What should my signal levels be?
 

A: Downstream (Rx) Power Level:

DOCSIS specifies -15 dBmV to +15 dBmV with 0 dBmV being the "optimal" level.

Most techs installing cable modems try to get downstream signal levels at the modem to be between -10 dBmV and +10 dBmV to allow some "wiggle" room in case of signal fluctuations.

Signal-to-Noise-Ratio (SNR):

Signal-to-noise ratio (SNR) levels should be greater than 25 dB for QAM 64 and greater than 30 dB for QAM 256. You need to know what QAM your cable provider uses.  When looking up your modem's signal readings you should see the QAM listed.

Occasionally some people are told their modem's SNR level is too high. This is just plain wrong and those that say that do not understand what SNR really indicates. Saying SNR is too high is similar to saying "I can't understand what your are saying because the background noise is too quiet". The signal-to-noise ratio cannot be too high.

Upstream SNR levels are also extremely important and are subject to more problems then downstream SNR levels, but they are only available to the cable provider as it is read from the CMTS (modem controller) side of the connection.

Tech Note on SNR: DOCSIS doesn't directly specify SNR levels as read by the modem. DOCSIS specifications explicitly specify acceptable carrier-to-noise (C/N or CNR) levels and bit-error ratio (BER) levels. These levels directly relate to the signal-to-noise ratio (SNR) levels indicated on most cable modems but due to differences in how they are calculated are not the same value and cannot be used in place of one another.

Upstream (Tx) Transmit Power (a.k.a. Return Signal) level:

DOCSIS specifications for upstream signal level are between +8 dBmV and +58 dBmV for QPSK modulated signals and between +8 and +55dBmV for QAM 16 modulated signals.

Most modems will not work if they are transmitting at +58 dBmV (+55 dBmV for QAM 16 systems) or higher (Com21 modems are the exception with some of them working up to +62 dBmV). If it is it usually means either your return path has a problem (requiring a service technician or line technician), you have too many splitters between the modem and the tap (a splitter causes loss in both directions), or you may have a wiring problem. A house (or drop) amplifier does not help this sort of problem and in the vast majority of cases will only make it worse.

Tech Note on Upstream levels: Many techs target upstream power levels are between +35 dB and +52 dB. Lower than 35dBmV may indicate some local cable system equipment may be adjusted incorrectly leaving the system open to return signal ingress and noise problems. Higher than 52 dBmV may cause problems in the future due to changes in the cable system equipment or environment related signal variations. 35-52dBmV is only an optimal target range and is subject to variation due to local cable system conditions and requirements.

Notes concerning all signal levels:
1. Signal levels outside of the specs listed above can cause bit errors which if bad enough, which will cause packet loss and slow speed due to constant retransmits of bad packets. The modem can also loose connection (also known as lost or no sync) to the CMTS (cable company's modem controller equipment) if enough errors occur.

2. Poor signal levels should not affect latency, so if your "ping" is 300ms with a downstream level of -7.0 dBmV, it is not going to get any better if your signal is increased to +1.0 dBmV. Latency is affected by a combination of physical network distance and network equipment "packet handling" time, not signal levels.

3. Temperature variations can cause wide signal level changes (2-3 dB in many areas) from day to night, so if your signal is "borderline" at one point during the day, it may be out of spec causing the modem to work poorly or not at all during another.

4. Most cable systems are designed is such a way that the optimum 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 which is connected to the "drop" line connected to the cable tap at the street. Kind of like this:


Modem <--RG-6 coax--> 2-way Splitter <--RG-6 coax--> Tap


Excess splits, bad connectors, and/or poor quality cabling will certainly affect cable signal levels and can cause problems.