Definitions
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).
Compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to the noise power expressed in decibels.
A ratio higher than 1:1 (greater than 0 dB) indicates more signal than noise. The SNR, the bandwidth, and the channel capacity of a communication channel are connected by the Shannon–Hartley theorem.
CNR (Carrier to Noise Ratio) levels:
Is the signal-to-noise ratio (SNR) of a digitally modulated signal. A digitally modulated signal is usually referred to as a carrier or carrier wave. Examples at QAM or PSK modulation. The term carrier-to-noise-ratio (CNR) is used over signal-to-noise-ratio (SNR) to express the signal quality when the signal has been digitally modulated.
The term is used to distinguish the CNR of the radio frequency passband signal from the SNR of an analog base band message signal after demodulation. If this distinction is not necessary, the term SNR is often used instead of CNR, with the same definition.
Downstream (Rx) Receive Power Level:
This is the amount of signal received by the modem from the transmitter in the cable company head-end.
Upstream (Tx) Transmit Power level:
This is the amount of signal transmitted by the modem to reach the receiver in the cable company head-end.
dbmV
The term decibel watts is used to track power levels. In cable modems the range goes from -15dbmV to +15dbmV. Typically +/- 0.1dbmV.
ATDMA and TDMA
Advanced Time Division Multiple Access. It is a channel access method for sharing either a network or transmission channel. It allows several users to share the same network or channel by dividing the signal into different time slots. Users transmit in rapid succession during their own time slot. This allows multiple stations to share the same transmission medium while using only a part of its channel capacity.
CDMA and SCDMA
Synchronous Code Division Multiple Access. Allows several transmitters to send information simultaneously over a single communication network or channel. This allows several users to share a band of frequencies, aka bandwidth. Both employ spread spectrum technology and a special coding schemes to permit multiple users on the network or channel.
CMTS
Cable Modem Termination System. Equipment that translates the RF signal from a cable modem to IP for the Internet. The setup is often referred to as Hybrid Fiber Coaxial (HFC) Cable network.
How it works
DOCSIS specifications list minimum CNR (carrier to noise ratio) levels not SNR levels. Not all QAM demodulator chipsets accurately calculate SNR levels. These levels may vary depending on which chipset and/or firmware is used in the equipment.
The three signal levels that need to be within the acceptable level are Downstream Power, Downstream SNR, and Upstream Power.
Downstream Power refers to the line signal sent from the cable provider to cable modem. Downstream SNR refers to the signal being sent in relation to the noise on the line. Upstream Power refers to the line signal from the cable modem to transmit data back to the cable provider.
A cable company’s headend typically has 1 or more CMTS units with a single CTMS servicing between 4,000 to 150,000 cable modems. Traffic destined for the cable modem from the Internet is known as downstream traffic, while the reverse direction is called upstream traffic.
Questions
What should my Signal Levels be?
Upsteam
- QPSK: 12 dB minimum. 15 dB or higher recommended.
- 16 QAM: 18 dB minimum. 21 dB or higher recommended.
Downsteam
- 64 QAM: 24 dB minimum. 27 dB or higher recommended.
- 256 QAM: 30 dB minimum. 33 dB or higher recommended.
NOTE: There is no upper SNR limit; however, 40 dB is the highest most people see. Going above 40 dB is possible though.
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. Only the provider can read the upstream SNR level. The upstream SNR level provided by the CMTS is not specific to any single modem, but is an averaged from all modems on that upstream channel on the upstream port.
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 signal loss.
What should my power levels be?
Downsteam (Rx) Transmit Power Levels
- -15 dbmV Minimum or Floor
- -11 dBmV to -14 dBmv Acceptable
- -10 dBmV to +10 dBmV Recommended Range
- +11 dBmV to +14 dBmV Acceptable
- +15 dBmV Maximum or Ceiling
Upstream (Tx) Transmit Power level:
+8 dBmV to +58 dBmV max for QPSK. (DOCSIS 1.0, 1.1)
+8 dBmV to +55 dBmV max for 8 QAM and 16 QAM. (DOCSIS 1.0, 1.1)
+8 dBmV to +54 dBmV max for 32 QAM and 64 QAM. (ATDMA, DOCSIS 2.0)
+8 dBmV to +53 dBmV max for SCDMA DOCSIS 2.0 (All Modulations)
Recommended upstream signal levels are +35 dBmV to +52 dBmV.
Important notes concerning signal levels
- 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 re-transmission.
- It’s recommended to have the modem’s signal levels at least 3dB away from the maximum/minimum levels due related signal variation caused by environmental factors.
- Signal levels that vary more then 3 dB in a 24-hour period usually indicate a problem that should be looked into.
- Excess splits, bad connectors, lose connections, and poor quality cabling will effect cable signal levels and will cause problems.
References
- https://arris.secure.force.com/consumers/articles/General_FAQs/SB6183-Cable-Signal-Levels
- https://www.cisco.com/c/en/us/tech/broadband-cable/cable-modem-termination-systems-cmts/index.html
- https://en.wikipedia.org/wiki/Carrier-to-noise_ratio
- https://en.wikipedia.org/wiki/Signal-to-noise_ratio
- https://en.wikipedia.org/wiki/Cable_modem_termination_system
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