Mom turn off the cake-blender, my ADSL line is dropping!!!

Άρθρο από Lefteris Karafilis Fri, 12/11/2010 - 13:01

If you are one of those ISP customers that having problems with your ADSL line, then you are in the right place.  You installed an ADSL connection recently and you don’t get the “speed” you expected. 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. ADSL speed depends on many factors I am planning to explain in the next few sections of this post.

Are you far far far away?

Have you ever heard about attenuation? Leaving out some of the technical jargon, line attenuation figure describes the distance between you and the exchange of your ADSL provider. 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 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 “speed” of your ADSL line is directly affected by attenuation. Unluckily, you can do nothing about low attenuation except moving to a new location closer to your ADSL exchange.

Is your line noisy?

Have you ever tried to speak to a friend of yours that is 100 m away in a noisy street? I bet he would 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 electromagnetic noise (cars,music,dogs barking) or ADSL exchange’s load (people talking in the street).

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

10dB and below is bad
11db – 20dB is OK
20dB – 28dB is excellent
29dB  and above is outstanding

Some routers instead (or additionally) of signal-to-noise ratio (SNR), display 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

The higher the number the better and 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 in a specific speed. You just know that above 6 dBs you are doing OK.

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

  1. Buy a router that is good enough to manage low SNR margins
  2. Install some good quality ADSL filters to your router and to each phone device sitting on the same line
  3. Try to change ADSL provider since 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 in it.
  5. Change in-building cabling. Poor, low quality telephone cabling can introduce noise to your lines. You can check if you have a problem with home/company cabling if you place your router (temporarily) directly to your building line intrusion point (probably at the basement) and check the stats.


The verdict

You need to consider both attenuation and SNR to determine the quality of your ADSL line. Your SNR margin should be above 6dB and the attenuation must meet some standards according to your line speed:

Speed Attenuation
2Mb below 69dB
4Mb below 56dB
8Mb below 41dB
16Mb below 25dB
21Mb below 20dB

Note that your router will probably display stats for upstream and downstream traffic. These stats may be different since upstream/downstream speeds are different. Also consider reducing the “purchased speed”  if you have an SNR margin lower than 6dB which causes you disconnections. That may improve SNR margin thus line stability.

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


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