Very low SNR Margin

[#1]

Hi,
Since my line got upgraded from 8 Mbps after BT upgrades in my exchange (about August 2012), I have had an issue where after a few days, the SNR margin drops to a very low value then the line resyncs back to a lower spped but higher SNRM value.
Currently  I have 10941 kbps at 43 db attenuation and 0.9 db SNRM.  The RCO is showing as 11784 kbps.
When the line drops, it comes back at about 9148 kbps / 43 db / 4.8 db SNRM, where it remains for a week or two before jumping back to nearly 11000 kbps
The router is a Netgear DG834PN plugged into the main socket with an ADSL faceplate fitted, no other equipment is plugged into the line.
Any ideas if it is a problem with my router, my line or the Openreach Exchange?
Thanks.

[#2]

Hi there,
I've looking at the connection logs (as below) the connection looks quite stable at the moment but I can see that the line rate has reduced to 9653kbps presumably due to the previous resynchs.
It's not appropriate for me to go out on a limb but it's less likely IMHO to be the router as you'd more likely be seeing a continual issue. Does your line have a test socket you could connect to whilst we do another test?

[#3]

Hello Adam,
I have removed the faceplate and plugged the router into the master socket.  The attachment lists the stats at my end after I reconnected.

[#4]

Interestingly the sync speed has increased by almost 1mb when you did that, what other devices apart from the router do you have connected to the lines? do you also have any extensions running from the master socket?
PS as the synch rate is still lower than the IP profile by a considerable degree I'm going to submit an SNR reset now for you.

[#5]

I usually just have a standard phone handset and a call reminder display.  There are no extensions from his socket.  
I've just seen the line drop and return at 9872 kbps at 7 db SNRM, which is now reducing to 5.4 db
I'll leave it for an hour then put the faceplate back, leave it another hour and plug the phone in, then leave it a further hour and add the call reminder back.

[#6]

Good Morning Adam,
Reconnected everything last night, and the speed & noise values seem to have remained pretty stable. I'll keep an eye on it for a few weeks, but the jump in speed when I connected to the master socket suggests the filter built-in to the baseplate may be faulty?  I'll look into getting a replacement.
Thanks.

[#7]

There is a program called Routerstats Lite that works with most routers and will monitor how the SNR changes with time (whilst your computer is on, that is).  Using this I was able to see that my SNR drops by about 1.5 dB at night and also dropped by about 1 dB when my wife switched on her computer - replacing the computer PSU fixed that issue.  I also see that when my router reconnects it starts at about 1 db above the target SNR but this 'extra' dB only last for the first day after a reconnection - I have no idea what that is all about.

[#8]

A slowly reducing D/S SNRM is something I've noticed but learned how to manage in my nearly 1 year with ADSL2+, my assumption being that it is due to the exchange managing my connection for minimum power consumption, although it's not possible to confirm that as my DG834v4 router does not report D/S output power.
Actually the situation is more complicated than that as the DLM persists in choosing a D/S SNRM of 3dB which drops to 1dB or even less at night with resulting huge error counts and sometimes disconnections, so I run it tweaked (via Routerstats) to 5dB, although it always resyncs at 7dB before slowly dropping back to the target figure over about a week.  
This is not something I feel I should have to do but I'm used to it now and the slightly reduced connection speed and throughput is a small price to pay for very good reliability.

[#9]

Your connection is already very good at 6dB, let alone 3dB! You sync at 9000Kbps approximately at a 6dB SNRM. On the same attenuation as you I get almost a meg slower.
BT's 21CN DLM is a strange beast...

[#10]

Thanks for all the feedback, good to know that I'm not the only one who sees their connection behave like this.
I replaced the ADSL faceplate with a new one (from PC Supplies Ltd, via Amazon)  and some tones above 328 returned, so maybe the filter built into this one is better than the previous one (that BT supplied and fitted) ?
I use routerstats and can see the SNR change over time, but haven't been able to tie the changes to anything in my household. Waiting to see what happens when the neighbourhood switch on their Christmas lights 😞

[#11]

Quote from: ammonyte

Hi, Since my line got upgraded from 8 Mbps after BT upgrades in my exchange (about August 2012), I have had an issue where after a few days, the SNR margin drops to a very low value then the line resyncs back to a lower spped but higher SNRM value. Currently  I have 10941 kbps at 43 db attenuation and 0.9 db SNRM.  The RCO is showing as 11784 kbps. When the line drops, it comes back at about 9148 kbps / 43 db / 4.8 db SNRM, where it remains for a week or two before jumping back to nearly 11000 kbps The router is a Netgear DG834PN plugged into the main socket with an ADSL faceplate fitted, no other equipment is plugged into the line. Any ideas if it is a problem with my router, my line or the Openreach Exchange? Thanks.

I have had a selection of Netgear routers and thomson routers on the same line and all the Netgear routers have displayed this sort of behaviour in that they hang on to the connection like grim death even though the noise margin has reduced to a point where data transfer has slowed down to a crawl. The thomson routers on the other hand take longer to re sync hang onto the connection quite well but if the noise margin falls they release the connection and re sync usually at speeds which are little different from those it was using prior to this time. Overall the most stable connections I had on a line that had a 63db attenuation and a sync that was about 2000kbps  came through the thomson routers whilst the fastest connections came from the netgear though the higher speeds were rarely ever held for more than a short time. In the end I put netgear equipment on one side.