Consumers - SB6121: Troubleshooting Information

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SB6121: Troubleshooting Information

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How do I troubleshoot connectivity issues with my SB6121?
If Internet connection issues with the SB6121 arise, troubleshooting may resolve the issue, or may provide insight as to what is causing the issue.  This document provides helpful information for troubleshooting connection issues with the SB6121.

This document contains the following sections, to be followed in sequence:
  1. Checking the physical connection
  2. Verifying the LEDs
  3. Verifying the cable signal
  4. Power Reset
  5. Factory Reset

If the solutions provided in this document do not resolve the issue, please contact the cable operator.

Checking the physical connection

  1. Verify that the SB6121 power cord is connected from the power port of the SB6121 to the power outlet.
  2. Verify that the coaxial cable is connected from the cable port of the SB6121 to the cable outlet.
  3. Verify that the Ethernet cable is connected from the Ethernet port on the SB6121 to the Ethernet port on the computer.


Verifying the LEDs

With the proper setup, the Online LED should be solid green, and the Link LED should either be amber or blue. If this is not the behavior that the SB6121 is displaying, please refer to this table for more information regarding LED behavior.


 OffFlashingSolid GreenSolid BlueSolid Amber
POWERNo power to deviceN/AModem is powered upN/AN/A
RECEIVEReceive channel not found

Scanning for a receive (downstream) channel connection.Downstream channel is connected.Downstream channel is connected with bonded channels.N/A
SENDSend channel not foundScanning for a send (upstream) channel connection.Upstream channel is connected.Upstream channel is connected with bonded channels.N/A
ONLINEInternet connection failedScanning for Network ConnectionConnected to the InternetN/AN/A
LINKNo connection to the modem's Ethernet port detectedTransmitting or receiving data on Ethernet portN/AComputer is connected via a 10 mbps or 100 mbps Ethernet portComputer is connected via a Gigabit (1000 mbps) Ethernet port

Verifying the cable signal

NOTE: If the SB6121 is connected to a router, it is recommended to disconnect the router and connect the SB6121 directly to a computer with an Ethernet cable.
  1. Launch a web browser, such as Internet Explorer, Firefox, or Safari.  Enter into the address box.  The Cable Modem Status page will appear.
  2. Click the Signal link on top menu.  The Signal page will appear
  3. In the Downstream section, verify Power levels are within the acceptable range of - 15 dBmV to + 15 dBmV for each downstream channel. 

    User-added image

  4. In the Downstream section, identify the Modulation and Power to find verify SNR levels are within the acceptable range for each downstream channel. 

    Acceptable SNR Levels (dB): 
    If QAM64SNR should be 23.5 dBmV or greater. 
    If QAM256 and DPL( -6 dBmV to +15 dBmV) SNR should be 30 dBmV or greater. 
    If QAM256 and DPL(-15 dBmV to -6 dBmV) SNR should be 33 dBmV or greater.   

    User-added image

  5. In the Upstream section, identify the Symbolic Rate to find verify Upstream Power levels are within the acceptable range. 

    Acceptable Upstream Power Levels (dBmV):

    User-added image

    Upstream Transmit Power Level
    ChannelUS Channel TypeSymbol RateAcceptable US Power Level
    TDMA1280 Ksym/sec+61 dBmV to 45 dBmV
    ATDMA2560 Ksym/sec+58 dBmV to 45 dBmV
    5120 Ksym/sec+57 dBmV to 45 dBmV
    TDMA1280 Ksym/sec+58 dBmV to 45 dBmV
    ATDMA2560 Ksym/sec+55 dBmV to 45 dBmV
    5120 Ksym/sec+54 dBmV to 45 dBmV

    Three or more
    TDMA1280 Ksym/sec+55 dBmV to 45 dBmV
     2560 Ksym/sec+52 dBmV to 45 dBmV
     5120 Ksym/sec+51 dBmV to 45 dBmV
  6. Refresh the page at least twice to identify variations in SNRDownstream Power, and Upstream Power.   

    NOTE: If the either of the signal levels are outside of the acceptable range, power cycle or factory reset the modem. 

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

Acceptable Signal Levels (per channel) 

Recommended Max Downstream
Power Level (DPL)
Recommended Min Downstream
Power Level (DPL)
+15 dBmV-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 Level
ChannelUS Channel TypeSymbol RateRecommended Max US Power LevelRecommended Min US Power Level
SingleTDMA1280 Ksym/sec+61 dBmV 45 dBmV
ATDMA2560 Ksym/sec+58 dBmV 45 dBmV
5120 Ksym/sec+57 dBmV 45 dBmV
TwoTDMA1280 Ksym/sec+58 dBmV 45 dBmV
ATDMA2560 Ksym/sec+55 dBmV 45 dBmV
5120 Ksym/sec+54 dBmV 45 dBmV
Three or moreTDMA1280 Ksym/sec+55 dBmV 45 dBmV
ATDMA2560 Ksym/sec+52 dBmV 45 dBmV
5120 Ksym/sec+51 dBmV 45 dBmV

Power Reset

  1. Shutdown the computer.
  2. Unplug the cable modem from the AC wall outlet and wait one minute.
  3. Plug the cable modem into the wall outlet.
  4. Wait until the Power, Receive, Send, and Online LEDs are solid.
  5. Restart the computer.

Factory Reset

  1. Disconnect the coaxial cable.

    NOTE: If the SB6121 is connected to a router, it is recommended to disconnect the router and connect the SB6121 directly to a computer with an Ethernet cable.
  2. Power cycle the unit by disconnecting the power cord for 10 seconds, then reconnect the power cord.

    User-added image
  3. Launch a web browser, such as Internet Explorer, Firefox, or Safari.  Enter into the address box. The Status page will appear.


  4. Click the Configuration link on top. The Configuraton page will appear.
  5. Click the Reset All Defaults button (as shown below).


  6. When asked, Do you want to proceed?, click the OK button.  The modem will reset to it's factory state, and will go through the initialization process.
  7. Connect the coaxial cable to the modem.

    NOTE: The initialization process may take 5 to 30 minutes​.



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