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Short Version:
    •Receive/Downstream: -15dbmV to +15dbmV
    •Transmit/Upstream: 30dbmV to 53dbmV
    •Signal to Noise Ratio(SNR): 33dB or Greater

Note: If your power levels are out of range, it can cause packet loss, re-transmissions which will eventually lead to speed loss; otherwise signals have nothing to do with your speed.

Long version:

Downstream (Rx) Receive Power Level:
This is the amount of signal received by the modem from the transmitter in the cable company head-end.

For all modems:
-15dBmV to +15dBmV maximum.
-12dBmV to +12dBmV recommended.

0 dBmV is the "optimal" level.

Upstream (Tx) Transmit Power (a.k.a. Return Signal) level:
This is the amount of signal transmitted by the modem to reach the receiver in the cable company head-end.

+8dBmV to +58dBmV maximum for QPSK. (DOCSIS 1.x)
+8dBmV to +55dBmV maximum for 8 QAM and 16 QAM. (DOCSIS 1.x)
+8dBmV to +54dBmV maximum for 32 QAM and 64 QAM. (A-TDMA DOCSIS 2.0)
+8dBmV to +53dBmV maximum for S-CDMA DOCSIS 2.0 modulation rates.

Recommended upstream signal levels are +35dBmV to +52dBmV.

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 upstream signal problem because most house amplifiers don't amplify the upstream signals, they only pass the upstream signal through with some loss.

SNR (signal to noise ratio) levels:
This is how clear the signal is at either the modem receiver (downstream SNR) or the receiver in the cable company head-end (upstream SNR).

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.

QPSK: 12 dB minimum. 15 dB or higher recommended. (often used in upstream channels)
16 QAM: 18 dB minimum. 21 dB or higher recommended. (often used in upstream channels)
64 QAM: 24 dB minimum. 27 dB or higher recommended. (often used in downstream channels)
256 QAM: 30 dB minimum. 33 dB or higher recommended. (often used in downstream channels)

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by drake See Profile edited by dbmaven See Profile
last modified: 2010-08-08 19:09:26