http://www.dslreports.com/faq/coxhsifaq
http://www.dslreports.com/faq/3692
For non-DOCSIS 3.0 modems:
Downstream power level (Receive): -15dbmV to +15dbmV
A value of -15 or worse indicates a poor downstream signal path. A tech would aim for a value close to the optimal 0 dBmV, but a good cable modem should be capable of working within the broader range of -15 to +15 dBmV, provided the downstream Signal to Noise Ratio remains good enough.
Upstream (Transmit): 35-52 dBmV. A value within the range +35 to +52 dBmV is within spec with the low to mid 40's the most common. If the cable modem is going offline, and the upstream signal strength is at or above +52dBmV, then a poor upstream path is probably the problem.
Carrier/Noise Ratio: 35 db and above The downstream Signal to Noise Ratio must be 35 dB or higher. The lower ratio the more noise and the poorer the performance. The Cable Modem will have to keep requesting retransmissions of packets with uncorrectable errors.
DOCSIS 3.0 modems:
Downstream power level (Receive): -15dbmV to +15dbmV
A value of -10 or worse indicates a poor downstream signal path. A tech would aim for a value close to the optimal 0 dBmV, but a good cable modem should be capable of working within the broader range of -10 to +10 dBmV, provided the downstream Signal to Noise Ratio remains good enough. Additionally all downstream channels should be within a +/-2db window with respect to each other.
Upstream (Transmit): 40-52 dBmV. A value within the range +40 to +52 dBmV is within spec with the low to mid 40's the most common. If the cable modem is going offline, and the upstream signal strength is at or above +52dBmV, then a poor upstream path is probably the problem.
Carrier/Noise Ratio: 35 db and above The downstream Signal to Noise Ratio must be 35 dB or higher on each channel. The lower ratio the more noise and the poorer the performance. The Cable Modem will have to keep requesting retransmissions of packets with uncorrectable errors.
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What you said
It should be in the mid 30's. If it's in the 40's or 50's then you have a cable side problem.
Also what you said
Sounds like you don't work the subscriber side of things.
I am a comcast level 4 install repair tech who also knows how to work on plant.
The cox tech who told you you have to be in the 30's may have meant your specific house due to the tap value you run off, or maybe
you misunderstood and he/she was talking about SNR which for down you want 35+ and up you want 30+, whatever the case, the info
as you posted it is incorrect for DOCSIS specifications, which are published and certified on equipment as meeting them.
DOCSIS 3.0 specification - its public for the world to see
http://www.cablelabs.com/cablemodem/specifications/specifications30.html
The specific part we want is here
http://www.cablelabs.com/specifications/CM-SP-PHYv3_0_110-111117.pdf
page 93 [their numbering not necessarily acrobats], you are concerned with qam64 upstream at this day and age, you will find 57 for multichannel mode disabled, and 54 for it enabled. That is the MODEM's actual capabilities, in order to be certified as a D3 modem, it is also the cable systems capabilities. Unfortunately that only works in a void, real world physics make the techs have to lower that range, hence why cox says 40 to 52, I am guessing their plant must run 23 to 14 value taps and then insert an LE or they EQ the heck out of it after that, which is a overall good design for managing noise/ingress. In either case, their actual install specs appear to exclude the range you said it should be in completely by starting at 40 and not including anything in the 30 range at all.
Now would being in 30's be that bad, that depends on you and your neighbors, but home owners tend to limp wrist cables when they put them on, and so they generate noise for the system, low value taps remove less signal going into the plant. A well places EQ can remove the low frequency return noise and force the modem to transmit higher improving not only your SNR on the upstream but the entire node's [more signal less noise] as well as less beats from over driven low frequencies, actually I am sort of envious of plant designed for 40 to 52, that would be a hell of a lot less noise sensitive than ours is. Regardless, the values you supplied and the conclusion you drew about what I do in cable appear incorrect.
I did not mean to offend you, simply correct a misconception you might of had either from a poorly trained tech or CSR or possibly a confusing conversation.