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Cable Modem Signal Levels

Cable modem signal levels

For VoIP to work consistently well the connection to the cable modem should be stable and within certain signal (dB) parameters.  The best way to determine if there are any problems with the cable signal levels, that could affect the modems connectivity is to view the cable modem's signal power levels by accessing an interior screen.  These levels can be read by accessing the modems internal pages in most cases.  Many modems have an IP address of which would be the address to use for access.  For more about Motorola cable modem access.
The cable modem's power levels should be within these ranges:
  • Downstream Power Level (Rx)- This level is the downstream signal strength and can be plus or minus and shows the amount of signal received by the modem from the cable companies' transmitter.  Typically the lower you go in the minus range the more the modem struggles to keep being acquired.  Getting down below -8 to -10 can result in periodic dB changes where the signal could degenerate even more to possibly -11, -14 or worse and loss of sync.  The range of signal strength can be:
    • -15dB to +15dB officially
    • -12dB to +12dB or better recommended*
    • 0dB would be the optimal level for downstream power Tx level
  • Signal to Noise Ratio (SNR)- This reading should be above 25/27, but in the 30s would indicate a nice clean signal.  SNR is the clarity or "cleanness" of the signal at the modem. Also known as Downstream SNR. There is an Upstream SNR, but the cable modem's pages diagnostic pages would not show this measurement, which can only be seen by the cable provider at their CMTS.  The SNR levels listed here are based on commonly recommended levels:
    • For 64 QAM 25dB would be minimum, but 27dB or higher would be recommended.
    • For 256 QAM 30dB would be minimum, but 33dB or higher would be recommended.
  • Upstream Power Level (Tx)-  This power level is the amount of signal transmitted by the modem back to the cable companies' receiver.  This reading should certainly be lower than 54, depending on specifics about your cable companies equipment, as an absolute maximum, but would be better to be in no higher than the high 40s.  Also known as the transmit level or return signal, as it gets into the higher regions the cable modem will have a harder and harder time keeping connectivity.  Additionally, a cable modem that is running a higher upstream modulation rate (many are now) could possibly in some cases lower that modulation rate if the Tx level is higher than the maximum signal level allowed.  This condition could cause slower speeds, packet loss, and even loss of connectivity.  The levels for Tx should be:
    • +8dB to +54dB
    • +35dB to +52dB recommended.

*Remember, that when checking the cable modem signal levels to remember that these readings are a snapshot at that time.  If you have issues on rainy days then compare the readings on a clear day to one when it rains.  A deterioration of signal strength would indicate a moisture problem in either a splitter, cable or connection degrading your signal.  Normally temperature changes can affect the power levels by plus or minus 3dB; a reason why you would not want signal levels close to the extreme limits.

Some steps that can be taken to improve your cable modem's signal levels and connection.

  • Cable line to modem should have the least amount of connectors, splitters or other items possible.
  • Move the cable modem's line to the lowest loss port on the splitter at the cable box.  For instance some three way splitters may have a 3.5dB loss port and two 5dB loss ports.  On this splitter place the line going to the cable modem on the 3.5dB port.
  • Place a 2way splitter first in line from the main connection coming from the cable company in the cable box.  Then connect the cable coming from the modem to one port of the two port splitter.  (A direct cable connecting directly to the cable modem would be best.)  The second port should then be used for the television lines  using an additional splitter after the two port splitter.  (TVs are more tolerant of lower signal strength than a modem.)

A signal amplifier will not help correct an upstream signal (Tx) issue because most house amplifiers do not amplify the upstream, they will only pass the Tx signal and then with some loss, which further degrades the modem's connection.  If a amplifier is needed to boost the signal for the TVs, then connect the amplifier after a two way splitter.  One cable from the 2-way splitter directly to the cable modem and the other to the amplifier and then to the additional splitter.

Cable provider-->Cable box-->2way splitter-->cable modem

                                                                     -->TV or second splitter-->TVs