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I'm currently using an ADSL connection. The SNR-Margin for the Downstream is usually around 6db for a Line-Attenuation of 54db. It's terrible and causes a lot of CRC errors. While, at the same time, the SNR-Margin for the Upstream is usually around 21db for a Line-Attenuation of 38db, and that is very good and almost never causes any CRC errors!

The line service technician told me that there are some high-voltage power cables near the telecommunication equipment cabinet that cause the noise. But, I'm confused:

If the noise is because of the high-voltage power cables, then why the upstream signal is that perfect, while the downstream is that corrupt?!

Please, shed some lights on this. Thank you in advance :)

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    Service technicians are, in my experience, rarely a reliable source of information. – Ron Trunk Feb 18 '18 at 16:44
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ADSL uses frequency-division multiplexing for transmitting and receiving, so the frequencies in either direction are quite different. The upstream uses lower frequencies, so very often it is less affected by noise.

There are several approaches to improving the situation, possibly all out of your reach:

  1. shorten the line - have ADSL termination moved to a POP located more closely to your premises
  2. replace line - possibly another line in the local loop is of better quality
  3. replace cable with better quality/screening (if there's a significant length of cable on-premises)
  4. switch to a more suitable protocol - e.g. Annex A ADSL leaves more low frequencies for data than Annex J, or multi-lane SDSL is more robust than ADSL
  5. switch technology - fiber or WLL may be available in your location
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