[Ignitionnet]

I'm quoting the DOCSIS standards. A signal hitting the modem at those high levels failing is an issue with the modems assuming the SNR is ok.

SNR is the overwhelming thing. High power levels of the wanted signal can indicate high power levels of the noise carried on the network as well resulting in a low SNR, high power levels can cause distortion at the modem, same as with a guitar amp, the distortion is caused by high power levels coming into the modem.

I would bet you'll find some modems are perfectly happy at those levels.

Knowsley / Langley whichever the power level guidance you've been given is a misnomer. So long as the signal is clean the power tolerances, assuming the modem is the same, should be equal. Whatever the network is doing it's how the modem handles it that's the main thing. Modems don't care what the name of the headend is, they only care about how high or low powered the signal is and the difference between the signal and the background noise. There's no reason why the ntl areas should struggle with some power levels while the Telewest areas are ok.

It seems a bit odd that the Telewest areas can apparently not cope with an SNR below 29dB which is actually below the DOCSIS standard for 256QAM which states 31dB is desirable, while they have a far wider power range they can tolerate. It's all about what the cable modem does with the signal, some cope with high power better than others, all will struggle with low SNR.