Bad SNR and instability of connection question

Connection issues, drop outs or speed related faults for ADSL and ADSL2+ services
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caspersky
Posts: 17
Joined: Fri Aug 15, 2008 3:43 pm
Location: Toowoomba

Bad SNR and instability of connection question

Post by caspersky » Fri Jul 10, 2009 5:32 pm

Hi

This is an ongoing problem rather than a suddenly happened one. I live in Toowoomba and about 3.5km from Middle Ridge exchange. I have always had the problem of low downstream margin which constantly fluctuates between 0-10db. I know that is bad so i'm asking some questions to see if it can be fixed:

1) If downstream margin suddenly changes from one number to another (eg. 2db -> 7db), does that mean drop outs occurred? I'm asking this as i can always see my downstream margin keeps changing all the time and i suspect every time it changes that indicates that a dropout has occurred.

2) Is there any possible solution to increase downstream margin? I know moving nearer to exchange works but that is not an option for me.

3) Is there anything you can do to fix the low SNR problem? I'm on Naked ADSL2+ and Exetel had already lowered my profile to 8mbps to stabilize the line and i don't want anything lower than that.

Below is a screenshot of my Linksys AM300 modem status:
Untitled.jpg
Untitled.jpg (26.66 KiB) Viewed 972 times
Note 9db is considered high for me, i sometimes get 2-3db even 0db.

The problem of low SNR has really been bugging me long, and the problem gets worse especially when i'm playing online games - they frequently disconnect. I really hope the problem can be solved and i'll be very grateful.

Thanks

Clevo89
Posts: 48
Joined: Thu Sep 11, 2008 10:41 pm
Location: Sydney

Re: Bad SNR and instability of connection question

Post by Clevo89 » Fri Jul 10, 2009 7:36 pm

Hi... I don't know how much I can help but I can pass on some observations.

I used to have the D-link504T and my downstream margin would, like yours, fluctuate between 3 & 10 db. My understanding of this fluctuation tends to describe more the noise aspect on the line. Therefore the higher the line noise the lower the db and the lower the db the higher the chances of the ADSL signal staying "true". Although it doesn't signify dropouts it's probably is one of the causes of drop outs...or a difficulty in staying connected.

since changing to the NB9WMAXX modem, I have a steady 6.5db ds margin and a slight increase in sync speed. Following the discussions on various forums I believe this has something to do with the Broadcom chipsets in the newer modem and the ability of those chipsets to "ignore the noise" and maintain some stability. With an attenuation of 43.5 I was never going to reach peak speads but I've had a steady, close to, 7Mbps and no problems with VOIP (plus I have a laptop connected wirelessly).

And with that...haven't had a single dropout unlike my experiences prior to to getting this modem.

The new modem seemed to help with my flaky line but I can't guarantee this will work for you.

caspersky
Posts: 17
Joined: Fri Aug 15, 2008 3:43 pm
Location: Toowoomba

Re: Bad SNR and instability of connection question

Post by caspersky » Sat Jul 11, 2009 10:57 am

Hi

Thanks for the input. Like you said, newer modems may have better design to stay stable even in an unsound condition. But, i previously stayed at another premise which connects to the Middle Ridge exchange via CMUX system(or something like that), and i didn't have such a problem connecting to Exetel via ADSL1 1.5mbps. Perhaps Telstra infrastructure is somewhat better than Optus? Not sure about that. Anymore input guys?

Dazzled
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Re: Bad SNR and instability of connection question

Post by Dazzled » Sat Jul 11, 2009 11:03 am

A CMUX may be thought of as a kind of small secondary Telstra exchange, built into a cupboard in the street. It possibly was quite close to your friend's place, whereas Exetel/Optus ADSL2 must be run all the way from the main exchange. There is a photo of one here- http://www.onthenet.com.au/broadband/pgSystems.asp

Optus street cabling from the exchange to your place is the same as Telstra's. Optus leases the exclusive use of a conductor pair from Telstra.

CoreyPlover
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Re: Bad SNR and instability of connection question

Post by CoreyPlover » Sat Jul 11, 2009 3:11 pm

caspersky wrote:1) If downstream margin suddenly changes from one number to another (eg. 2db -> 7db), does that mean drop outs occurred? I'm asking this as i can always see my downstream margin keeps changing all the time and i suspect every time it changes that indicates that a dropout has occurred.
If the downstream drops to 0dB, then the modem is forced renegotiates a connection because 0dB noise margin means that noise levels have increased to the point where they impede upon the ADSL signal. If noise margins increase dramatically, the modem often chooses to renegotiate a conenction because it thinks that it can capitalise on the higher margin and sync at a higher speed. So in short, yes: fluctuations in noise margins result in drop outs or at least, modem re-negotiations.
caspersky wrote:2) Is there any possible solution to increase downstream margin? I know moving nearer to exchange works but that is not an option for me.

3) Is there anything you can do to fix the low SNR problem? I'm on Naked ADSL2+ and Exetel had already lowered my profile to 8mbps to stabilize the line and i don't want anything lower than that.
Well, the noise margin represents a buffer against fluctuations in background noise. The higher it is, the more tolerant and stable your connection is, but a higher noise margin also makes it sync slower. You can easily get a higher noise margin if you are willing to sacrifice speeds. For example, a 1.5Mbps profile will give you a higher noise margin but you have indicated you don't want this. What it sounds like you want is a way to reduce the noise fluctuations in order to get a more stable connection. To achieve this, you must identify and fix the source of the noise interference and unfortunately this is not easy.

It might be a connection at your exchange, it might be poor quality copper running from your exchange to your house, it might be bad wiring in your house, or as Dazzled says, it might be the modem itself. So you can easily test the modem and your house wiring by trying replacement modems and different wall points and ensuring you use short phone cables to connect the modem to the wall point. You can also request a technician to test your phone line back to the exchange. If they find a fault, they fix it, otherwise they charge $220 for the call out, so this is a decision and risk you will have to accept if you proceed with it (and if so, you can request Exetel arrange it for you via residentialsupport@exetel.com.au).
caspersky wrote:i previously stayed at another premise which connects to the Middle Ridge exchange via CMUX system(or something like that), and i didn't have such a problem connecting to Exetel via ADSL1 1.5mbps
The reason you didn't have a problem is because the 1.5Mbps profile is a much lower sync speed, therefore has a lot more buffer to withstand noise fluctuations. Also, this might have been a completely different (and higher quality) phone line. In general, the "infrastructure" is identical because telcos lease Telstra laid phone cables and they simply connect their own infrastructure at the exchange.

caspersky
Posts: 17
Joined: Fri Aug 15, 2008 3:43 pm
Location: Toowoomba

Re: Bad SNR and instability of connection question

Post by caspersky » Sun Jul 12, 2009 4:02 pm

Thanks for the answers Corey, cleared my doubts.

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