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Standard User Andrew_Curry
(newbie) Fri 12-Aug-11 19:11:42
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Can Someone Please Explain SNR Target To Me?


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As I understand it SNR (Signal to Noise Ratio) is a ration of how much signal there is to noise on the line (no ¤¤¤¤¤¤ sherlock )

Basically the higher the better, the higher the number the more signal you have relative to noise.

Okay understand that, basically if you have a signal to noise ratio of 6 you have 6 times as much signal as noise.

What about the target. My router stats show that I (used to) initially synch with BT's equipment at about 7.8Mb/s. This used to start out as a snr as 6.0. However, in the evening it seemed to fluctuate and the number dropped to as much as 3.0. Apparently this is common in the evening. The extra noise on the line is coming from outside the house because I have the router well away from any electrical devices, and even tried switching some nearby devices on and off to see if it made a difference, it didn't.

Anyway, my Netgear DGN2000 router seemed to be able to cope with the fluctuations no problem, so not a problem.

However, recently it has started to sync up at 8.8Mb/s and the initial SNR has changed to 3.0 There are many people online talking about SNR target or margin. Many of the threads have people with initial SNR values of excess of 6, which I would assume is a good thing, very little noise on their line.

Anyway, now that my router is initiall syncing up with a value of 3.0 it seems to drop down to less than one some evenings.

Why do we have a snr margin/target, as from what I can see it is causing me problems which I didn't have before.

Can someone please explain why we have this and what it is supposed to be doing for my connection.
Thanks.
Andrew.
Standard User MHC
(legend) Fri 12-Aug-11 22:56:53
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Re: Can Someone Please Explain SNR Target To Me?


[re: Andrew_Curry] [link to this post]
 
A 6dB SNR is actually 4 times. The deciBel scale is logarithmic - 3dB is double, -3dB is half, 10 dB is a factor of 10, 20dB 100 times ...

The router will sync at a speed where the target SNR is met - in most case 6dB unless that has been overridden and is, 3dB, 9dB, 12dB &c - this can happen for a variety of reasons. The 2wire 2700HGV will sync at 3dB on what is seen to be a good line.

The SNR will fluctuate throughout the day - get Router Stats Lite and watch it for 24 hours.

The increase in noise in the evening is due to various factors including radio transmission, street lights, and other factors. The router will hold the sync until the error level is too high at which time it will resync - the 2700HGV for example can hold sync at 0dB or slightly below but most struggle at 2 or 3dB.

If you are having problems - try a manual resync just after dawn and see if it becomes stable. Or ask your ISP to force the system back to 6dB.





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M H C


taurus excreta cerebrum vincit
Standard User XRaySpeX
(knowledge is power) Fri 12-Aug-11 23:11:55
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Re: Can Someone Please Explain SNR Target To Me?


[re: Andrew_Curry] [link to this post]
 
You are correct about what SNR is, but it is measured on a logarithmic scale.

So a reading of 3 deciBels = 0.3 Bels = log(2) Bels means Signal is twice as strong as Noise. Or for your example 6 db means Signal is 4 x as strong as Noise.

However the figures you see reported by routers are never the SNR but the SNR Margin, which is the margin which by which the noise level can rise before connection is lost.

On most good lines a SNRM of 6 dB is sufficient to give a good stable speed. On very good lines a SNRM of 3 dB will give fast speeds and still be very stable. If it starts dropping below that you have noise events that may start causing disconnections.

See Roberto for a fuller explanation.

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 17 Meg Untweaked 19 Meg Tweaked WBC


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Standard User Andrew_Curry
(newbie) Sat 13-Aug-11 08:16:04
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Re: Can Someone Please Explain SNR Target To Me?


[re: MHC] [link to this post]
 
In reply to a post by MHC:
A 6dB SNR is actually 4 times. The deciBel scale is logarithmic - 3dB is double, -3dB is half, 10 dB is a factor of 10, 20dB 100 times ...

The router will sync at a speed where the target SNR is met - in most case 6dB unless that has been overridden and is, 3dB, 9dB, 12dB &c - this can happen for a variety of reasons. The 2wire 2700HGV will sync at 3dB on what is seen to be a good line.

The SNR will fluctuate throughout the day - get Router Stats Lite and watch it for 24 hours.

The increase in noise in the evening is due to various factors including radio transmission, street lights, and other factors. The router will hold the sync until the error level is too high at which time it will resync - the 2700HGV for example can hold sync at 0dB or slightly below but most struggle at 2 or 3dB.

