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I've reciently tested ADSL network. Alongside parameters that I'm familiar with their meaning (SNR marin, Line Attenuation etc.) I come across Tx Power. I tried to google it, but there was no clear explanation, my greatest achievement in this matter was the decoding abbreviation to "transmit power".

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It is the transmit power (≈ voltage) with which your device sends its signal to the cable. It effectively means, how strongly your device "shouts" on the cable to the receivers (which is mainly an ADSL router by your ISP).

It is in decibel. Decibel is a

  • relative
  • logarithmic quantity.

Relative means, that it shows the proportion of the measured quantity, compared to a reference value.

Logarithmic is an uncommon thing. For example, in a linear unit, like meters, 2m means 2 times higher distance as 1m. In the case of decibel, +10 decibel means a 10 times of increase.

Thus, if you increase the Tx power with 10, you will get 10 times higher power. If you increase with 20, you get 100 times higher power.

Note, a 10 dB difference means a 10 times growth in the voltage, and thus a 100 times growth in the power (because the power depends on the voltage in these cases mostly quadratically). This is not important from a network engineering view (but it is from an electronic engineer view).

Tuning the TX parameter can be useful if you have to deal with too long or noisy cables. These cause mainly exponential losses with the cable length, this is why the logarithmic tuning is needed.

It may be also useful to suppress noise sources.

Similar tuning is useful also on other network types, where noise is a significant problem (for example, wifi).

Because it is a logarithmic quantity and the loss grow exponentially with the cable length, typically the required decibel has to be tuned linearly with the cable length (i.e. if you require, for example, 10 dB length grow for a cable length growth of 20m, you need again 10 dB for the next 20m).

Most L0 electronic can tune the TX on need.

In my experience, if you have to tune this value manually, the chances are high that it won't work very well with any setting (this is why the automatical tuning failed).

| improve this answer | |
  • Umm no, a 10 dB difference means a 10 times growth in the power, which means about a 3.2 times increase in the voltage, – Peter Green Jan 19 at 1:20
  • @PeterGreen It contradicts what I know, and also contradicts this. I would not close out that here we have some special case, could you please cite some reference about it? – peterh - Reinstate Monica Jan 19 at 15:47

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