sounds like your modem / router wasn't up to the task.

I think it's a combination of mis-configuration and BT's 10 day training period giving me problems.

I'm going to try the line-upgrade again as it is no loss to me (I'm on Virgin plan 3

Possibly, but I don't think so. When I got high speeds they were reliable. I just couldn't be sure of getting a high speed at any particular time. My DSL transmissions have always shown 0 errors.

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The SNR margin is more difficult to describe because unlike the SNR it's a figure that's relative to the sync speed, and the SNR margin is actually just an estimated value. The key point is that the lower the SNR, then the lower the sync speed that the modem can maintain. So if the modem synced at the highest maintainable sync speed, then it would lose sync when the SNR dropped. To make this less likely it leaves a safety margin. The SNR margin is difference between the actual SNR and the (lower) SNR that the modem estimates that it requires to maintain the current sync speed. The smaller the margin then the less stable the line.

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Thanks very much for that as I often get mixed up when trying to understand the difference between SNR and SNR margin.

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Thanks very much for that as I often get mixed up when trying to understand the difference between SNR and SNR margin.

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Don't worry, so do many router manufacturers

So if we have two routers of the same model and one has a stated noise margin of 8.6db whilst the other has 12.4db we can deduce that the latter is attached to a noisier line..or at least a line that has a history of noise?

Generally the higher the figure the better. It's easier sometimes just to think of them as Signal to Noise Ratios.

I don't think you can deduce anything about the absolute level of noise on the line from the margins. All you can say is that 12.4 dB is better able to cope with noise on the line compared to the lower figure.

The figures don't have history. They are the level at the last measurement. I think some modems update every 15 seconds or so.

You would need to take account of the line rates and power output levels also.

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So if we have two routers of the same model and one has a stated noise margin of 8.6db whilst the other has 12.4db we can deduce that the latter is attached to a noisier line..or at least a line that has a history of noise?

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Noise figures are pretty meaningless on ADSL Max without comparing them to the sync speed. As the sync speed increases, the SNR will drop, because the modem will be using higher frequencies which are more susceptable to noise. Incidentally, the modem isn't measuring "noise", but the signal-to-noise ratio, which is the ratio between the wanted signals and the unwanted signals on the line, expressed in a logarithmic format. With ADSL Max, the modem negotiates a speed which should give a stable connection over a 24 hour period, and to establish that may mean inceasing the noise margin, because noise increases at night.

Edited by deleted (Fri 13-Apr-07 09:17:14)

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As the sync speed increases, the SNR will drop, because the modem will be using higher frequencies which are more susceptable to noise.

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Sorry, but that's very misleading. Firstly it's the SNR margin that drops when sync speed increases on a particular line (or vice versa). The SNR itself stays the same. Secondly this isn't to do with the modem using higher frequencies, but rather it's because the modem is using the same frequencies to communicate at a higher speed (more bits per second per frequency channel).

A simple way to think of it is that there is a specific SNR for a particular line (which is largely speed independent by the way), and the modem splits this between the margin and the sync speed. Unfortunately modems usually display the margin rather than the SNR, which I think is where a lot of confusion comes from.

Edited by cahaddras (Fri 13-Apr-07 17:49:54)