SNR Margin adjustments:

[ranierofas]

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
I’m wondering what the ISP really changes when it modifies the margin

If it is A or B
Any idea?

[Philip]

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 needed by noise fluctuations 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 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 when 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: https://www.speedguide.net/faq/what-...margin-snr-355

[Phil Shepherd]

Hi Philip,
So, can you please explain me what SNR margin and line attenuation are?

[Philip]

SNR margin, as explained above, is the difference between the actual SNR and minimal SNR required to sync at a specific speed. It can be simplified to: the difference between actual signal and signal required to sync.
See: https://www.speedguide.net/faq/what-...margin-snr-355

Line attenuation (signal loss) is a measure of how much the signal has degraded over distance between the DSLAM and the modem. Lower dB is better, each 3dB halves the strength of the signal.
See: https://www.speedguide.net/faq/what-...ttenuation-371