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Knowledge Base document #3830
FRITZ!Box 5690 Pro
This support document is available for the following products:

The fiber optic connection drops frequently

The internet connection between your FRITZ!Box and the fiber optic connection drops frequently drops frequently (several times a day). During this time, the Power/Fiber LED on the FRITZ!Box flashes and then stays lit as soon as the internet connection is re-established. The following error message is displayed in the event log of the FRITZ!Box:

"The fiber optic connection is interrupted.
Starting fiber optic connection synchronization (training)"

Cause

  • The stability of the fiber optic connection depends on the condition and the quality of the access line. Faults in the cabling or problems with the fiber optic connection can also affect it.

Simply proceed as described below. After each measure, check whether the problem is solved.

1 Installing the latest FRITZ!OS for the FRITZ!Box

Install the latest version of FRITZ!OS because it contains improvements for different fibre optic connections that can resolve the error:

  1. Manually update FRITZ!OS.

2 Checking the event log of the FRITZ!Box

  1. Click on 'System' in the FRITZ!Box user interface.
  2. Click on 'Event Log' in the 'System' menu.
  3. Check whether events also took place before the time when the error occurred, or whether only events are shown that took place after the error occurred.
  4. If events are also displayed that took place before the time the error occurred, proceed with the next section.

3 Checking the cabling

Errors in the cabling often go unnoticed. Therefore, make sure that the FRITZ!Box is correctly connected to your line:

  1. Remove all cable extensions and adapters between the FRITZ!Box and the fibre optic socket.
  2. Use an undamaged cable to connect the FRITZ!Box to the fibre optic socket. The fibre optic cable may not have any kinks or be damaged.

    Note:The SFP modules included with the FRITZ!Box have LC/APC sockets (compact connector, angle polished). Depending on the fibre optic socket, you may need to use a fibre optic cable with an adapter or a cable with different plugs on both ends. Normally the cable length in the home does not matter.

4 Testing a different cable

The cable that connects the FRITZ!Box to the fibre optic connection could be defective. Therefore, test a different cable:

  1. Use a different fibre optic cable (for example from a different fibre optic modem) to connect the FRITZ!Box to the fibre optic socket.
  2. If the Power/Fiber LED lights up constantly after 10 minutes, the fibre optic cable used before is defective and must be replaced.

    Note:Fibre optic cables can be purchased from electronics retailers. You can find information on the different types of plugs and polishing in the guide FRITZ!Box: Which fibre optic cable do I need?.

5 Checking the fiber optic connection

There may be a problem with your fiber optic connection. Since it may take a while for your internet service provider to check your line and you may be charged for the service, proceed as follows:

  1. Remove the FRITZ!Box from the fiber optic connection.
  2. Connect a different fiber optic modem to the fiber optic connection.
  3. Test whether the fiber optic connection remains stable when you use this fiber optic modem.
  4. If the connection continues to drop, have your provider check your line. There is a problem with your fiber optic connection.

    ATTENTION!You may be charged for checking. Ask your provider about charges in advance!

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