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I've started getting periodic packet loss on my DSL connection lately, and am trying to understand what conditions are causing this.

My DSL modem has a stats page with values for "SN margin (dB)" and "line attenuation (dB)". I think SN refers to signal to noise ratio, but I would expect that to be a unit-less value, since it's a ratio.

What are the important details I should look for in a good signal, and what do they refer to?

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Brief

You want the Line Attenuation as low as possible and the Signal to Noise Margin as high as possible.

A signal-noise margin of less than ~8dB may be problematic.
Attenuation of above about ~40dB will probably start to limit your speed.

Line Attenuation

Attenuation is how much power / "volume" of the signal is lost between your router and the exchange.

The higher attenuation is the quieter the signal is and the harder it becomes for the router to "hear" correctly.

Is caused by being futher away from the exchange, having a poor microfilter, excessively long extension cables, poor cabling, EM interference, etc.

Signal to Noise Margin

The signal to noise margin is the difference in amplitude between the noise and signal - how much "louder" the signal is compared to the background noise. Very similar to signal to noise ratio (as you mentioned) but is an absolute measurement, not relative.

If this is small it is difficult to tell the signal and noise apart.

True noise is much harder to control (see this Wikipedia article for some info on electrical noise) as "noise" does not technically include interference - but you modem will not be able to tell the difference. So instead check for sources of possible interference - other devices on the phone line, object generating EM interference over your extension leads, poor microfilters, etc.

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Is this a new problem?

High SNR, low attenuation is best. I think the exact specs depend on the type / kind of DSL Modem and DSLAM. Other things that can affect the performance is distance and quality of wiring.

Can you run test from the DEMARC? If so you can isolate if the problem is related to the house wiring. I would also try a new modem / splitter pair.

The obvious, all you are seeing is your end so you would need to talk to the DSL provider to find out what the link looks like from their end.

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  • It is a new problem, and the only thing which changed was upgrading the service from 3Mbit to 6Mbit, which I don't believe should have an impact. I have a feeling that it could just be the modem beginning to fail, since at one point it went into a reset loop for a while. +1 because comparing the signal numbers with the modem connected to the box outside the house is a really good idea.
    – Dave Andersen
    Commented Aug 22, 2010 at 6:22
  • @Dave - ah, upgrading your service is the culprit! Increasing your bandwidth will normally result in an decrease in your SNMargin. If you're not now quite getting the full 6Mbit it's because your line can't sustain 6Mbit without the SNM dropping too low, so it'll reduce the bandwidth to improve the SNM a bit.
    – DMA57361
    Commented Aug 22, 2010 at 9:04
  • @DMA57361, that's good to know! However after having the connection completely fail for most of the day (with the modem periodically rebooting on it's own or reporting no DSL link), we finally tried connecting with a new modem, and that seems to have solved the problem. Perhaps the old modem was over-stressed by the increased speed? Or maybe it was just its time...
    – Dave Andersen
    Commented Aug 30, 2010 at 5:25

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