DSL Line Testing

In this page we will discuss the nubmers that come back from DSL line tests.

Many types of modems will give you the same numbers if you log into them and check the status of the line.

Here's a typical customer line test from Frontier:

Distance is 14,000ft, bit rate 3000/864Kbps and noise margins 16/11dB. 


Noise (dBm) includes crosstalk from other lines, radio frequency interference, distortion, etc.
Signal-to-noise ratio (SNR) is defined as the power ratio between a signal background noise.
6dB or below SNR is bad, you will experience no sync, or intermittent sync problems
7dB-10dB is fair but does not leave much room for variation in conditions
11dB-20dB is good with little or no sync problems (if no large variation)
20dB-28dB is excellent
29dB or above is outstanding

The more commonly used SNR margin, as described below is sometimes abbreviated as simply SNR as well.

SNR margin (dB, a.k.a. noise margin) is the difference between the actual SNR and minimal SNR required to sync at a specific speed. It is normally measured in decibels. SNR margin is often confused and used interchangeably with SNR. Some NAT wireless routers, for example (notably dd-wrt open source router firmware) use SNR margin, only denoted as "SNR".

For example, to calculate SNR margin:
If actual measured SNR = 45db
SNR to sync at 8Mbit/s = 35db
SNR margin = 45-35 = 10db

Higher SNR/SNR margin numbers repesent cleaner/stronger signals, with less background noise. The higher the SNR margin the more stable the connection. In some instances interleaving can help raise the noise margin to an acceptable level.

Note that there may be short term bursts of noise that may drop the margin, but due to the sampling time of the management utility in your modem, will not necessarily show up in its interface.

Some DSL routers display both the actual SNR, and the signal-to-noise margin (SNR margin) as a separate value, which (again) is the difference between the actual SNR and the SNR required to sync at a specific speed. As with actual SNR, the higher that SNR margin number, the better (stronger signal over background noise).

For DSL, the further you are from the exchange, the lower your SNR and the higher your attenuation will be.
At peak times, the noise may increase as your provider's DSLAM becomes congested.
Fluorescent lights and other sources of EMI close to the modem can affect the SNR as well.