Information on signal levels: http://homepage.ntlworld.com/robin.d.h.walker/cmtips/signal.html#signal

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.