Last updated 11 Jan 2013
Noise Margin is the non-technical term for Signal to Noise Ratio Margin (SNRM). Domestic standard modems and ADSL routers often use the terms Noise Margin or SNR when reporting on its value.
The value reported by these, unless they give both, is never the SNR (Signal to Noise Ratio).
The Signal to Noise Ratio, as its name implies, is the ratio between the strength of the signal and the level of noise on the line. That ratio is a major factor in determining the connection speed, as the higher the ratio the higher the possible speed.
The SNRM is a margin which by which the noise level can rise before connection is lost.
So what does that mean?
Given for example an SNR of 40dB then that might allow a connection speed of perhaps 6000kbps. The trouble is that as soon as the noise increases for any reason, and it varies almost all the time, then that connection speed would no longer be possible so the connection is lost. The modem/router then reconnects at a lower speed commensurate with the higher noise level (lower SNR).
That would be a pain in the neck. This is where the SNRM comes into play.
Taking the default (target) value of 6dB applied by most ISPs, instead of the exchange equipment and the modem/router negotiating a speed on the basis of an SNR of 40dB they work on the basis of 40dB - 6dB = 34dB and work out what speed that will support.
As a result the SNR can fall (in theory) by 6dB to 34dB and the connection holds. The SNRM will fall in line with this, from 6dB to 0dB. The figure shown by your modem/router is not doing anything, it is just telling you how things have changed since you connected.
If the noise on a line varies a lot causing frequent disconnections then one of the first things that happens on BT Wholesale-based connections is that the target SNRM is raised, in steps of 3dB. This lowers the connection speed as explained above and the line becomes more stable.
For more information on BT line management please see BT DLM in the menu.
On LLU systems the increase in target noise margin is normally achieved by the user contacting support who set it manually. See also the next paragraph.
To see ways of improving the stability of your line see the Troubleshooting page.
Very stable lines.
On very stable lines some ISPs will lower the target margin to 3dB on request. At the time of writing BeThere allow users to alter it themselves on their Unlimited and Pro products. This of course gives increased download speed.
A modification to the BT Wholesale DLM during 2012 allows their target margin to be lowered automatically to 3dB. For most users this seems helpful, but there have been some reports of it being too optimistic when doing this, resulting in see-sawing between 3dB and 6dB settings.
With certain modem/routers the user can “tweak” the target margin using free software or through the Command Line Interface (CLI or Command Prompt). For more on that please see Tweaking.
Note that on FTTC Openreach state very clearly that the noise margin should not be tweaked. Nor anything else.
On ADSL such as ADSL Max each rise of 3dB in the target value costs between 500kbps and 750kbps of connection speed. On ADSL2+ the difference can be 1200kbps or more.
On virtually all lines the background noise rises as the sun falls and falls again as the sun rises. This mainly due to the effect the sun has on the radio-reflecting layer of the atmosphere, particularly medium-wave signals. So the noise margin (SNRM) will fall near dusk and not fully recover till well after dawn. Other causes are covered in Miscellaneous Nasties on the Troubleshooting pages.
The effect of this higher background noise at night is that if a connection is made at these times the sync speed will be lower than in the daytime when the noise is lower.
See also “High SNRM/Noise Margin” in the Troubleshooting pages.