Noise margin

PuncH · Jun 13, 2009, 08:36:48

Never really understood what it meant, so I was wondering if someone could give me a quick explanation?

Is a low number good or bad? ie 6.1db

DAB Badboy · Jun 13, 2009, 08:47:34

It indicates how higher the level of your signal is above the background noise. IE shows how much you signal would have to drop before it was lost in the noise.

My Noise Margin (downstream) is 9.6 dB - which is a good 3dB better than yours, but having said that 6.1 is perfectly acceptable.

Steve · Jun 13, 2009, 08:47:57

The default noise margin is 6db and if you have a quiet line it will remain connected without issue. If noise occurs on our line which is greater than your margin it will cause a disconnect. When this happens the automated BT sytems will raise your margin in steps of 3 to 15 until a stable connection is achieved. However whilst raising your margin to counteract noise your sync rate will be reduced for each incremental increase in noise margin.

Whilst this rule applies for many if however you have a full sync with your local exchange your margin will often be higher than 6 and in this case the higher the better as you have an increased margin before noise effects your sync rate.

Rik · Jun 13, 2009, 09:31:46

What's been said, Jonathan. Basically the noise margin is the amount of headroom you have to work in before the signal becomes too degraded to be usable. It shouldn't be confused with the more common signal to noise ratio, as there's no ratio involved in this.

PuncH · Jun 14, 2009, 20:19:14

Is it normal for the noise margin to flucuate a lot during the course of the day?

I've been keeping an eye on it on routerstats and it's largely pretty stable, but this afternoon it went a bit crazy and was up and down for about an hour. It never dropped low enough to cause the connection to drop however as it was only dropping by about 1db or so, but it was doing it a lot.

Actually it's doing it now as my wife is ironing and it's plugged into the socket next to the phone socket so I guess this could cause the noise margin to flucuate?

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Dopamine · Jun 14, 2009, 21:19:08

A movement of only 1dB is normal and will not cause problems. As a contrast, the graph below is of a long line I monitor. It represents the fluctuations experienced this afternoon and is representative of how it's been for over a year. BT have examined the line numerous times and declared that it's as good as it will ever be. Needless to say, a line suffering bad noise like this suffers from a 15dB default margin and a very low profile.

Sebby · Jun 14, 2009, 22:40:56

Quote from: PuncH on Jun 14, 2009, 20:19:14
Is it normal for the noise margin to flucuate a lot during the course of the day?

I've been keeping an eye on it on routerstats and it's largely pretty stable, but this afternoon it went a bit crazy and was up and down for about an hour. It never dropped low enough to cause the connection to drop however as it was only dropping by about 1db or so, but it was doing it a lot.

Actually it's doing it now as my wife is ironing and it's plugged into the socket next to the phone socket so I guess this could cause the noise margin to flucuate?

Don't worry, the fluctuation you're seeing is nothing to worry about.

PuncH · Jun 14, 2009, 23:05:44

Quote from: Sebby on Jun 14, 2009, 22:40:56
Don't worry, the fluctuation you're seeing is nothing to worry about.

Cool daddio 8)

Lance · Jun 14, 2009, 23:09:06

I'll bring my ironing up the a12 for your wife to do and we can keep the experiment running for longer!

PuncH · Jun 14, 2009, 23:24:20

my first reaction was "phhhh....i don't think so!"

however, my second reaction is "how much?"

mind you, i'd probably have to hide from her for a week as she's 32 weeks pregnant! lol!

drummer · Jun 15, 2009, 01:32:11

Couple of quick questions:

If 6db is the default noise margin, why is higher better?

How do people discern their SNR?

I probably won't understand the answers because none of the responses to PuncH's original question make any sense to me.

"The cat sat on the mat" describes my level of numeracy but I really do want to get to grips with this ADSL lark.

Constantly posting this question will eventually yield results...

rireed3 · Jun 15, 2009, 02:59:11

I'll have a quick go before falling asleep.

In electronic communications all the world is noise -- until someone sends a signal strong enough to be detected over the noise.

Before thinking about margin, there is signal-to-noise ratio or SNR. Sometimes people here will shorten "SNR margin" to SNR, but they always mean SNR margin. I'll spell out signal-to-noise to try to clear that up.

