Reduced Downstream speeds

[#1]

Hi,
Over the last couple of days I seem to have suffered a significant decrease in my downstream speeds. Before Wednesday I was getting speeds of between 10-12M and now I am getting 5M. Yesterday I was struggling to get 1M.
My router was synching at approx 12000 and now only seems to synch at 6655.

ADSL Link	            Downstream	       Upstream
Connection Speed	     6655 kbps	        1057 kbps
Line Attenuation	     29.0 db	          14.8 db
Noise Margin	         19.4 db	         6.8 db

I can see that the Noise Margin has increased quite a lot on my line as this was about 6db in the past. So this may explain it.
Any reason why this may have happened?

[#2]

Hi newb000,
The increase in noise margin is entirely the reason for your drop in speeds.  Also it is a considerable increase.
Can you double check your filters and your internal wiring?  For an increase like that, it would suggest that there's a considerable amount of interference on your telephone line - definitely also check your telephone itself to see if you can hear any static.

[#3]

Hi,
I have removed all phones from my internal lines and the only thing I have pugged into the master socket is the Router/Modem via a microfilter.
When I removed all the phones from the sockets the Noise Margin actually got worse and is now at 22.4dB.
There does seem to be a bit of interference on the phone line but I am not sure if this is any worse than before.
Nothing has changed in my telephone wiring since the start of this trial. I have 1 other phone on an extension which also has a microfilter.
How do I check my filters? Is it a case of buying some new ones - do they go wrong?
Could the issue lie with the BT line or the exchange ?
Brian

[#4]

What was the sync speed

[#5]

The sync speed is still 6655 even with 22.4dB noise margin

[#6]

Forgive me for thinking that 22.4 noise margin is good, it means you can set your target noise margin lower and get a huge increase in speed. Setting it for 6 to 9 dB. Except I notive the upstream noise margin is lower than the downstream 

[#7]

To make sure I understand what you are saying - you unplugged the router and moved it to the master socket then plugged it in again. and the speed stayed the same with a higher noise margin
Or it was plugged into the master socket then you removed all the other phones without telling the router to resync and the noise margin rose from 19.4 to 22.4
If the latter then you need to do a resync with all the phones disconnected as disconnecting them improved the noise margin by 3dB (about twice as it is a logarithmic scale)

[#8]

The router is always plugged into the master socket. I removed all the phones from the circuit, re-synced the router and the Noise Margin increased to 22.4dB which is 3dB worse not better but the sync speed stayed the same!!
I have plugged all the phones back in, re-synced the router and the Noise Margin is still 22.1dB and the sync speed is still 6655.

[#9]

I have to say I am totally baffled - it seems as though this fancy new system has put a speed cap on your line.
I am going to wait for the experts on this one as it really shouldn't do that
Note an increase in noise margin in this context is better and does tend to indicate that there is a problem with one of the phones or filters. If it wasn't for this assumed speed cap you would have got significantly higher speeds with them unplugged

[#10]

OldJim - I think you may be getting confused between SNR and Noise Margin. With SNR higher is better but with Noise Margin lower is better. So my line is getting worse.
Anyway I guess I still need an answer to my previous questions

Quote

How do I check my filters? Is it a case of buying some new ones - do they go wrong? Could the issue lie with the BT line or the exchange ?

Brian

[#11]

I'm not - noise margin is SNR (Signal Noise Ratio)

[#12]

OldJim - my apologies I am sure I read somewhere that Noise Margin was different to Signal to Noise Ratio so now I am confused as to why James says an increase in my Noise Margin should lead to a decrease in my sync speed and to an increase in noise on my line

Quote from: Jameseh

Hi newb000, The increase in noise margin is entirely the reason for your drop in speeds.  Also it is a considerable increase. Can you double check your filters and your internal wiring?  For an increase like that, it would suggest that there's a considerable amount of interference on your telephone line - definitely also check your telephone itself to see if you can hear any static.

If as you suggest an increase in Noise Margin is good then my sync speed should have gone up when my Noise Margin increased from 6dB to 19dB.
Even more confused now. 

[#13]

It's complicated
Your Target SNRM is set by the BT equipment at the exchange during initial sync.  (although this can be overriden by forcing your router to negotiate a different margin in some cases)  This is usually 6, 9, 12 or 15 for stock ADSL - not sure what they are on 21CN yet.
Your Noise Margin is the *actual* noise margin displayed at any point in time.  As noise levels vary over time, so your noise margin will.
When referring to the Target SNRM, lower is better as a general rule of thumb.  A 6dB target is normal, and higher values indicate a noisy line.
Once the initial sync is made, any upward deviation in your noise margin indicates that the signal is becoming much stronger than the noise, and is a good indicator.  Any downwards deviation indicates the noise is becoming stronger than the signal, and is a bad indicator.
You can see your Target SNRM immediately after a sync (as it's the noise margin that your router just negotiated with the exchange). 
Large variations in noise margin indicate bursty noise on the line, and in general is not good news.  Standard deviation is perhaps 2-3dB either way.
B.

[#14]

I will try to explain.
For a given line length and attenuation the effect of noise on the line is to reduce the margin between the signal your router can see and the total signal.
Also higher speeds require a stronger signal at the higher frequencies which is where the loss of signal is more noticeable.
If you are on MaxDSL the normal rule is the lower the noise margin the better as this gives the highest speeds.
If you are on a fixed speed line (one of the old products such as 2Mb/s) then the higher the noise margin the better as this means you have a better usable signal.
The difference between the two scenarios is that in one case the speed is fixed so an increase in noise margin won't affect it. The equivalent situation on MaxDSL is where your synch speed is 8128kbps (non interleaved) where the speed can't go any higher then an increase in noise margin just reduces errors and gives more stability.
In your case the synch speed didn't alter when you rebooted the router despite the noise margin being very high. This indicates that the speed is being capped and hence the improved noise margin (or reduction in noise) isn't being reflected in higher speeds.
The reason I said that the  higher noise margin was better was that it increased by 3dB when you disconnected the phones. Normally this would have meant that the synch speed would have increased while the noise margin remained more or less constant. As you saw this didn't happen and that is what needs investigating.
Is there any chance of trying a different router to see if the router is causing the problem. I have no idea why it could but it's worth a check.

[#15]

Thanks for the explanation - I am afraid I don't have access to another router so I can't check that. Would BT be capping my speed - can PlusNet check this?