What do Margin and Attenuation figures indicate?

My ADSL modem/router reports the following:

Upstream Rate (Kbps) 288
Downstream Rate (Kbps) 1152
US Margin 29
DS Margin 31
Modulation MMODE
LOS Errors 0
DS Line Attenuation 17
US Line Attenuation 17
Path Mode Fast Path

In simple terms (that a software engineer can understand) what do the Margin and Attenuation figures indicate. Is the line capable of a faster speed?

The DS (Downstream) figures are the really important ones. The lower the attenuation the better, think of attenuation as frequency dependent resistance, yours is good. The higher the SNR Margin the better, this is the difference between the noise floor and the signal level. If you were to go to MAX, where SNR Margin is generally more important than attenuation, I would expect at least 4Mbps+, maybe even 5Mbps+ if your lucky. If you try the BT number checker it should give you a best guess based on your line, though this should be only taken as a guide.

Thanks.

I think I understand attenuation - it seems to be used in the normal English sense of loss over distance.

Margin still puzzles me. I always assumed high SNR was good. But is high SNR Margin good? i.e. I think I understand what Signal to Noise Ratio is but don't understand what Margin means in this context.

Yes attenuation is loss over distance but is frequency dependent, IIRC it is measured at 300KHz for an ADSL line, but don't quote me.

A high SNR Margin (SNRM) means that there is a big margin between the noise floor and the signal level on the line so the higher the margin the more room there is to cope with both any increases in floor level or background noise as well as noise bursts. SNR is simply a ratio between the noise and signal level while SNRM is the actual gap or margin between the noise floor and the signal in db.

Note, that if you go onto MAX your SNRM will drop as SNRM is inversely proportional to frequency, thus for a higher frequency you will see a lower SNRM, all else being equal. MAX works to a target SNRM of 6db and will give the best speed possible for this target SNRM. It was from the difference between your present DS SNRM and the target DS SNRM of 6db that I crudely calculated your possible speed on MAX.

Also note that if your line won't stay stable at the target SNRM of 6db BT increases this target SNRM in 3db steps until a stable line is reached. This increase in target SNRM is at the cost of speed. I.e. if a higher target SNRM is required for stability this will be at the cost of some speed.

You'd probably easily get 6mb+ with those stats on an upto 8mb product.

SNR margin is the difference between the actual SNR and the SNR required to achieve a bit error rate of 1 in 10^7 so more SNR means more margin but more speed means less margin.

In reply to:

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A high SNR Margin (SNRM) means that there is a big margin between the noise floor and the signal level on the line so the higher the margin the more room there is to cope with both any increases in floor level or background noise as well as noise bursts. SNR is simply a ratio between the noise and signal level while SNRM is the actual gap or margin between the noise floor and the signal in db.

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I know what you're trying to say, but this is not quite right. The ratio between the noise and signal is the same thing as the margin between the noise floor and the signal. These are both the SNR. Put simply, the SNR is simply a measure of how much louder the signal is than the noise.

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.

Cheers, I realised looking at it again just now that I had explained it badly.

I think I understand the terminology now.

Off topic but,

I only asked because I briefly upgraded to "MAX" but cancelled when the line wasn't stable after a couple of weeks. I'd rather have guaranteed 1MB than problematic 6MB.

When I upgraded my router ALWAYS showed a DS connection at 8128 Kbits and US at 448 Kbits (I guess this is what you call SYNC speed).

Unfortunately I never bothered to look at attenuation/margin figures during this period.

My short-lived upgrade experience was

2 days not able to log-in (user name/password rejected by CHAP).
8 days of having to dis-connect/re-connect every day to get a working internet link and speeds of between 6Mbits and 30Kbits.

p.s. currently I still have to dis-connect/re-connect but at least the link is 1 Mbits once I've done so.

I am surprised as your figures indicate a half decent line. But that only goes to show how lines can vary, even when they have the same type of figures. I assume you did check your internal wiring to eliminate any noise coming from any extensions and the like. As MAX, working at a higher speed, i.e. frequency, is far more suscptible to noise. But it is surprising how a few easy steps can also improve matters, at least if the source is internal to your location. However, if the noise is external to you, an industrial park that your line passes through as one example, then there is not a lot that can be done.