Router synch speed fluctuation

[#1]

May I please ask the excellent knowledge base of these forums advice on something that I’ve noticed?
My router sync speed has been constant for perhaps a month to 6 weeks at 5219, however, on returning yesterday from four great days with the family in the New Forest, my synch speed was 6816, and checking a few minutes ago was 5984. I’ve noticed similar fluctuations over longer periods but have always had a speed of about 4.5mb to 5.5mb. If I didn’t occasionally check the router synch speed I probably wouldn’t even notice these fluctuations, but why does this occur? 
Many thanks
Rob

[#2]

MaxDSL is a rate adaptive service where your sync speed is the maximum that your line will support. The sync speed is determined by a target noise margin set by the equipment at the exchange. For maxDSL that target can vary in 3dB steps from 6dB to 15dB with the default being 6dB (signal 4 times noise level). The DLM can vary the target noise margin to maintain a stable connection with minimal transmission errors. Interleaving is also used for this.
Noise levels are usually lower during the day, so that higher sync speeds (e.g. 6816kbps) are achieved then. However after dark noise increases and it is possible that sync will be lost. It is then regained at a lower speed (e.g. 5984kbps) to restore the target noise margin. If the router is left on that sync speed will be retained, potentially for weeks. However if the router is restarted this "cycle" could occur again, depending on the time of day when it is restarted.
Hope this helps.
David

[#3]

6dB is 4 times noise level but a 6dB SNR margin is not. Each tone operate at approximately 35dB minimum so with a 6dB SNR margin the system will sync at a speed with a SNR of 41dB.

[#4]

Many thanks for the replies guys.
@David…….I leave the router (Netgear DG834, not sure which version and can’t check as I’m at work) on 24/7, does it periodically re-synch? This would result in the fluctuations if these re-synchs have happened at differing times of the day. 
@itsme………I do genuinely appreciate the reply, however, I’m afraid it was too technical in language for me to take any benefit from it 
Thanks
Rob

[#5]

Sorry it's me being pedantic over this statement 'default being 6dB (signal 4 times noise level)'. A 6dB SNR margin increase the SNR by 4 it does not mean that the signal is 4 times more than the noise.
Lets say that the minimum SNR that the adsl will work is 40dB. 40dB is 10,000 (10^4) So the signal is 10,000 time more than the noise. With a SNR margin the modem will sync close to 46dB and this is 39,810 (10^4.6) So the signal is 39,810 more than the noise and this value is 4 times more than the minimum SNR.
If you what to know where I got the 40dB from if you look at any of the DMT graphs that have been posted on the forums and refer to the SNR per tone it can be seen that these commonly go from 30 to 40dB. http://community.plus.net/forum/index.php/topic,71238.msg604041.html#msg604041 is an example.

[#6]

Quote from: randpwar

@David…….I leave the router (Netgear DG834, not sure which version and can’t check as I’m at work) on 24/7, does it periodically re-synch? This would result in the fluctuations if these re-synchs have happened at differing times of the day.

The router would normally only re-sync if the connection was lost due to the noise margin becoming insufficient to sustain the connection. Normally this is likely to happen only after dark when noise margins are at their lowest.
Sync is then usually regained at a lower speed. However once a sustainable (minimum) speed has been reached sync at that speed can be sustained for weeks. It would require a manual restart of the router to push the speed up again.
David

[#7]

Thanks David,
As I say, as a rule I leave the router on all the time, however, looking at the logs (example below), it appears to lose connection or restart every 2 weeks or so. If, as you say, this is ‘due to the noise margin becoming insufficient to sustain the connection.’ and re-synch after dark, would that not always result in lower speeds? As you’ve noticed before, mine seems to go in ‘cycles’?
Sun, 2009-03-29 23:36:54 - Receive NTP Reply from time-g.netgear.com
Sun, 2009-03-29 23:35:09 - Router start up
Mon, 2009-03-30 06:53:46 - LCP down.
Mon, 2009-03-30 06:53:48 - Initialize LCP.
Mon, 2009-03-30 06:53:48 - LCP is allowed to come up.
Rob

[#8]

Power dips are another possible cause? Are you in an area vunerable to such things? The other occasional cause is Static from lightning.