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:
-15 dBmV to +15 dBmV maximum.
-12 dBmV to +12 dBmV 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.
+8 dBmV to +58 dBmV maximum for QPSK. (DOCSIS 1.x)
+8 dBmV to +55 dBmV maximum for 8 QAM and 16 QAM. (DOCSIS 1.x)
+8 dBmV to +54 dBmV maximum for 32 QAM and 64 QAM. (A-TDMA DOCSIS 2.0)
+8 dBmV to +53 dBmV maximum for S-CDMA DOCSIS 2.0 modulation rates.
Recommended upstream signal levels are +35 dBmV to +52 dBmV.
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)
There is no upper SNR limit, although in practice 40 dB is about the highest seen.
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.
Important notes concerning signal levels:
1. Signal levels not within the specifications listed above can cause slow speeds, connection problems, and connection loss due to packet errors, packet loss, and/or constant packet retransmission.
2. Its 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. Most cable systems are designed is such a way that the optimal signal levels for the modem are achieved if the modem is connected to a dedicated "homerun" RG-6 grade cable line straight to a 2-way splitter at the demarcation or ground point. This is then connected to the "drop" line connected to the cable tap on the utility pole or at the street. Like this:
4. Excess splits, bad connectors, and/or poor quality cabling will certainly effect cable signal levels and will cause problems. Make the cable run to the modem as clean as possible, the less junk in the way the better.
5. Poor signal levels should not affect latency, so if your "ping" is 300ms with a downstream level of -14.0 dBmV, it is not going to get any better if your signal is increased to +1.0 dBmV. Latency is caused by a combination of physical network distance and network equipment "packet handling" time, not signal levels.
If your wondering how to get these levels from your modem, see this page: /faq/5861.
CableLabs DOCSIS Specifications
Spectrum Analyzer CNR Versus CMTS SNR. Ron Hranac. Communications Technology, Sept. 2003
»www.ct-magazine.com/arch ··· and.html
More on CMTS SNR. Ron Hranac. Communications Technology, Oct. 2003
»www.ct-magazine.com/arch ··· and.html
How to Increase Return Path Availability and Throughput. Cisco Systems.
»www.cisco.com/en/US/prod ··· ad.shtml
Sunrise Telecom Online Learning Seminars
»www.sunrisetelecom.com/b ··· ng.shtml
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