What causes a bad SNR/How to fix a bad SNR?

[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.

Originally Posted by Broadbandings

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.

Its the question of the downstream power levels I asked about in my previous post, could you please clarify why the modems should be able to handle the much higher power levels you say they are capable of, but tend to go off line and require a Tech visit if they actually reach such levels.

[Ignitionnet]

So long as the modem is good enough and fully compliant to standards no reason why it can't deal with the power levels I mentioned.

High power levels can cause issues with modems, not so much from the power of the cable modem signal but the overall power of the RF hitting the modem's receiver causing distortion by overdriving it.

Not so much the cable modem signal but the other few hundred MHz of RF hitting the modem receiver and how well it isolates channels it's not tuning.

[AbyssUnderground]

The technician came this morning at 12.05pm, he said between 12pm and 4pm so he was very early! I offered him a drink, to which he accepted with a lot of thanks! He must be deprived by most people, the poor soul!

He hooked his test equipment up and it gave him a flat battery warning... What a great start! Back to the van for his charger. He also brought a new modem in with him, a lovely black 256 modem. He hooked that up, phoned to get it activated and et viola, my SNR is now 39-40dB and rock solid like it should be.

So, it seems that the modem was to fault. After 8 years of 24/7 service its finally given up the ghost. And the first time we've ever had Virgin Media technicians to come to the house too and it was certainly a good experience.

The techy was also interested in the software I used to graph the SNR etc. It turns out he'd never even heard of the modems having a web interface! Ah well, I guess his hand held equipment does pretty much the same job doesn't it.

Anyway, all in all a great turnout. 5 stars to VM and the guy who visited me today!