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Author Topic: Low SNR Problem  (Read 7510 times)

RedCoat

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Low SNR Problem
« on: August 24, 2006, 01:14:53 AM »

Ahh well basically here is the whole story of my internet connections and the problems i have at the moment..

I had 512kb/s from BT for quite a few months after upgrading from dial-up, about 6 months i had 512 for and it ran strange until i re-installed the whole BT Broadband package and i never had any trouble with it ever again.
 
Upgraded to 2mb and had that for a month, no problems, no disconnects, nothing. Had that for like a month before the thought of 8mb tempted me to upgrade again (plus it was only like ?2 extra with BT).

Upgraded to 8mb, connection was dropping late at night on the very first day and has continued to do so to this day. I had got in touch with the BT desktop tech support after 10 days and they said to wait for them to finish testing the line for the maximum speed it could handle (the test takes 15 days). So i said  okay and let 15 days pass, then 20, then 25 and so on and so forth. Each time I got in touch with them either via the desktop help or over the phone i always ended up speaking to some indian guy (who i must say was very professional and polite) who kept telling me the same thing over and over about electronical interference.

I made sure there was nothing electrical interfering with my connection and went back to them. He suggested I phone up BT and speak to them and he gave me a smart code.  Gave my smart code to the guy on the phone and he did a line test which brought up no problems and he then said he had no idea about what to do.

So thats when I started scouring google for answers and came across a few sites including this one. I found out what I think is the problem. Its low SNR. My connection ONLY drops after 11pm at night, it doesnt even flicker during the day. After 11pm i can expect connection drops at least 3 times every 30minutes, sometimes more.

My SNR stats look like this at 1.10am in the morning:-

http://img220.imageshack.us/img220/2553/snraw4.jpg

And my local exchange is literally 200metres away
(http://www.samknows.com/broadband/exchange.php?ecode=SWLJ)

I am in the process of ordering a new Netgear router (I currently use a BT Voyager ADSL 205 router) and apart from that I'm not sure what else I can do.

Thanks.
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roseway

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Low SNR Problem
« Reply #1 on: August 24, 2006, 08:06:24 AM »

Those stats aren't particularly bad. By default ADSL Max is set up to sync at the best speed it can with a downstream noise margin of not less than 6dB. You're syncing at the highest possible speed with 7.5dB downstream margin, and the router should be able to handle that. So if you're getting dropouts it's probably due to intermittent noise spikes, which can come from all sorts of sources.

The Netgear router may give you an improvement (the DG834 can hold sync down to very low noise margins) but you can probably get improvement by checking out your internal telephone wiring. Have a read of http://www.kitz.co.uk/adsl/lowSNR.htm for a lot of useful information.

Eric
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  Eric

RedCoat

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Low SNR Problem
« Reply #2 on: August 24, 2006, 02:24:16 PM »

well during the day my SNR is 8.5 so i'm not sure what else i can do
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kitz

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Low SNR Problem
« Reply #3 on: August 24, 2006, 02:59:04 PM »

If you say it only happens after 11pm then it does indeed sound like you are getting interference from an outside source. :/

This sort of thing can be one of the hardest to track down, because it could be anything from street lighting, heating, faulty lighting etc etc.
Is there anything that you can think of that gets switched on at that time.. or possibly a neighbour if are in the same *building*.

mr_chris will be able to tell you about such things as he has something similar..  its like a neighbour turns something on .. and the SNR Margin immediately plummets.  Hes not been able to isolate exactly what it is yet, but his SNR Margin can drop from say 8dB to 0db instantaneously.
The only reason he reckons he stays connected is because of the Netgear DG834G.
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mr_chris

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Low SNR Problem
« Reply #4 on: August 24, 2006, 05:01:05 PM »

Yes, tis true...

Currently connected and remaining so with an SNR margin of exactly 0dB :D

You can see my graph of woe at http://home.chrishodder.co.uk/mrtg/snr.html

It's totally bizarre. As you can see today it plummeted at 11am and is still down. I went out before 11am today so it's nothing I'm doing. I don't have central heating and all my heaters are off (as should everyone else's be!).

I have the DG834v2 (based on the Texas Instruments AR7 chipset). The currently shipping DG834v3 is based on a Broadcom chipset, same as the DG834GT, and the BT Voyager 2100 I think, and some other routers. That said, I've seen someone report that a GT holds the line at 0dB SNR margin, so perhaps it's something specific to the netgear firmware that is more tolerant to disconnects.

