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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): +8dbmV to +58dbmV.
A value within the range +25 to +55 dBmV is within spec with 40's the most common. The lower this figure is, the better the upstream path to the UBR. If the cable modem is going offline, and the upstream signal strength is at +58dBmV, then a poor upstream path is probably the problem.
Carrier/Noise Ratio: 30dB to 36dB .
The downstream Signal to Noise Ratio must be 23.5 dB or higher, and should ideally be 30 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.
reference: same link as above