I had Comcast (local cable internet supplier) service my exterior line today.
The serviceman used my browser to display the Motorola modem readings at the end of my house line.
The Downstream Power Level is the most important reading (just remember that -4.0 is better than -10.0). You might still be able to use the internet with a -13 reading but it should be above -10.
SNR (signal to noise ratio) is also important it should be above 33dB.
Readings outside this working range can be caused by a poor signal from the cable supplier or from bad connections (too many splitters) or too long a cable run in your house. Each splitter cuts your Power Level in half. 50 feet of marginal quality or poor connection on the cable might drop the signal by 4 dBmV.
Motorola Surfboard SB5120 QAM 256 (Working range)
Down Power (between -10 and +10 dBmV)
SNR (higher than 33dB)
Up Power (between 30 and 55dBmV)
Thanks to xpcman for this tip on the forum.