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Author Topic: SNRM - what is a normal decrease ?  (Read 1801 times)

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SNRM - what is a normal decrease ?
« on: October 30, 2009, 11:02:46 AM »

I'm on O2 LLU with a Target SNR of 6dB, synching at around 11000kbps with an attenuation of 38dB (Premium package).

During the day the SNRM normally fluctuates on downstream between 6dB and 7dB but there are several periods where it remains static within those parameters. Upstream stays more or less static on 7dB for most of the day. For example today since 9am downstream has been static at either 6.5dB or 7dB. But the upstream has been static at 7dB since yesterday morning even during last night.

After 5pm the downstream, as expected, begins to fall and usually stabilises at 2dB - 2.5dB at 9pm - 10pm. Upstream remains more or less static at the day time figure of 7dB.

"Surprisingly", since some mysterious work carried out at my local exchange on 7 October by O2, my TG585v7 has behaved itself perfectly with no unplanned disconnections.

My questions : Is the 4dB drop during dark hours normal ? Is the static upstream normal ?
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kitz

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Re: SNRM - what is a normal decrease ?
« Reply #1 on: October 30, 2009, 12:08:22 PM »

Its very hard to define 'normal' and it also depends on several other factors such as line length and power output and how hard the line is being pushed and whats going on in your area.

For eg when on Max it was very seldom that I saw any change at all in my SNRM.
On adsl2+ fluctuations of around 0.5dB are normal..  but sometimes..  say during holidays 1dB is normal.  This week Ive been seeing a bit more than usual fluctuations in my downstream.

On long lines with high attenuation then 9 or even 12db could be normal.

Its kinda just one of those things that you have to judge when looking at all the line stats and I guess over time observing the characteristics of the line.

>> But the upstream has been static at 7dB since yesterday morning even during last night.

Funny you should say that about the upstream..  
My upstream generally tends to be a tad more erractic than my downstream.
But about about 2 weeks ago my upstream flatlined completely which I thought was highly unusual.  Look what happened last night though :/

>> since some mysterious work carried out at my local exchange on 7 October by O2,

Hmmm..  guess what - I had a text about a couple of weeks ago from Be which would have more or less co-incided with when mine flatlined.
I dont have the text anymore to check the exact date..  but iirc it said something about a short period of downtime whilst they made some improvements blah blah blah.

Until you mentioned it today I never even put the 2 events together.
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roseway

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Re: SNRM - what is a normal decrease ?
« Reply #2 on: October 30, 2009, 12:30:46 PM »

Some routers don't seem to report upstream noise margin in real time. With my DG834GT the reported upstream margin stays absolutely steady from one sync to the next.
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  Eric

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Re: SNRM - what is a normal decrease ?
« Reply #3 on: October 30, 2009, 12:46:19 PM »

So from what you have said, for a 38dB attenuation, a 4dB drop in downstream SNRM may not be immediately indicative of any problems.

My only thought is that if the TG585v7 doesn't hold on at 2dB - 2.5dB which is regularly achieved between 10pm and 2am then I will see the routine disconnections return.

Prior to 7 October maintenance the router would disconnect 3/4 times a week so I can foresee a Target SNR of 9dB being requested if the disconnections return.

@Roseway : I do see an occasional blip of 0.5dB, say 3 or 4 times a day, but otherwise it's a straight line. Prior to 7 October it was always on the move, up to 2dB over 30 minutes was not unusual. Whatever O2 did on the 7 October it certainly improved the stability of the TG585v7  :clap2:
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philip_l

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Re: SNRM - what is a normal decrease ?
« Reply #4 on: October 31, 2009, 11:32:57 AM »

Hi

I'd agree your drop sounds quite normal.

Upstream uses lower frequencies that travel very well and uses much fewer of them so overall as a target for noise, they don't get hit or affected very much so you normally see much less variation in SNRM.  For my line on Be I use a 3.5 db profile so I get a bit extra of upstream as that runs at 3.5db which doesn't change much, then tweak the downstream SNR locally so it runs at around 7db as it would normally to give enough margin for the swings.

On the downstream what will cause SNR fluctuation is the higher frequencies, these are attenuated much more, and are spread out over a bigger chunk of frequency space so get hit by all sorts of noise as night approaches.  This is one reason while when on ADSL1 and a fixed line rate say 1 or 2Meg, which on most lines the higher frequencies are not needed as there is enough capacity in the lower range, SNR swings were much less, unless you were on a very long line.

Switching to ADSL2+ which unless you live next door the exchange means your line is working at it's absolute maximum, SNRM starts moving around a lot more.

Regards

Phil

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