Why does my SNR drop

Just a query really as my Netgear DG834Gv5 has always done this. Doesnt cause a disconnect but not sure why it does it

When i reboot the router it syncs with a noise margin of 9db as thats what ive set it to (on LLU so can manually configure noise margin). However after a few week without a resync, my noise margin will slowly drop overtime. For example now my noise margin is sitting at around 7db after 20 days without a reysnc and even during the afternoon wont go any higher. If i left it another week or so, the noise margin would be 6db and so on. Just slowly drops and drops over many weeks. If i reboot the router it goes back to 9db with the same sync.

So is it some router bug that slower drops my noise margin? Doesnt effect anything - speeds, errors, sync etc all fine. Just seems odd that it doesnt maintain the same noise margin

Edited by bobble_bob (Sat 18-Jul-15 08:59:00)

If you have software that can examine the bit-loading graph, the old DMT monitor does have, then I think you might find that over time a fair number of tones have lost a lot of allocation or even been dropped.

Edited by RobertoS (Sat 18-Jul-15 11:48:58)

So it that normal behaviour and/or something out of my hands?

Not normal. It is noise affecting the relevant (probably group of) tones (bins/frequencies).

As they get written off, bits are swapped into other bins to compensate, until they become full. At which point your noise margin starts to decrease.

A re-sync starts from scratch, with all bins used to their maximum capacity.

When I say "full", IIRC a bin can hold 14 or 15 bits, but attenuation on that frequency will for many make the actual maximum on it lower. Easily seen on a graph.

Edited by RobertoS (Sat 18-Jul-15 12:17:31)

Hi

I think it is normal, I found the same issue. It is bit-swapping that causes it. Noise can be quite random, so bins get affected by noise, the data they carry is swapped to other bins, if the bin is effected so much it can't carry any data it is turned off and never comes back into play again. Of course at some point the random noise may go away, but the bin is now permanently out of action. The same goes on with other bins, so over time there are less bins but the same amount of bit-rate being forced over the remaining ones so margin is eroded, and I guess that is one reason we start off with some margin in the first place.

If the noise isn't too random than the affected bins get shut down, and it becomes a survival of the fittest, the remaining bins are good quality and strong and not getting hit by the noise, so even though the margin has been eroded, overall the line isn't any less stable because of it.

A re-sync is a fresh start so all the shut down bins are back in play until they get knocked out again.

SRA (seamless rate adaptation) doesn't even re-enable the bins, but it works around it by re-negotiating a slower sync-speed which gets some margin back that way.

When I visualised the bins on my line being affected this way on ADSL it was always the tail end ones, the higher frequency bins that started off already borderline.

Regards

Phil

Edited by deleted (Sat 18-Jul-15 17:28:08)