Guide to Cable Modem Signals
Guide to Cable Modem Signals
Checked the archives to verify this data, it looks like the site isn’t functioning properly anymore. Sad times for my former colleague (thanks for the reminder). I explored the internet archive just to confirm that this information would remain accessible for troubleshooting Docsis cable connections. It’s uncertain how well this will perform with Docsis 4.0, but I expect the signal quality to improve as it did in Docsis 3.1. What are the ideal signal settings?
Downstream (Rx) Receive Power Level: The strength of the signal coming from the transmitter at the cable company’s end. For all modems supporting DOCSIS 3.0/3.1:
-7 dBmV to +7 dBmV – "Recommended"
-8 dBmV to -10 dBmV – "Acceptable"
-11 dBmV to -15 dBmV – "Maximum"
Below -15 dBmV & above +15 dBmV – "Out of Spec"
SNR (signal to noise ratio) levels: This indicates how clear the signal is at both the modem and the company’s end. DOCSIS defines minimum carrier-to-noise ratios for different modulation types. Actual chipset performance may differ, so these numbers might not match every device exactly.
*No upper limit exists. For DOCSIS 3.0/3.1:
- 256 QAM: at least 30 dB, preferably higher
- 33 dB or more – ideal
- 64 QAM: 24 dB minimum, 27 dB recommended
- 27 dB or more – preferred
- 16 QAM: 18 dB minimum, 21 dB recommended
- QPSK: 12 dB minimum
- 15 dB or higher – best practice*
Downstream SNR is displayed on the modem’s downstream channel and can be checked via its diagnostic tools. Upstream SNR is shown at the CMTS on the upstream channel—not the modem or screens—and isn’t viewable directly by the end user. Only the provider can report the upstream SNR, which reflects an average across all connected devices.
Upstream (Tx) Transmit Power: The strength of signal sent from the modem to reach the cable company’s transmitter.
- Recommended: +35 dBmV to +47 dBmV (DOCSIS 3.1)
- DOCSIS 3.0/3.1: +35 dBmV to +49 dBmV
- OFDMA (DOCSIS 3.1): 50 dBmV max, A-TDMA, TDMA & SC-QAM: 53 dBmV max
- S-CDMA: 54 dBmV max
- 32 QAM and 64 QAM: up to 55 dBmV max
- 8 QAM: up to 58 dBmV max
- QPSK: up to 60 dBmV
If the upstream signal exceeds these limits, the modem may switch to a lower rate (e.g., from 64 QAM to 16 QAM) to stay within specs. This can lead to slower speeds, packet loss, or disconnections depending on channel conditions.
House amplifiers usually don’t boost upstream signals—they just pass them along with some loss.
Key points about signal levels:
- Deviations beyond the listed ranges may cause slowdowns, errors, or drops.
- It’s wise to keep signal strength at least 3 dB above the recommended limits, especially if temperatures fluctuate.
- Consistent changes greater than 3 dB over a day often signal an underlying issue.
- Issues from split splitters, poor connectors, or low-quality cabling can also impact performance.
Feedback on this guide was shared by a member of BroadbandReports. This answer isn’t official from Comcast, but it reflects community knowledge. Any questions?
Thanks a lot for sharing this! It’s often useful when checking family and friends’ internet issues, and I’m disappointed when the site goes down. Haha