Sync losses, negative numbers on SNR margin?

The router has a SYNC-light which shows if the ADSL link to exchange is up. There is also a NET-light which shows if logged into ISP and any traffic. Also LAN-lights for each lan port.

The router is set to connect-to-ISP on command. If I'm at computer and look at router seeing only SYNC-light on then to get online I initiate a 'ping xxxx' in a terminal. I see the LAN-light flicker and a couple secs later the NET-light comes on and I see the ping result, off I go using the internet.

Now, I happen to look at the router and only SYNC-light is on but then it goes off for about 10secs, flashes slow then fast, maybe repeats and then goes steady. This means that something has caused a re-sync or IOW I had a sync loss.

This time when I initiate a 'ping xxxx' I see the LAN-light flicker and......nothing. I try another command which should get the router to connect-to-ISP but nothing here either.

I fire up browser and log into router. I go to Maintenance and page which can show tech details including the UP/DOWN line attenuation, power and SNR margin. I see that the SNR margin downstream is '-x' and 'x' can vary, in the times this has happened, from 1 to 8. So the SNR margin is showing a _negative_ db figure.

When things are normal I see it vary between 6 and 11. Otherwise the other parameters always stay much the same within +/-1.

These are the figures for majority of time.

noise margin upstream: 25 db
attenuation upstream: 12 db
output power upstream: 9 db

noise margin downstream: 6 db [between 6-11]
attenuation downstream: 25 db
output power downstream: 23 db

This page has an adsl reset button, so I click on that. After a few secs the SYNC-light goes off and does the slow/fast flash then goes steady. I recheck the details and it's usually returned to a positive figure. I say 'usually' because some times when I've been online and suffered a few sync losses inside 30mins or so, I do the reset and it still comes back with a '-x' number. Maybe the second or third reset will fix it.

And note that I get the same Down SNR margin '-x' trying 2 other different devices when there seems to be a period of sync losses. And one of them will show a 10 digit +ve number 2147365248 at times.

And note this happens in the Test Socket too. Standard phone (or none) and no wireless.

I'm about 1.5Km from exchange and lines are underground.

This can happen more so when some days of very wet and windy weather but just as likely to get a chunk of re-syncs when it's fine. Times of day vary too. The phone line gives a very gentle hiss though at the stormy times I did hear a mild level increase and the odd 'spzz' and 'shhkrss' but not, I'm sure, what would warrant a BT fault.

Started a few months back when I got moved to 21CN. Had been rock solid beforehand with an 8128 sync. The sync now shows 14333.

Is it still too "general" to point the finger at a certain part of the chain, or what do people think?

an increase in noise will drop the SNR margin and cause a retrain, if the noise is erratic the retrain may struggle to lock on until the noise has passed (if it's a short burst).

The internet connection can be slow to re-establish if the PPP session doesn't spot the break in connection and needs to reconnect. "Nailed up connection" may be a better option if available.

You could try forcing it to G.dmt / G992.1 to stick at 8128 sync ??

I erred in not adding the question wrt the post title.

What significance is there in seeing negative numbers for the SNR margin?

There's no way to change the adsl mode. I'd either have to revert to an older device or see if ISP can put me back. I know what you mean by "nailed-up". I have used that as well as having a long time-out.

A negative SNR means that the background noise is greater than the signal - albeit averaged across the spectrum. In general ADSL will not sync or pass data when the signal is nearly lost in the noise - although some parts of the spectrum may be fine and you will get some data across. There are some modems - 2wire2700 series for example that will hold sync and operate with negative SNRs but that is down to a good front end.

In reply to a post by MHC:

A negative SNR means that the background noise is greater than the signal - albeit averaged across the spectrum. In general ADSL will not sync or pass data when the signal is nearly lost in the noise - although some parts of the spectrum may be fine and you will get some data across. There are some modems - 2wire2700 series for example that will hold sync and operate with negative SNRs but that is down to a good front end.

No, No!

You are correct in saying "A negative SNR means that the background noise is greater than the signal". However the value being discussed is the SNRM not the SNR which most firmwares erroneously call it.

A negative SNRM does not mean the noise is greater than the signal. You can have an SNR of 40dB quite happily with an SNRM of -1, it just means the original averaged SNRM figure at sync time was not spot on.

What significance is there in seeing negative numbers for the SNR margin

it means the noise has increased to the point where the error rate is likely to be higher than that defined with the SNR at 0dB margin (1e-7 from memory).

What model is the router ?

Haven't been at home for a bit, reluctant about logging in on public access.


The router's a Zyxel 661 and older model tried is a Dlink624.