Changing Sync Speeds

Plusnet managed to get BT to change the target SNR on a line from 15dB down to 6dB last night. Since the change the line shows no "disconnect" (ie it has been up for over 19 hours with no disconnections).

The strange thing is that the speed has been changing during this time and the SNR staying permanently at 5/6dB. So, initial sync rate was 5344 and has then be changing to 4992, 5504, 5568, 5344, 5792, 5440, 5664, 5728, 5792, 7104 and currently up to 7296. So, it seems to be holding a stable SNR but changing the sync rate to maintain that SNR (and as I say, without ever actually showing a disconnection event).

So - the question is - why is it doing this? I didn't think the BT IPStream solutions did automatic sync rate changes? It is standard IPStream (exchange not LLU or 21CN). I have seen occasional events like this before but never a line holding a 6dB SNR and adjusting the sync 11 times in 19 hours to maintain that. If it were dropping the line I could understand it but it isn't...

Have others seen this behaviour? Is it something new? I would think it was the line training had been turned back on but I didn't think this happened even during that? Has BT accidentally enabled something that is not normally provided? Will be interesting to see what happens tonight as line normally fluctuates by around 4 or 5dB during the night so will be interesting to see if it adjust down the same way it is adjusting up.

Also - 7Mb is unheard of for this line - it has been running for about 4 years and never got over 5Mb.

Edited by ian72 (Tue 24-Nov-09 10:33:48)

The sync speed is changing because you are losing your connection. This is a worrying sign and it is quite possible your default noise margin will increase unless you can find the cause.
The fact that your router log isn't showing the disconnection is probably down to the router not logging it

Edited by deleted (Tue 24-Nov-09 11:41:39)

The uptime is not changing, there are no disconnections logged anywhere. Usually when a netgear loses connection it resets the timer for the interface - that is not happening.

I agree that a Netgear would usually show this.
Have you tried another logging program such as Routerstats.
One other thing - are you on a fixed IP address or a variable. If variable is there any indication that it has changed
You could also raise a ticket with Plusnet to ask for the connection log over the same period.

It's a fixed IP. Not looking to speak to Plusnet if I can avoid - was more interested if anyone else had had this before. Will see what happens overnight tonight.
In the past if it has reset it has reset the timers (and logged it) and normally would then stay at that speed until SNR dropped below 0dB - I do not believe the SNR is dropping that low and therefore do not believe it is actually disconnecting. It looks more like seemless rate adaption - but that shouldn't be happening on the BT IPStream product.

The PPP session may not be dropping if the modem is resyncing quickly, i.e. burst of noise is so short and resync time fast that it can do some before the PPP session timesout.

Seen a few modems do this.

It is doing it about every hour. There has not been a single event that the netgear DG834G has recognised as a resync. No logs that the PPP session has been renegotiated. And it always comes back as being 5 or 6 dB SNR - both before and after the event (I'm taking stats every 5 minutes and seeing no variation of SNR outside of that 5-6dB). The strange thing is that it is increasing speed as well as decreasing - normally you would see a period of higher SNR before a resync to a higher speed but I am not on this link. It is very unusual.

Tonight will be interesting as most SNR swing on the line occurs during the hours of darkness so will be interesting to see if there are any proper resyncs or just speed changes but keeping the same SNR.

I had expected that once the connection settled to it's highest stable sync rate it would stay there until a reboot (this is what used to happen). But, this seems to change as soon as there is any minor change in margin that either gives it a little more or a little less speed. It really doesn't look like resyncs due to noise bursts (not saying it isn't but the pattern feels completely wrong for this).