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Author Topic: SNR Margin question??  (Read 4526 times)

jid

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SNR Margin question??
« on: April 19, 2009, 08:17:02 PM »

Hi all,

If I were to resync my router using 10% DMT tweaking.

And it would sync at say 7000kbps and around 6.2dB, after the noise would it shoot up to say 7dB because the line is capable of more speed and the snr?

And then the next time noise comes along would it cope better - or would errors still go through the roof?

Sorry if its confusing but it was a hard one to explain :-[

Thanks

Jamie
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Jamie

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kitz

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Re: SNR Margin question??
« Reply #1 on: April 19, 2009, 11:46:59 PM »

umm if I understand what youre saying.. 

>> sync at say 7000kbps and around 6.2dB, after the noise would it shoot up to say 7dB because the line is capable of more speed and the snr?

Yep :)

>> And then the next time noise comes along would it cope better - or would errors still go through the roof?

The next time the noise came, then you would expect the SNRM to fall back down to say 6.2dB. Is that what you mean?  If so then no it shouldn't have any adverse effect on the errors because youve still got the 6dB SNRM.  The lower your SNRM, then the more chance you have of getting errors.
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jid

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Re: SNR Margin question??
« Reply #2 on: April 20, 2009, 08:34:06 AM »

Thanks Kitz,

Reason I ask is the connection dropped (again ::)) last night and was thinking of ways to make more margin available when needed.

I did indeed resync last night (I had too, synced back at 5550) and got 7232kbps and at the time had 6.3dB margin.

Woke up this morning to 8.1dB and no CRC overnight (a change as there usually are). Pretty straight line on Router Stats at the moment...

I guess I just have to balance the margin out so I ensure I get the mixture of speed and margin right so noise bursts go un-noticed

edit, Still lovely and straight! Added some stats too...

(just noticed I am back to my old ways of posting stats lol!)
« Last Edit: April 20, 2009, 09:31:04 AM by jid »
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kitz

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Re: SNR Margin question??
« Reply #3 on: April 20, 2009, 10:30:34 AM »

I really dont mind you posting your stats on this occassion since I know that youre now quite astute to whats going on and you just wanted to query this latest episode and whats happening.

>> and was thinking of ways to make more margin available when needed.

Probably the best way is depending on the time of day you resync..SNRM is normally always lower in the evenings, therefore an evening resync will give you a higher SNRM the next morning, but still sufficient to cover the next evenings without any drop outs.

What sometimes happens is users reboot their routers in the mornings.. this gives a good sync speed, but doesnt always allow sufficient SNRM for the evenings so the router will drop connection, the router gets a lower sync speed.  They turn off the router that night and then the next morning they reboot get higher sync speed, evening comes SNRM falls and then we start going round in circles.

This is why we often say leave the router and let it find its own level and only reboot if the sync is much lower than anticipated.

If youre tweaking the SNRM for best performance and stability and best speed, then a reboot in the evenings with your chosen SNRM is probably best as it will then ensure theres sufficient SNRM to cover the next evening... otherwise you stand the risk of drop outs and/or a high error rate which can cause slowness.  Take the SNRM tweaking slowly, steps at a time to ensure that you dont go too low at once and observe your line behaviour a few days before even attempting to take it down further.
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jid

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Re: SNR Margin question??
« Reply #4 on: April 20, 2009, 11:00:46 AM »

So far ..... so bad >:(

CRC have gone to 253 and SNR has dipped to 6.5dB.

As I said in a previous post, I don't notice the problems of the daytime as I am usually in school, so will leave it as I am back there tomorrow, just need to see this evenings outcoming.

I am worried though if there is a serious noise issue in the area that will be there no matter what sync and what adsl mode ::)

CRC are currently averaging around 10 per minute...

edit, just had a big burst of noise, 2000 CRC in a minute and connection went :(

Resynced back with 12dB profile and something like the attached. Those dips in the frequencies are the noise I guess? There seem to be more gaps now as well? Is this normal?

« Last Edit: April 20, 2009, 11:07:39 AM by jid »
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jeffbb

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Re: SNR Margin question??
« Reply #5 on: April 20, 2009, 03:34:20 PM »

Hi

The pattern of missing /lower bits/tone seems to be very similar in both graphs . What appears to have happened is that your target SNR has gone up and so the synch rate has gone down. Looking at the two graphs it looks as if 1 or 2 bits have been shaved off to accommodate the new margin ,if that makes sense .
Have you got routerstats ? very useful to chase intermittent burst of noise as you can graph CRCs and other errors covering 15 minute periods over as long a period as required .