If you are having problems - try a manual resync just after dawn and see if it becomes stable. Or ask your ISP to force the system back to 6dB.


It used to sync (initially) at 6db, and then in the evening would drop to almost 3.
Recently it's starting up after sync showing a noise of 3db. Thing is, occasionally in the evning that means it's dropping to almost 0.
(It has only done that once so maybe nothing to worry about yet)

What I don't understand is why a router with a higher snr, and therefore a stronger signal, will sync at a lower speed.
This has given me almost a 1Mb/s sync increase, so could be good, but surely it's going to give me other problems when that low?
Standard User jchamier
(knowledge is power) Sat 13-Aug-11 10:42:50
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Re: Can Someone Please Explain SNR Target To Me?


[re: Andrew_Curry] [link to this post]
 
In reply to a post by Andrew_Curry:
It used to sync (initially) at 6db, and then in the evening would drop to almost 3.
Recently it's starting up after sync showing a noise of 3db. Thing is, occasionally in the evning that means it's dropping to almost 0.
(It has only done that once so maybe nothing to worry about yet)

What I don't understand is why a router with a higher snr, and therefore a stronger signal, will sync at a lower speed.
This has given me almost a 1Mb/s sync increase, so could be good, but surely it's going to give me other problems when that low?


Assuming you're on a BT connection, the "dynamic line management" (DLM) changes the target SNR Margin. You start at a 6db margin which means the router negotiates with the DSLAM 6db of "spare" between the minimum needed to make that sync speed work.

So if the noise in your line increases (so the margin decreases) you don't get disconnected

If your line never has enough noise to cause the margin to decrease ; then the DLM may decide to set a lower target (e.g. 3db) and resync you. This results in an increased sync speed. However there is much more risk that as noise occurs the line will get swamped and disconnect. If that happens enough the DLM notices and increases the target SNR margin and forces a resync.

Many LLU providers don't have this dynamic line management, and you can request a certain target SNR margin with customer services or on their web portal. The issue is that if you have a regular occurrence of noise that swamps the normal margin, you get constant disconnections.

The idea of SNR Margin is to keep you connected whilst the noise on the line changes. Electrical noise is not normally audible, it includes lots of things that can be "picked up" on the copper wire running down the road.

Many people like to "tweak" and force their router to negoitate a lower target SNR margin than the DSLAM instructs. This is where tools such as DMT are talked about. Doing this can give more speed, but can make the line unstable so you get disconnected (due to resync's) many times a day.

James - be* pro - on THFB - sync about 17.2mbps - BQM
Standard User XRaySpeX
(knowledge is power) Sat 13-Aug-11 17:34:02
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Re: Can Someone Please Explain SNR Target To Me?


[re: Andrew_Curry] [link to this post]
 
You didn't read what I said frown
In reply to a post by Andrew_Curry:
What I don't understand is why a router with a higher snr, and therefore a stronger signal
It is not a higher SNR; it is a higher Margin for the noise to increase w/out swamping signal, so it does not indicate a stronger signal.

You will never know the actual SNR unless you have a particularly informative router, which are rare.

1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 17 Meg Untweaked 19 Meg Tweaked WBC

Edited by XRaySpeX (Sat 13-Aug-11 17:37:09)

Standard User broadbandjockey
(member) Sat 13-Aug-11 18:38:38
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Re: Can Someone Please Explain SNR Target To Me?


[re: MHC] [link to this post]
 
In reply to a post by MHC:
A 6dB SNR is actually 4 times. The deciBel scale is logarithmic - 3dB is double, -3dB is half, 10 dB is a factor of 10, 20dB 100 times ...


Careful, you're talking about power ratios there, not voltage.

Power Ratios are expressed in dB as 10[log [P1/P2]] Hence 3dB is a 2:1 ratio

Voltage Ratios are expressed in dB as 20[log[V1/V2]] Hence 6dB is a 2:1 ratio

The SNR in this context is a voltage rather than power domain, so 6dB is double.
Standard User MHC
(legend) Sat 13-Aug-11 22:56:40
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Re: Can Someone Please Explain SNR Target To Me?


[re: broadbandjockey] [link to this post]
 
In reply to a post by broadbandjockey:
The SNR in this context is a voltage rather than power domain, so 6dB is double.



Since when?

The POWER level from both DSLAM and local modem is measured in dBm, the attenuation is in dB and the total spectral envelope is measured in POWER. Why would teh margin be measured in dBmv?





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M H C


taurus excreta cerebrum vincit
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