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SNR and CNR Levels

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SNR and CNR Levels

Hi all,

Is there anyone who can tell me what the values of SNR and CNR must be to have an good signal.

And if my levels are bad, what will happen?
e.g. will my upstream drop from 16qam to qpsk, or will it yust be dead?

I now have: SNR 22.6dB and CNR 20dB. I expect thats a bit low ?


The answer is 'It depends',

The answer is 'It depends', on docsis version, modulation type etc...

Here is an older document on Cisco's site with what they say is good.

According to this document (and the original spec) your numbers look quite low.

With Ciscos cmts your upstream will not change types, but will likely happen is you will have serious upstream probs, such as modems not locking online, modems dropping offline, longer than normal sync times, slow speeds, lots of retransmissions etc.

We recently had a bad blade we tried to put on our CMTS which had a 26 CNR straight out of the blade, while most of the modems did indeed lock online there was ~10-15% that never locked online. We RMAed the blade and got a new one and whallla it works like a charm now.

Everyones mileage will vary with these values for us what has worked well was >35 DB for CNR (downstream) amd we strive for >30 db on SNR (upstream) (though most average 25-27 db)

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