Regards Jeff
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jid

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Re: SNR Margin question??
« Reply #6 on: April 20, 2009, 05:57:56 PM »

Yes I have been tweaking with DMT, hense the change in speeds...

I also have Router Stats running, not massive drops but they must effect the errors somewhere
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jeffbb

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Re: SNR Margin question??
« Reply #7 on: April 20, 2009, 07:48:40 PM »

Hi

The latest version of routerstats gives the ability to plot the bitswap. I am still trying to understand the data . It looks as during quiet periods there is little bitswapping . That makes sense . There are periods when bitswapping seems to increase quite a lot . It also seems that most bitswapping happens at the higher frequencies .Generally the number of swaps is fairly evenly distributed over those channels . There are however 3 channels where there are 10  times more swaps . My guess would be that for whatever reason those channels are experiencing some repeated noise . for example channel 211 has 114   132 bitswaps in just over 1.5 hrs . That channel has only 4 bits loaded and 26db attenuation (~14db margin).

What this means I am not so sure .But my gut feeling is that very heavy bitswapping would indicate some problems with noise that eventually  would lead to errors ??   :-\

might be of interest for you to have a look and see what you make of it
regards jeff


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jid

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Re: SNR Margin question??
« Reply #8 on: April 20, 2009, 08:00:22 PM »

Bit swapping is used to combat noise bursts.
http://www.kitz.co.uk/adsl/adsl_technology.htm#bit_swapping

Indication that bit swapping has been used means there is noise at that time.
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Jamie

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jid

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Re: SNR Margin question??
« Reply #9 on: April 23, 2009, 04:09:20 PM »

Last post on here now.

Connection up     2 days, 20 hours, 25 minutes, 45 seconds

Stats are shown, very very happy with SNR rate, for some reason which seems odd my line like this sync and SNR combination :-\

Target SNR I believe was only 7dB as I resynced in evening, however it shoots up to about 10DB on occasions, but as you can see there is some noise about somewhere almost all the time.

Thanks to everyone once again for your help its most appreciated.
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kitz

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Re: SNR Margin question??
« Reply #10 on: April 23, 2009, 06:19:10 PM »

>> hose dips in the frequencies are the noise I guess?

Yes they are :/

>> There seem to be more gaps now as well? Is this normal?

Nope it indicates a problem (noise) at those frequencies.  It is one of the things that was pointed out to you very early on and why some of us thought there either may be a fault on the line or REIN.  iirc even a couple of BT engineers whom looked at your stats thought there may be an issue.. the problem was getting Tiscali to recognise it.

Those dips are what is stopping you get better speeds.  Each bar in that graph is the no of bits loaded..  obviously the more bits loaded then the higher your sync speed.

Its when you get a problem such as this that the the 'better' ISPs come into their own, as they (unlike the mainstream ISPs) often know how to interpret linestats and can see there is a problem.

>> But my gut feeling is that very heavy bitswapping would indicate some problems with noise that eventually  would lead to errors

yep it can lead to either that or loss of sync.  Depends on how much noise though, and also wether or not there is sufficient spare to cope with the bit swapping process.
Once the SNR in a subchannel gets too low, then the router will mark it an unusable.

>> Last post on here now.


I really do hope not - honestly I dont mind you posting when you do have a problem.... which that line obviously does have somewhere.
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jid

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Re: SNR Margin question??
« Reply #11 on: April 23, 2009, 09:13:29 PM »

Another quick question, it is perfectly fine to leave my router on if I am not at home is it?

They're designed to be left on aren't they? :-\

I don't really want to loose that sync so...

Other than the gaps, I think the line is working quite well, about 3CRC this evening which averages out quite well I guess.

Thanks again for the advice Kitz, as you say there is most likely a fault along the line somewhere, but the saying is "don't fix want ain't broke" so that is what I will do :D


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kitz

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Re: SNR Margin question??
« Reply #12 on: April 23, 2009, 09:20:13 PM »

>> Another quick question, it is perfectly fine to leave my router on if I am not at home is it?

Yep, its probably best to do so... and they are designed to be left on 24/7
Ive had adsl now for 6 years and I never turn my router off - although obviously I have swapped makes/brands.
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jid

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Re: SNR Margin question??
« Reply #13 on: April 23, 2009, 09:32:58 PM »

OK thank you Kitz.

 :)
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