DSL line readings: Signal to Noise and Attenuation

If you are having problems with your DSL, it may be useful to check and see what the line readings are, to see if the telephone line carrying your signal is performing correctly.


Most modems have the ability to give you numbers like Line Attenuation and Signal to Noise Ratio. We will have instructions for a couple of types of common modems. If yours isn't listed here, read them anyway, yours is probably similar.

Other device

If you have some other type of device such as a Westell, Arris, or other brand of modem/router, you can look at the setup instructions for that device, try to find the line tests, and then come back here to see what the numbers mean.

What the line readings mean

The combination of attenuation (signal strength) and signal-to-noise (noise level) will tell us if our signal is strong enough to carry a fast dsl signal. If the distance to the central office is short, the signal should be strong. If the wires are good quality the signal to noise ratio should be high. Both are desireable.

Signal to Noise

Signal-to-noise ratio (often abbreviated SNR or S/N) is the ratio of a signal power to the background noise. If there is too much noise, your modem won't get a clear signal and the DSL will work badly. The bigger the number, the better your DSL will work.

Signal to Noise Line Quality
29db Outstanding. Your line would probably support a speed upgrade.
20db-28db Excellent. Your line will work well.
11db-20db Good. You should have little or no sync problems.
7db-10db Fair. You will not have much room for variations in conditions.
5 db or below Very bad. You will have sync problems.

Line Attenuation

Attenuation is the loss of signal strength due to distance. The longer the wires, the larger the percentage of signal that is lost. The distance is the distance from your location to the phone company local central office, which hopefully is not more than a couple of miles away. If the distance is greater than about three miles the signal doesn't work at all. In general, lower Attenuation is better; the signal is 'stronger'.

Attentuation Line Quality
20dB and below Outstanding. Your line would probably support a speed upgrade.
20dB-30dB is Excellent.
30dB-40dB is Very good.
40dB-50dB Good.
50dB-60dB iPoor. You may have reliability issues.
60dB or above Bad. You will definitely have reliability issues

Distance and Speed

The closer you are to the Central Office, the stonger the signal, and the faster your line.

The following guide (distance vs. attenuation vs speed) tells us what speeds to expect.

Distance Attenuation Max Speed
3000 13.81dB 23Mb
4500 ft= 20.7dB 21Mb
6000 ft= 27.6dB 18Mb
7500 ft= 34.5dB 13Mb
9000 ft= 41.4dB 8Mb
10500 ft= 48.3dB 6Mb
12000 ft= 56dB 4Mbit
13500 ft= 62.1dB 3Mb
15000 ft= 69dB 2Mbit
5.0km = DSL usualy won't work