cancel
Showing results for 
Search instead for 
Did you mean: 

Attenuation and noise margin question

Community Veteran
Posts: 1,160
Thanks: 1
Registered: 01-08-2007

Attenuation and noise margin question

I'm currently trying to troubleshoot wildly fluctuating sync speeds - from well under 1000 to just over 6000 (have been on MaxDSL a while so it's not 'training'), with queries to both BT and PlusNet support.

One thing I can't see anywhere is about whether or not I should be seeing changes in attenuation, which I thought just indicated distance from the exchange.

I know that the lower the noise margin the more likely the speed is to drop in order to maintain a workable noise margin but I'm also seeing a variation in attenuation (quite apart from the phenomenom where attenuation occasionally appears to drop to 15.5, which I understand is some BT glitch).

Is this normal? It's only a few db but I wonder if it's a cause or an effect of the fluctuating speeds.

Thanks if anyone can enlighten me!
Plusnet user since November 2003
Currently on Unlimited Fibre Extra and Unlimited UK & Mobile Calls
8 REPLIES
N/A

Attenuation and noise margin question

I've only ever seen this attenuation bug on the DG834GT - but that's all it is, a reporting bug. The attenuation is always constant (give or take a few dB depending on how the BIN's are used).

The fluctuating sync speeds sounds like you have a fault with your line or extension wiring (probably the latter).

Have you tried plugging a filter into the test socket behind the BT master and then your modem into the filter? (just to make sure it's not your extension wiring causing this)

Craigiusmaximus
Grafter
Posts: 212
Registered: 30-07-2007

Attenuation and noise margin question

Has the bell wire been disconnected from in the main socket??

there is a thread somewhere ill have a look and post a link to it.
Craigiusmaximus
Grafter
Posts: 212
Registered: 30-07-2007

Attenuation and noise margin question

pacem
Grafter
Posts: 175
Registered: 07-09-2007

Attenuation and noise margin question

Once you've eliminated other extensions causing the problem you may want to keep a diary of the weather. Water around a faulty line can cause big changes in sync speed. BUT it can take several days for surface water to reach to affected cable, hence the need for a diary.

Note on SNR:
Unless you are very close to your exchange your SNR shouldn't change (much): As line quality reduces your SNR will reduce until it reaches your minimum SNR target (probably 6 or 9 dB) after that your router will negotiate a lower sync speed to maintain the SNR at your target.

[Ponders] I wonder if a valid line quality coefficient can be calculated from SNR and sync speed. [Starts another thread]

Paul.<><
Community Veteran
Posts: 1,160
Thanks: 1
Registered: 01-08-2007

Attenuation and noise margin question

Thanks all for your replies.

I've done the plugging into the back of the master socket bit and it makes not a scrap of difference either to synching or to the quality of the telephone line.

The line is noisy in wet weather and I've been keeping a note of that in my fault ticket that's currently open, which shows the relationship between wet weather and low speeds. Seems to me to be water getting in somewhere pretty fast when the synch rate drops and the sun drying it out as it gradually increases. BT checked the poles near the house but I don't think they spent the time to have looked anywhere else. They seem to be in denial about there being a fault, which is why I've passed it on for PlusNet to investigate too.

I think the bell wire is disconnected on the extensions as there are a couple of wires from each cable that aren't wired into the front bit of the master socket. But the readings on the router are the same whether it's plugged into the back of the master socket or into an extension.

My SNR generally stays around 5 db - if it goes lower I know the speed will drop and when it goes higher the speed will eventually increase. So, from what you say I understand I shouldn't take any notice when the attenuation is just a few db extra when the connection speed is slower?
Plusnet user since November 2003
Currently on Unlimited Fibre Extra and Unlimited UK & Mobile Calls
Nocturne
Grafter
Posts: 140
Registered: 01-08-2007

Attenuation and noise margin question

Having been 'upgraded' four months ago I have very similar problems:

Speed drops from 6500+ down to 160 kbs
Noise on the phone following rain
Attn up from 43 dB to 57 dB when speeds low
Phone drops ADSL
20 dB SNR drop in 15 mins - Not sure what effect a phone call would have at these times

CS confirmed they will get interleaving turned on.
Not sure how much that was expected to achieve.
Interleaving has been on for at least two months anyway so rather limited progress so far.

Hope you get sorted soon and that I'm not far behind you.
In the meantime I'll follow your post with interest.
N/A

Attenuation and noise margin question

I have been messing for months with my Maxed. My snr can be 19 at 4 meg but as low as 5 if I connect at 6 meg. The other night it was 2 for hours but kept going. Some points I have noted which may help
I have removed the bell wire no diferance.
Pluging into the master socket means I connect at 6.5 meg but the connection drops after 1 min 40 secs.
If I connect with the old filter I connect at 4.6 meg snr of 8 if I then plug in the new filter snr goes up to 18.

If I connect with the good filter I connect at 5+ meg but it will drop the line at about 6-7 oclock(pressume it traffic)

If I plug a old phone in without a filter I connect at 2 meg and I sometimes have to use this if its gets silly connecting.

So a good filter ups the snr a lot but gives you a faster connection which you may not keep when it get buzy.

Some thing which effect the connection are the weather ski tv and the cable to it(if its near the phone line it pumps out loads of noise) Dec phone.
Nocturne
Grafter
Posts: 140
Registered: 01-08-2007

Attenuation and noise margin question

I'm a little unclear about some of the advice you were given.

Quote
Have you tried plugging a filter into the test socket behind the BT master and then your modem into the filter? (just to make sure it's not your extension wiring causing this)


Accessing the Test Socket removes any extension by default.
My understanding is that you can plug your Router straight into the Test Socket.
Its the phone that needs to be filtered when used in conjunction with ADSL.
The Router is connected unfiltered to the BT line.

A Filtered ADSL Faceplate allows you to connect a 'phone' extension to filtered terminals and an ADSL extension to unfiltered terminals.

You could of course connect a phone and Router direct to the appropriate sockets on the Faceplate.

Having said that your symptoms are almost identical to mine and you seem to have eliminated your own wiring.