Understanding sync speed, SNR margin and Errors

[#1]

Hi folks,
I've been trying to understand my line speed issues that I've been having. If I reboot my router, then it syncs at around 4.5Mbps with a SNR margin of around 6db, but then over the next few days the sync speed drops down gradually until it gets to about 2.5Mbps with a SNR margin of around 12-14. As I understand it, the exchange tries to keep your SNR at about 6db and if you get connection issues then it will raise the SNR whilst dropping the sync speed, but does the sync speed ever go back up again? I've had a connection for about 5 days and it's just sat at 2.5Mbps/13.9SNR at the moment. I've never noticed any connection issues/disconnections so am just after something to explain what's happening.
For the record, the full stats (at the moment) are:
Link Information

Uptime: 5 days, 7:34:04
DSL Type: ITU-T G.992.1
Bandwidth (Up/Down) [kbps/kbps]: 448 / 2.592
Data Transferred (Sent/Received) [GB/MB]: 1,99 / 456,75
Output Power (Up/Down) [dBm]: 12,1 / 19,6
Line Attenuation (Up/Down) [dB]: 30,0 / 48,0
SN Margin (Up/Down) [dB]: 21,0 / 13,8
System Vendor ID (Local/Remote): TMMB / ----
Chipset Vendor ID (Local/Remote): BDCM / ????
Loss of Framing (Local/Remote): 8 / -
Loss of Signal (Local/Remote): 1 / -
Loss of Power (Local/Remote): 0 / -
Loss of Link (Remote): -
Error Seconds (Local/Remote): 1.273 / -
FEC Errors (Up/Down): 334 / 45.935.540
CRC Errors (Up/Down): 137 / 1.386
HEC Errors (Up/Down): -37 / 1.386 / 1.682
This seems to be quite a high level of FEC errors, but there's no noise on the line that I can hear (BT quiet line test). The router's plugged into the master socket via a splitter and that's all that's plugged in - no extensions etc. It's annoying as I have to disconnect/reconnect the router every week or so to bump the speed back up to over 3M. I don't know whether these errors are normal or not or whether they are causing anything and whether I should report them as a line fault or not. The exchange is due to be upgraded to 21cn (not even on ADSL2+ yet!) soon but I have no idea whether that will help or not
Thanks!

[#2]

Then after a router reboot to re-set the sync speed, I get:
Link Information

Uptime: 0 days, 0:01:04
DSL Type: ITU-T G.992.1
Bandwidth (Up/Down) [kbps/kbps]: 448 / 4.832
Data Transferred (Sent/Received) [kB/kB]: 69,35 / 95,18
Output Power (Up/Down) [dBm]: 12,4 / 19,9
Line Attenuation (Up/Down) [dB]: 30,0 / 48,5
SN Margin (Up/Down) [dB]: 21,0 / 8,1
System Vendor ID (Local/Remote): TMMB / ----
Chipset Vendor ID (Local/Remote): BDCM / ????
Loss of Framing (Local/Remote): 0 / -
Loss of Signal (Local/Remote): 0 / -
Loss of Power (Local/Remote): 0 / -
Loss of Link (Remote): -
Error Seconds (Local/Remote): 0 / -
FEC Errors (Up/Down): 0 / 0
CRC Errors (Up/Down): 0 / 0
HEC Errors (Up/Down): - / 0 / 0

[#3]

I think you need to monitor what is happening more carefully.
If your router is supported can you run Routerstats http://www.vwlowen.co.uk/internet/files.htm and keep it going continuously (don't switch the computer off or into standby over night)
What I think is happening is that the default noise margin is 6dB which gives you the initial speeds you are reporting but then there are major bursts of errors accompanied by a significant increase in noise on the line. This causes a resync, again with a 6dB noise margin which actually gives you a lower sync speed. The noise/errors then go away and the noise margin increases to a higher level. It stays like that for a while and then you get another burst of noise/errors which causes a resync with an even lower speed.
This may repeat a few times until the speed reaches a point at which the noise margin during the periods of noise/errors is enough to hold sync albeit with a heck of a lot of corrected errors.
If you can get routerstats to work you may be able to see a what time iof day the problems occur and just possibly identify the cause.
You say that the router is connected via a splitter into the master socket and there are no extensions.
Can you please remove the lower plate of the master socket and see if there are any wires connected to it - there just may be an extension you don't know about.
You say you have connected the router into a splitter to the master socket.
This sounds wrong - unless what you are describing as splitter is in fact a filter
The only reason for having a splitter is to allow multiple phones etc. to be connected and from you description what you have is the following
Master Socket
Splitter
Phone or other things attached to the splitter
Filter attached to the splitter then to the router
Assuming I am correct then you have unfiltered items on the phone line which will cause all sorts of problems and almost certainly could explain your problem
What you should have is
Master socket
Filter
Router attached to the filter
Phone attached to the filter
If you need more than one device attached to the phone line then fit the splitter to the filter and then the phone etc.

