Quick question on signal to noise ratio

Hi there,

So for all the DSL based connection like ADSL, VDSL/VDSL2. One has to deal with signal to noise ratio.

Essentially my understanding is that the lower the signal to noise ratio is, the more information can be sent through the cables, and the download/upload speed will increase. However, the connection will be less stable, and you might lose connection altogether.

So, assuming that is correct, my question is why is that? The best answer I have found, but that I would like to check here. It is because as you increase the signal by increasing the bandwidth, meaning using higher frequencies for conveying information there will simply just be more noise. Especially because the higher frequencies get attenuated more and there will relatively be more noise for those bandwidths.

So is this correct or at least partially correct. I would really like to try and understand this concept?

If you are talking to someone on a quiet roadside - you can hear each other as you talk perfectly normally... moderate signal, low noise (high signal to noise)

Then a truck comes past - if you continue talking at the same volume, the noise will tend to swamp it, but you can still hear it if you concentrate. Moderate signal, moderate noise.

Now a phalanx of motorcycles comes past - if you continue at the same volume there is now no chance of hearing even if you concentrate. Moderate signal, high noise. (high noise to signal)


Similar with your DSL broadband - the signal level is essentially constant, if the noise increases it has to "work harder" (do more error recovery/correction) and the protocols ramp up the amount of error recovery/correction information sent per databit (which decreases the data capacity of the line - and also increases the latency).

The available bandwidth on a DSL line is divided up into "bins" - basically a slice of the frequency range. Each bin carries some bits - noiseless bins carry more bits that noisy ones.

You're right that as bandwidth is increased then the upper limit of the frequency range is increased but thanks to the laws of physics the higher the frequency, the higher the attenuation and that means the received signal is lower (and the noise proportionately higher) the higher frequency you use - so the higher frequency bins carry less datasimilar applies to long lines (and you lose the higher frequencies faster than the lower ones as the line gets longer too)

You can see both of the above here - https://www.thinkbroadband.com/broadband/hardware/re... in the "Connection Tweaking and DSL feedback" section.

Add on to this other sources of interference - like broadcast AM radio - if you live near the Radio 4 longwave transmitter, and look at the spectrum of your DSL line there's a very obvious gap around 200kHz - which is not unexpected because R4LW is on a198kHz carrier. That puts those bins out of use and decreases the information throughput.

Thank you for your explanation! It is very useful.

Btw another thing I have thought about, is how the signal is transmitted? Is the information conveyed it like pulse (extremely short pauses in between)? Or is it more like on continuous stream between the router and whatever is at the other end?

It's an analogue representation of a digital signal... using many "interesting" forms of modulation - you can read more in the Wiki article on DSL generally: https://en.wikipedia.org/wiki/Digital_subscriber_lin... - I think most in Europe is OFDM.

Cheers!