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harald

join:2010-10-22
Columbus, OH
kudos:1
reply to pflog

Re: [Signals] Should SNR swing as much as 1dB with temperature?

No cable modem is capable of reading SNR.

What they display as SNR is a calculated value based upon the ratio of packets with correctable errors to good packets, using the formulas derived by Claude Shannon in the 1040's. The details of how this is done is proprietary to Broadcom and other chip makers. Inferences about what will cause the SNR to change are pretty shaky.

The same is true for upstream SNR. The cards in the CMTS also use similar chips.

So it is best to think of the displayed SNR as just an indication of the error rate in terms that techs are used to understanding. The estimate is also impacted by modulation errors.



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said by harald:

No cable modem is capable of reading SNR.

What they display as SNR is a calculated value based upon the ratio of packets with correctable errors to good packets, using the formulas derived by Claude Shannon in the 1040's. The details of how this is done is proprietary to Broadcom and other chip makers. Inferences about what will cause the SNR to change are pretty shaky...

I have seen references to that as well, but that does not explain why Channel ID 6 in my SB6121 does not have a much higher DS SNR than the other three channels, instead of all of them reading 38dB.



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harald

join:2010-10-22
Columbus, OH
kudos:1

What is missing is an understanding of what time frame is used for the calculation. Here is a white paper from Broadcom that explains the issues, but does not even hint at what the averaging parameters are:

»www.broadcom.com/collateral/wp/C···01-R.pdf