[#4]

Jim,
thanks for the reply. You are correct in that what I was describing as a splitter is indeed the ADSL filter (it's getting late and I should go to bed!).
I've had the faceplates off the master socket a few months ago when I was removing the old extensions and running cat5e instead so I can confirm that there are no other wires attatched to the master socket either in front of or behind any of the faceplates. It's just socket --- filter --- Router/Phone. That's it.
I've downloaded Routerstats and left it running so hopefully I'll have some better stats for you tomorrow.
Steve

[#5]

Here's a graph from routerstats. Unfortunately when I got up this morning it only had the previous hour-and-a-bit of graph so I lost most of the data overnight.
I would have expected the SNR to raise/lower gradually rather than do a step-like jump. I don't have any reason as to why the connection seemed to drop out at about 7:20 this morning, or why the SNR was then low, then high, then low, now a bit higher.
Also, I couldn't find a graph for Errors over time - just the current total (which I can't get to at the moment as I'm not at home and just using the web interface to get that graph)

[#6]

That is bad.
Do you have anything on a timer which comes in at that time.
Did the router resync at any of those step points
Regarding the graph
Click on the gearwheel on the top menu, then on graphs, then change the points per page - mine is set to sample every 60 secs and 1500 points per page - gives 25 hours recording
I wouldn't bother about errors over time.

[#7]

No, I have nothing that comes on via a timer at those times. I assume that the step-behaviour in the SNR is not normal. The dishwasher/washing machine wasn't running this morning and the boiler's gas powered and only on between about 6:30 and 7:30am. Would a fridge clicking on/off do this? Though I'd be surprised if the fridge's coolant was active for about 4 hours.
How do I check if the router resynced at any point? Just by checking the connection time and seeing if it drops to 0?
Could it be anything outside the house/neighbours that could be affecting their line and causing interference on mine?

[#8]

Quote

Could it be anything outside the house/neighbours that could be affecting their line and causing interference on mine?

I've known some older security lights to cause issues with this yes, in fact any (usually older) faulty electrical equipment can cause interference. With that in mind I'd have a look at what devices might be in use close to where the line runs in your property.

[#9]

I am suffering much the same sort of problem though in honesty I get a smaller step in SNR. I believe that mine comes in from one of the houses around me on the estate. I even suspect that it is the woman up the road watching Wimbledon on a duff tele as the times often seem to match but I have not convinced myself enough to tackle her yet. So yes it could be something outside your house but it is well worth checking your house out carefully as you might just get lucky and find it is something in your control and therefore easy to correct. Best of luck in sorting it.

[#10]

I do have a security light, but why it would be causing interference for such a long period during the day I don't know. As for where the line runs into the property, I don't really know. The socket's in the middle of the house at the bottom of the stairs and the cable just disappears downwards in the middle of the wall from there. The house was built in the early 80s and there are no overhead wires on telegraph poles in our street (cable must come in underground) so if anyone has any advice on how to chase the cable from the master socket outwards then I'd gladly appreciate it.
As an addendum, routerstats seems to have found the overnight data. Seems that it was happily stable until this morning.

[#11]

The next time the REIN occurs try the BT engineers' trick with an offtune AM radio.  See here:
http://www.kitz.co.uk/adsl/rein.htm
The radio aerial is directional so pointing it in different directions will help locate where the noise is coming from.  It worked for me - in my case it was the power brick for a neighbour's PC monitor.

[#12]

ffox - sounds like a good tip! I'll have a go at that and see if I can find a cause of the noise (as it does seem to be something turning on/off given the sharp increase/decreases). Thanks!  
Now I just need to find such antiquated technology as an AM radio!
Is the interference likely to be just where the ADSL router is, or could it be anywhere along the phone-cable that could be going anywhere out into the street and beyond?

[#13]

See my post here on how to do it...... it's reply No 3
http://community.plus.net/forum/index.php/topic,105744.msg902285.html#msg902285
ffox
How did you approach your neighbour over the problem, and were they interest/cooperative etc
I've narrowed my own 'problem' to one of three houses.
Snag is I don't know the owners of any of them.
I know exactly when the interfence is happening and not and could write all this down in a diary form.
However I fear that if I
a) put a polite note through their door(s) enquiring whether the patten matches anything in their houses being switched on -  it will go in the bin.
b) knock on their door I'll be told to sod off - or worse.

[#14]

I'm on good terms with them so that helped! 
I knocked on the door with the radio sounding like a motor boat in my hand and asked if they had turned anything on in the last 10 minutes.    They replied "Yes, the PC".    I asked if they would mind turning it off for a few seconds.  The noise disappeared immediately and came back when the PC was switched on again.  By this stage they were intrigued and helped me turn off each module to isolate the cause.    Luckily I had a spare monitor which I lent them as a temporary replacement. 
This took about five months to sort out because they only used the PC a couple of times a week for an hour so.

[#15]

ffox - that graph looks very much like mine so I suspect that one of my neighbours (or someone else along the cable-route) has something that interferes. The only problem now is that I don't have an AM radio! Have asked friends if any of them have such old technology so may have to wait a few days 😞
I'm on good terms with the neighbours so hopefully if I can find a source then they'll approachable