Or they've simply set the SNR margin reasonably high and therefore a 0dB SNR margin on the Netgear may equate to say a -3dB SNR margin on another router.. just another thought as to why.. whatever it is, I'm quite glad of it though!
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Chris

RedCoat

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Low SNR Problem
« Reply #5 on: August 24, 2006, 05:02:36 PM »

Well the only thing i can think of would be the street light directly outside of my pc room window. No heaters on or anything like that and the street light turns on before 11pm.
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dbames

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Re: Low SNR Problem
« Reply #6 on: September 16, 2006, 06:07:26 PM »

I too have been experiencing similar problems on my MAXed line.  It was updated in May (I think), and would initially drop around 8 times from around 11pm-5am.  More recently (as the night draw in!), drops can start as early at 8.30pm and run through to around 5.45am.

Initially I wanted to blame it on the streetlights, but did some investigations with the extension wiring in my house.

Currently I've got an disused extension removed (this terminated in the kitchen at the back of a fridge/freezer unit), and the bell wire removed on the extenion that is used.  This appears to have done the trick.  I've now managed a full 48hours of synch (before I would have expected around 60+ drops in that time frame).

I think (in my case), it's a combination of low SNR and additional interference in the evenings and that just pushing my line over the edge.  It's early days, but I'm quietly confident I've resolved the issue, if not I'll ask for my synch speed to be capped at a lower figure, which should increase the SNR and therefore the stability of the line.

I'm using a Thomon SpeedTouch 510v4.

fyi, my current stats are:
                                  Downstream        Upstream
Margin       [dB]     :              6.0             24.0
Attenuation  [dB]     :             24.0             13.0
OutputPower  [dBm]    :             19.0             10.5

Available Bandwidth                 Cells/s           Kbit/s
  Downstream          :            18566             7872
  Upstream            :             1056              448
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soms

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Re: Low SNR Problem
« Reply #7 on: September 16, 2006, 07:10:24 PM »

Your line stats aren't so terrible. You should look at further internal line improvements if possible to remove as much interference as possible. Your basically getting the full speed there and I envy you! Your attenuation and margin would suggest the line is pretty reasonable. As you say, stuff like street lights can be a factor, but you should see the stats vary if things like this are the cause. If I were in your boots (most of us have been in a similar position), i'd try some monitoring of the line stats at different times of day and try out further wiring enhancements, it has worked for me to a certain extent.
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dbames

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Re: Low SNR Problem
« Reply #8 on: September 17, 2006, 10:23:59 PM »

My problem was, that even with the stats I posted, I would get disconnected >33 times a night.  Since disconnecting an disused extention from the NTE, along with the removal of the bell wire I have greatly improved the stability of the line in the evenings.  This has been done without any apparent change in stats.
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soms

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Re: Low SNR Problem
« Reply #9 on: September 18, 2006, 08:29:14 PM »

Indeed, i would suspect that suggests something located nearby your extension was giving you interference. At least it was a quick fix  :)

I would always recommend using shielded cat5e cable for extensions if you need an extension, as they do offer some resilience to interference.
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roseway

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Re: Low SNR Problem
« Reply #10 on: September 19, 2006, 07:43:50 AM »

The only trouble with shielded Cat5e cable is that it's difficult to hide. A better solution generally is to use a filtered faceplate on the master socket, then the telephone extensions can be wired with any thin telephone cable without interfering with the ADSL. Of course this solution presupposes that your master socket is in a place where you can connect the modem/router without lots of trailing wires, and that's my current problem...

Eric
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  Eric

soms

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Re: Low SNR Problem
« Reply #11 on: September 19, 2006, 03:42:48 PM »

Indeed I absolutely agree with roseway. I am also in the same situation - the NTE5A is located in a place inappropriate to use DSL direct from it.

I have a modified faceplate in place, and all the wiring I can get to, be it DSL or just telephony is using cat5e where possible. of course this might throw most engineers who might lay some super-duper new 2 pair cable instead.

(In all fairness 2 pair reg cable is fine for all telephony applications, but if laying new cables and with DSL being such a big thing I would just put in cat5e cable for both data networking sockets and telephone/DSL side by side.
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