Consumers - T25: Cable Signal Levels
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T25: Cable Signal Levels

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Question
What are the acceptable cable signal levels for the T25 Cable Modem?
Answer
Cable signal levels for all DOCSIS cable modems need to be at acceptable levels for the modem operate properly.  The three signal levels that need to be within the acceptable level are Downstream Power Level, SNR (Signal to Noise Ratio), and Upstream Power Level
  • Downstream Power Level: cable signal sent from the cable provider to cable modem.  
  • Downstream SNR (Signal to Noise Ratio): cable signal being sent in relation to the noise on the line.  
  • Upstream Power Level: cable signal from the cable modem to transmit data back to the cable provider.
This document describes how to check the three signal levels to determine if they are within the acceptable level for the modem operate properly. 


Checking Signal Levels

NOTE
: If the T25 is connected to a router, it is recommended to disconnect the router and connect the T25 directly to a computer with an Ethernet cable.

  1. Launch a web browser, such as Internet Explorer, Firefox, or Safari.  Enter http://192.168.100.1 into the address box, and press the Enter key.  The Status page will appear.

    NOTE: A security or private risk alert may appear on the web browser advising users to proceed with caution when accessing the Web Manager.  There is no risk in proceeding to the Web Manager.  For instructions to by-pass the alert screen, refer to article # 18181 - Alert Message for Web Manager Access.
     
  2. In the Downstream QAM section, verify Power levels are within the acceptable range of - 15 dBmV to + 15 dBmV for each downstream channel.

    User-added image

     
  3. In the Downstream QAM section, identify the Modulation and Power to verify SNR levels are within the acceptable range for each downstream channel.  
     
    Downstream Signal To Noise Ratio (SNR)
    ModulationDS Power Level (dBmV)Acceptable DS SNR (dB)
    64QAM-15 to +15 +23.5 or greater
    256QAM-6 to +15 +30 or greater
    -15 to -6 +33 or greater

    User-added image
     
  4. In the Upstream QAM section, identify the number of Upstream channels and Symbolic Rate to verify the Power levels are within the acceptable range for each upstream channel. 

    Right click on the image to save it to your computer.

    Acceptable Upstream Power Levels (dBmV)
     
    Upstream Transmit Power Levels
    ChannelSymbol RateAcceptable US Power Level
    Single (1)1280 Ksym/sec45 dBmv to 61 dBmV
    2560 Ksym/sec45 dBmv to 58 dBmV
    5120 Ksym/sec45 dBmv to 57 dBmV
     Two (2)1280 Ksym/sec45 dBmv to 58 dBmV
    2560 Ksym/sec45 dBmv to 55 dBmV
    5120 Ksym/sec45 dBmv to 54 dBmV
    Three or more (>=3)1280 Ksym/sec45 dBmv to 55 dBmV
     2560 Ksym/sec45 dBmv to 52 dBmV
     5120 Ksym/sec45 dBmv to 51 dBmV
  5. Refresh the page at least twice to identify variations in SNR, Downstream Power Level, and Upstream Power Level.   

    NOTE: If either of the signal levels are outside of the acceptable range, power cycle or factory reset the modem.  Factory resetting the modem will erase any customized settings.

     
  6. Contact the cable provider to adjust the signal level to the acceptable level range.

    Acceptable Signal Levels

     
    Acceptable Downstream Power Level
    -15 dBmV to +15 dBmV
    Downstream Signal to Noise Ratio (SNR)
    ModulationDownstream Power LevelAcceptable DS SNR
    64 QAMn/a23.5 dB or greater
    256 QAM
     
    -6 dBmV to +15 BmV30 dB or greater
    -6 dBmV to -15 dBmV33 dB or greater
    Upstream Transmit Power Levels
    ChannelSymbol RateAcceptable US Power Level
    Single (1)1280 Ksym/sec45 dBmv to 61 dBmV
    2560 Ksym/sec45 dBmv to 58 dBmV
    5120 Ksym/sec45 dBmv to 57 dBmV
     Two (2)1280 Ksym/sec45 dBmv to 58 dBmV
    2560 Ksym/sec45 dBmv to 55 dBmV
    5120 Ksym/sec45 dBmv to 54 dBmV
    Three or more (>=3)1280 Ksym/sec45 dBmv to 55 dBmV
     2560 Ksym/sec45 dBmv to 52 dBmV
     5120 Ksym/sec45 dBmv to 51 dBmV

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