[AusNOG] DSL Line Stats

[Ross Wheeler]

> Usually its the SNR which is the cause (knob to tweak), and the sync speed is the effect.

I'm possibly arguing semantics here, but without the signal, there can be 
no SNR. So SNR *MUST* be the result rather than the cause.


> When the modem and DSLAM first connect and go through the SHOWTIME negotiation, each
> one measures the received signal level (and calculates the SNR) in each bin and then
> reports these figures to the other end. They then negotiate and tell each other how
> many bits-per-second they will try to stuff down each channel - high noise channels
> (low SNR) are allocated a low bits-per-second datarate in order to cut through the
> noise, low noise channels get more bits-per-second.

So they generate signals, measure how much they recover at the other end, 
calculate SNR.... then, compare the SNR they've achieved at that known 
bitrate to the acceptable SNR and adjust bitrate accordingly.


> The lower the SNR floor is permitted to go by configuration, the more bits-per-second
> the modem and DSLAM will try to push down each channel and the higher the achieved

Yes, SNR is the "measure" - the result. More bits-per-second reduces the 
ratio of signal to noise (in a given environment). Typical ADSL2 "minimum 
acceptable" SNR of 6dB determines how fast they can operate, not the other 
way around. With a "stability profile", aiming for a minimum of 12dB 
provides a more stable connection (because there is more noise margin), 
but will (by necessity) result in a lower speed.


> Set a high SNR target figure, and then each channel is allocated a conservativly low
> bits-per-second rate that is easily distinguished by the receiver, and the link should
> be rock-solid.

And that's exactly my point: "Set a high SNR target figure" - SNR is the 
RESULT. The "measure", not the "cause".