Router – SNR Margin and Line Attenuation

adslrouter

I'm currently using an adsl connection and last week an engineer from my ISP came and told me that my SNR margin and line attenuation is not good, and they need to replace the main telephone cable to provide cable tv. I tried researching on SNR margin and line attenuation but I'm not sure how accurate the information are. So, can anyone explain me what SNR margin and line attenuation are?

Best Answer

The SNR margin is the Signal to Noise Ratio margin.

Signal to Noise Ratio is the relation between the data signal level and the noise level. Even the most perfect cable absorbs some noise. This "noise" is electromagnetic interference produced by:

  • other cables running near the cable.

  • Faulty connectors.

  • Motors or transformers near the cable.

  • Radio systems.

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If the noise is strong then the signal cannot be recognized.

Higher ratios means better cables. Below 10dB is very bad and more than 20dB is good.

At higher ratios, more speed can be achieved and lower ratios mean error-prone cable and lower speeds.

The SNR margin is the difference between the SNR of the cable and the SNR needed to get an specific speed.

Suppose that the SNR of the cable is 34dB and the SNR to sync at 6Mb is 30dB, then the SNR margin is 34-30 = 4dB

Attenuation

Every cable on earth suffers of attenuation. It is a measure of how the signal loses strength when running through the cable.

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Suppose you put 5 volts in the end of a 1 Kilometer cable. If you measure the voltage in the other end you get, for example 4.5 volts. There was an attenuation of the original voltage. If the cable was longer, for example, 5 kilometers then the voltage on the far side could be very low.

For data signal this is key because if the original signal is strongly atternuated it could be gibberish for the equipment.