from a new subscriber to FTTC. You can never discount the addition of more subscribers at any time.

Good idea with limiting the rate though. You probably want to set it so that the ES value stays very low.

In reply to a post by WWWombat:

Possibly crosstalk from a new subscriber to FTTC. You can never discount the addition of more subscribers at any time.

Good idea with limiting the rate though. You probably want to set it so that the ES value stays very low.

Thanks. I've an unlocked HG612, are there any stats in there that could indicate cross talk? I've been keeping logs so can look for changes.

an increase of QLN I think is an indicator. It only refreshes on a new sync event. So e.g. if your modem stayed synced when the snrm reduced, your QLN will not have updated.

Thanks.

The SNRM change occurred immediately after a DLM resync. I was connected at 34999Kbps (because DLM put me on banded profile) with an SNRM of 9.6dB.

DLM then resynced my line to 38796Kbps with an SNRM of ~6.5dB (I know from previous logging that this should be around 7.9dB SNRM).

Looking at the QLN logs, you can see a bit of a difference towards the end of the graph, any idea what this indicates?

Before: http://cl.ly/XeUD
After: http://cl.ly/Xf6q

My max attainable rate is currently higher than it was on the 5th September. Here are the stats from 5th Sept, Today and what I usually see with this sync with the example taken from 7th Sept after applying --maxDataRate.

5th Sept: Max: 50888Kbps, Sync: 41490Kbps, SNRM: 6.6dB
7th Sept: Max: 51180Kbps, Sync: 38690Kbps, SNRM: 7.8dB
Today: Max: 51164Kbps, Sync: 38696Kbps, SNRM: 6.7dB

So I've got a slightly higher max attainable today compared to the 5th, and pretty much exactly the same stats as the 7th, except my SNRM is the lower than the 7th.

I've never looked at QLN so please forgive my ignorance, but a quick Google shows it's a measure of noise on the line? Any help in interpreting the graphs?

Your help is much appreciated

Edited by deleted (Sun 21-Sep-14 09:13:14)

Those QLN graphs don't help much, as they only cover the ADSL tones (tone 300 is around 1.3MHz) rather than the full set needed for VDSL2 (up to 17MHz, or around 4000 tones).

I can't tell you what to do with DSLstats - I monitor mine using the other package (HG612 Modem Stats) from the same forum, and have recently started using mydslwebstats too: http://forum.kitz.co.uk/index.php?board=46.0

QLN does indeed show you the "Quiet Line Noise" - which is the amount of "noise" measured on your line prior to the modem squawking at each other to synchronise.

A level of -140 would appear to be the base for a very quiet line at all frequencies. I have seen this *once* on my line, while the rest of the time I see a regular pattern with values between -120 and -140. For the one "odd" time, the best guess is that the DSLAM restarted that time, so the QLN measurement was made with no neighbouring VDSL2 modems running.

Other ways to look at what has happened on your line:
- HLog graph: shows the quality of your line. A non-"smooth curve" to the line might suggest problems with the wiring topology, such as a bridge tap, while significant changes from sync to sync might suggest an intermittent problem in some joints.

- SNR per tone: big changes (drops) here might suggest an increase in noise - which could be plain interference, or crosstalk.

- Bits per tone: should reflect the SNR per tone graph.

Over a year ago, I made a post on a simiar vein, where I animated my 18-month history of these graphs. You can see the post & animations here:
http://forums.thinkbroadband.com/fibre/4241788-attai...

Result: There may have been issues with the modem, and what it was reporting at the time. The behaviour I saw disappeared when the modem firmware was updated in October 2013.

I now run at a speed of 79Mbps, and an SNRM of 6dB; the bottom line is that you *will* get gradually-reducing SNRM values over time. You will also get a gradually-increasing rate of errors, until DLM is triggered; I am now in the region where this can happen, and has happened (I get around 72Mbps during DLM intervention).

Personally, I think that a drop of ~1.5dB is not actually that much, and you can expect much more. In Ireland, where they have turned on vectoring (to counter crosstalk), people have seen increases in SNRM of 10dB or more.

Thanks, I'll have a look at all that and post back if needed.

It was more the fact that the 1.5dB SNR drop was instant.

I'd synced the day before at the same speeds, with the correct SNRM, but then the next morning there is a sudden 1.5dB difference in SNRM.

Expect to see crosstalk arrive in steps, when each new customer is activated & connected, rather than a gradually-sloping reduction.

The steps themselves can vary in height, depending on how close the wires are to yours within the cable bundle. Some of these steps could be a lot higher than 1.5dB, but most are probably lower.

The lesson is to never recommend the service to your neighbours

If the stepped reduction in SNRM happened during normal working hours, there's every chance it was a new user being connected.

I have described my own experience of gradually increasing crosstalk here:-

Crosstalk Effects



You would need to be logged in to see the attached montage.

Well DLM resynced me this morning and the SNRM is now reading 7.8dB again. Maybe it was a new subscriber being installed when the resync happened (it was within 9-5) and it messed up the stats temporarily.