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SNR margin, line attenuation, ADSL and a slow internet speed

Slow internet speed is often a result of a bad attenuation and SNR margin figure. If you are one of these users who have problems with the ADSL line, then you are in the right place. Have you installed an ADSL line recently and you get a slow internet speed? Sometimes the line is dropping and you are wondering… why God!?!?!

Actually God has nothing to do with it. ISPs are offering ADSL speed up to some bandwidth; it could be 1MB, 2MB, 4MB, 8MB – 24MB. Unfortunately the term up-to is not chosen by luck. Internet speed depends on many factors which I am planning to explain in the next few sections of this post.

Slow internet speed and distance

Have you ever heard about line attenuation? Leaving out some of the technical jargon, the line attenuation figure describes the distance between you and the exchange of the ADSL provider. It is expressed in dBs and you can check this number on your ADSL statistics of your home / corporate router (show dsl interface atm (something) for Cisco). In short, the lower this number is, the better:

20dB and below is outstanding
20dB – 30dB is excellent
30dB – 40dB is very good
40dB – 50db is OK
50dB and above is bad

The “slow internet speed” of your ADSL line is directly affected by attenuation. Unluckily, you can do nothing about a low attenuation figure, except from moving to a new location closer to the ADSL exchange.

Bottom of Form

Is your line noisy? Check SNR and SNR margin!

Have you ever tried to speak to a friend of yours that is 100 m away in a noisy street? I bet he would be struggling to hear you. If the street was empty, his hearing capabilities would be drastically improved. Like a street, ADSL lines are susceptive to noise. This could be because of the electromagnetic noise (cars, music, dogs barking) or the ADSL exchange’s load (people talking).

Signal-to-noise ratio (SNR) is another figure you can check in the ADSL stats of your home / corporate router (show dsl interface atm (something) for Cisco). This number is also expressed in dBs and describes the relation between your speaking strength (signal) and the street’s noise strength (noise). The higher this number is, the better, as your speaking voice outperforms the noise.

8dB and below is bad
11dB – 20dB is OK
20dB – 28dB is excellent
29dB and above is outstanding

Some routers instead (or additionally) of the signal-to-noise ratio (SNR) figure are displaying the signal-to-noise margin (SNR Margin)  which is the difference between the actual SNR and the SNR required to sync at a specific “speed”:

Actual SNR = 44dB
SNR to sync at 8Mb = 35dB
SNR MARGIN = 44-35 = 9dB

A higher number is better and the numbers bellow 6dB may cause problems. It is easier to understand if your line is in an acceptable SNR level, by using the margin figure, because you don’t need to know the SNR dB value you should have, for the line to perform at a specific speed. You just need to know that above 6 dBs you are doing just fine.

As you are probably thinking, noise is a subject of change according to various sources. On-peak hours, the noise increases as your provider’s DSLAM becomes crowded (people talking in the street) and the SNR margin gets worse. Florescent light sources, telephone devices installed on your phone line, your mom’s cake-blender, all these may decrease the SNR margin. Luckily, there are some things you can do to improve the SNR margin:

  1. Buy a router that is good enough to manage low SNR margin figures
  2. Install a good quality ADSL filter to your router and to each phone device installed on the same line
  3. Try to change the ADSL provider, as some providers are less crowded than others. You can check out with your neighbors to see how they perform with their ISP.
  4. Change the faceplate of your line with a better one and check that the cable terminates properly to it.
  5. Change the in-building cabling. Poor, low quality telephone cabling can introduce noise to the line. You can check if you have a problem within-building cabling, just by placing the router (temporarily) directly to the intrusion point (probably at the basement) and by checking the router’s statistics.

The verdict

You need to consider both attenuation and SNR margin figures to determine the quality of your ADSL line. The SNR margin figure should be above 6dB and the attenuation figure must meet some standards.




below 69dB


below 56dB


below 41dB


below 25dB


below 20dB

Note, that the router will probably display statistics for the upstream and the downstream traffic. These statistics will be different, as upstream/downstream speed is different. Also consider reducing the “purchased speed”, in a case where you have a SNR margin lower than 6dB which may cause frequent disconnections. That may improve the SNR margin thus line stability.

Finally, there are some error statistics that a router may display. The most important are LOSES, CRC and HEC errors. The CRC is an error correction code that is used to detect and retransmit corrupted packets. The HEC is a form of a CRC on the header of an ATM cell. If these errors are high, within a period of time, it will slow or even disconnect the line. The LOSES refer to a loss of signal which can indicate a problem with noise, in the case where the number is high.