Signal-to-noise ratio is expressed as a logarithm, decibels or 'db'. It takes 2db signal-to-noise ratio to transmit anything at all on a Digital Subsriber Line (dsl).

Bits can be encoded and use up signal-to-noise ratio down to some 'floor'. I don't know the exact number here, but lets say it's 2db. Then if your line had 20db signal-to-noise, you would have 18db with which to encode bits.

This relates directly to your connection (sync) speed. The bits of your data are encoded on hundreds of radio-frequency tones in 'frames' sent thousands of times per second. It takes 3db of signal-to-noise ratio to encode a bit on one tone. You can get a rough idea of your line speed if you know the signal-to-noise ratio of each tone before the bits are encoded, but no one ever does.

What we see is how much "SNR margin" is left over 'above the floor' after all the bits are encoded. The more signal-to-noise you can use and squash down the "SNR margin" without letting a lot of errors in to be corrected, the faster you can send bits, so less "SNR margin" means more bits/second encoded, but more "SNR margin" means better resistance to errors, for a given line.

Each line has a different 'original' signal-to-noise before encoding, and this is determined by the length of the line. A longer line will have a weaker signal. This length is measured by 'attenuation', a different db value, higher meaning weaker or 'more attenuated'. My line is fairly short so my attenuation is 24db, and I can sync at around 19 Mbps and still have 5db "SNR margin". A long line at 60db 'attenuation' will have trouble getting more than about 4.2 Mbps.

Hope that helps,
Richard

Sebby · Jun 15, 2009, 09:06:15

Quote from: drummer on Jun 15, 2009, 01:32:11
Couple of quick questions:

If 6db is the default noise margin, why is higher better?

How do people discern their SNR?

I probably won't understand the answers because none of the responses to PuncH's original question make any sense to me.

"The cat sat on the mat" describes my level of numeracy but I really do want to get to grips with this ADSL lark.

Constantly posting this question will eventually yield results...

It's a bit of a weird concept. The default "target" SNRM is 6dB. The exchange will aim to connect you at the fastest rate to achieve this target margin. Higher is better - it's just that most lines are not good enough to sync at full speed and have a margin higher than 6dB.

rireed3 · Jun 15, 2009, 09:33:34

QuoteHigher is better

Higher is more resistant to noise.

At this point BT Wholesale put their oar in. Their Dynamic Line Management monitoring system (DLM) can decide that at "SNR Margin" 6db your line has too many errors to stay connected reliably. They will put your target up by some multiple of 3db, and your connection sync rate (from the number of bits you can encode in a frame) will be less.

In the past, DLM has been accused of being over-enthusiastic, raising targets too high unnecessarily, so cutting speeds too much. Time will tell, but it looks to me like 24M ADSL2+ has a better DLM system than 8M Max did.

In any case, it usually pays not to have your line re-sync unnecessarily and get DLM excited. I do any fooling around with my router configuration with the phone line disconnected until the only thing left is to connect. Some router setups may make that more difficult.

Richard

Rik · Jun 15, 2009, 09:35:58

Sound advice, Richard, I do the same.

drummer · Jun 16, 2009, 01:59:10

Thanks all for the surprisingly detailed replies - karmas are in the post along with the cheques...

For the first time ever, I'm saving a hard copy of a forum discussion in order to print it as I'm still none the wiser having read it several times on my monitor. On paper, it may actually make more sense to me.

I'm a bit of a plank when when it comes to numbers though, so pay me no heed.

Rik · Jun 16, 2009, 10:28:58

To keep it as simple as possible, Drummer, noise margin is the amount of headroom in which ADSL has to work. When it hits 0 (or in Netgears a bit below 0), the router cannot distinguish the signal from the background noise and so loses sync. While a larger noise margin the better from a stability standpoint, the trade-off is speed (which is why fixed rate products used to have much larger noise margins). For most routers, 6db is adequate to sustain the connection. On some lines, though, there's simply too much noise at that level, so the NM is increased, in increments of 3db, in order to achieve stability. As NM increases, it stops the router using the higher frequencies (and ADSL signal is actually a bundle of individual frequencies, each carrying some of the data which makes up the throughput). As those frequencies get chopped off, so the sync speed decreases. My own line, for example, becomes stable at 9db, with interleaving on, and that costs me about 1M in throughput.