Downstream (Rx) Receive Power Level:
For all modems:
SNR (signal to noise ratio) levels:
DOCSIS specifications list minimum CNR (carrier to noise ratio) levels not SNR levels. The SNR levels listed here are based on commonly recommended MER levels for digital cable signals. Not all QAM demodulator chipsets accurately calculate SNR levels that approximate actual MER levels, so these levels may vary depending on which chipset and/or firmware is used in the equipment.
Downstream SNR levels are read at the modem on the downstream data channel and can be viewed using the modem diagnostic screens.
Upstream SNR levels are read at the CMTS on the upstream data channel, not the modem or the modem diagnostic screens. The end-user cannot get the upstream SNR directly. Only the provider can read the upstream SNR level, directly from the CMTS. Also, the upstream SNR level provided by most CMTSs is not specific to any single modem, but is an averaged, aggregate level from all modems on that upstream channel on the upstream port.
Upstream (Tx) Transmit Power (a.k.a. Return Signal) level:
A cable modem running a higher upstream modulation rate may downgrade itself to a lower modulation rate (i.e. 64 QAM to 16 QAM or 16 QAM to QPSK) if the upstream transmit level is higher than the maximum signal level allowed for the higher modulation rate and the CMTS is configured to allow such a change. This downgrade can cause slow speed, packet loss, and connection loss issues depending on the condition of the upstream channel.
A house or drop amplifier will NOT fix an upstream signal problem because most house amplifiers don't amplify the upstream signals; they only pass the upstream signal through with some loss.
Important notes concerning signal levels:
2. It's recommended to have the modem's signal levels at least 3dB away from the maximum/minimum levels listed above due to normal temperature related signal variation. If the modem's signal levels are at the maximum or minimum limits, they may be out of spec. if the temperature changes significantly. Signal levels that vary more then 3 dB in a 24-hour period usually indicate a problem that should be looked into.
3. Excess splits, bad connectors, and/or poor quality cabling will certainly effect cable signal levels and will cause problems.