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T25: Troubleshooting Internet Connection

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Question
How do I troubleshoot Internet connection issues with my T25 Cable Modem?
Answer
If all the client devices in the home network have lost Internet connectivity, troubleshooting may resolve or provide insight on the cause of the issue.  This document provides helpful troubleshooting steps for Internet connectivity issues with the T25.  

Troubleshooting on the T25 is not needed if the following scenarios are applicable.  
  • Compatible Cable Provider
    Xfinity is the only compatible cable provider for the T25.  The T25 is not compatible with other cable providers, please check the cable provider's website for compatible ARRIS modem and gateways.
     
  • First Time Setup
    For first time installation of the T25 on the Xfinity home network, refer to article # 18063 - T25: First Time Installation and follow the installation steps.

The following troubleshooting steps are to be followed in sequence.
  1. Check the Power Supply
  2. Check the Modem's LED Light Status
  3. Power Cycle the Modem
  4. Check the Cable Signal Levels

Check the Power Supply
Use the power supply included with the T25.  Overall functionality and performance cannot be guaranteed if the incorrect power supply is used when installing the T25.  When using the incorrect power supply, the T25 may get online and function, but firmware upgrades will not complete successfully.  


Check the Modem's LED Light Status
First step in troubleshooting is to check the 3 LEDs located on the front of the T25.  Reference the table below to identify the possible issue for each LED status and perform the steps listed under Possible Solutions.  The detailed troubleshooting steps for power cycle and check cable signal levels are available below the LED Light Status table.      
 
LED StatusPossible IssuesPossible Solutions
PowerOff - No power on the modem
  • Bad power cord
  • Power cord disconnected
  • No power on the electrical outlet
  • Modem power on failure
  1. Check for damages on the power cord.
  2. Connect the power cord from the modem and to an electrical outlet.
  3. Check that the electrical outlet is working; if controlled by a wall switch, confirm the wall switch is on.
  4. Try a different electrical outlet.
US/DS
 		Blinking - The modem receives low quality or no cable signal
  • Loose or disconnected coax cable 
  • Cable signal outside acceptable range
  1. Securely tighten the connectors on each end of the coaxial cable by hand. 
  2. If applicable, check the television service to ensure cable service is operating properly.
  3. Power cycle the modem by disconnecting power cord for one minute and connecting it back to the outlet.
  4. Check the cable signal levels in the Web manager of the T25.
OnlineBlinking - The modem is scanning for an Internet connection
  • Modem activation is needed
  • Cable signal outside acceptable range
  1. Power cycle the modem by disconnecting power cord for one minute and connecting it back to the outlet.
  2. Check the cable signal levels in the Web manager of the T25.
  3. Contact the cable provider to reactivate the modem.

Power Cycle the Modem
Power cycling the modem is one of the easiest troubleshooting methods that may resolve connectivity issues.  It allows the T25 to establish connection while retaining its current configuration settings.  Follow the steps below to power cycle the T25.
  1. Shutdown the computer.
     
  2. Disconnect the power cord from the AC wall outlet and wait one minute.
     
  3. Connect the power cord into the wall outlet.
     
  4. Wait until the Power, US/DS, and Online LEDs are solid white.

    NOTE: It may take up to 15 minutes for the T25 complete the boot-up sequence and connect to the Internet.
     
  5. Restart the computer.

Check the Cable Signal Levels
All DOCSIS cable modems require that the cable signals fall within a very specific range to be an acceptable level for the modem to connect to the Internet.  The Downstream Power, Downstream SNR (Signal to Noise Ratio), and Upstream Power levels must be within the acceptable ranges for the T25 modem to operate properly.

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 Login page will appear.

    NOTE 1: 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.

    NOTE 2: On firmware prior to version 11.02, the Status page will appear and the admin Login page is unavailable.
     
  2. On the Login page, enter admin in the Username field.
     
  3. Enter a customized password in the Password field and then click the Login button.  The Status page will appear.

    NOTE: If this is the first-time access to the Web Manager Access, refer to article # 18100 – T25: Web Manager Access. 

    User-added image
     
  4. 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
     
  5. 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
     
  6. In the Upstream QAM section, identify the number of Upstream channels and Symbol Rate to verify the Power levels are within the acceptable range for each upstream channel. 
     
    Upstream Transmit Power Level
     Symbol Rate (kSym/s)Acceptable US Power Level (dBmV)
    Single
     
         		1280 +45 to +61
    2560+45 to +58 
    5120 +45 to +57 
    Two1280 +45 to +58 
    2560 +45 to +55 
    5120 +45 to +54
    Three or more1280 +45 to +55
    2560 +45 to +52
    5120 +45 to +51 
    Right click on the image to save it to your computer.
     
  7. 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.
     
  8. Contact the cable provider to adjust the signal levels to the acceptable level range.

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