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Thread: SNR Margin adjustments:

  1. #1

    SNR Margin adjustments:

    I understand that SNR MARGIN is the difference between
    A) actual line SNR
    B) SNR required to synchronize
    My provider has an option to lower the SNR MARGIN from 12dB to 6dB
    Iím wondering what the ISP really changes when it modifies the margin

    If it is A or B
    Any idea?

  2. #2
    Administrator Philip's Avatar
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    SNR margin is the difference between
    A) actual SNR (signal to noise ratio)
    B) signal level the modem requires to sync. SNR is the ratio between signal and background noise.

    You also have to keep in mind the following:
    1) SNR (A) fluctuates over time, noise on the line varies because of crosstalk, radio frequency interference, distortion, time of day, DSLAM congestion at the time, etc.
    2) SNR to sync (B) is different depending on the speed your modem syncs at, i.e. lower SNR means you sync at lower speed, and/or you are further from the DSLAM.

    You can think of the SNR MARGIN as a safety buffer before the modem drops the connection (and resyncs at lower speed that requires lower SNR). If the modem is set with 12db margin, it is less likely to drop the connection when the noise increases - when (A) and (B) get closer together, as you have 12db of buffer between the two values before the modem disconnects and has to resync, possibly at a lower speed. With a 6db margin, the modem will be more likely to resync if the SNR value fluctuates.

    You can ask them to change the value to 6db if the SNR does not fluctuate much, I would monitor it over time (at different times of day/night) and see how much it changes. If it varies a lot you may be better off with the 12db, if it is pretty stable you can have them lower it to 6db. In essence, 6db margin allows you to sync more on the edge of what your line allows, the modem may even sync at a higher speed, but it will be more likely to drop the connection when SNR fluctuates.

    I hope this makes sense.

    There is some more info in the FAQ on the main site here:
    Linux is user friendly, it's just picky about its friends...
    Disclaimer: Please use caution when opening messages, my grasp on reality may have shaken loose during transmission (going on rusty memory circuits).

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