FRITZ!Box 7490 Serwis - Baza wiedzy

FRITZ!Box 7490 Serwis
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Crackling noise when conducting landline calls on an ISDN line

You always hear an annoying crackling sound when conducting landline calls on an ISDN line. It does not matter whether the telephone is connected to the FRITZ!Box or directly to the ISDN NT.

Simply proceed as described below. After each measure, check whether the issue persists.

1 Checking the cabling

Mistakes in the cabling can often be overlooked. Therefore, make sure that the FRITZ!Box is correctly connected to your line:

  • Check the cabling to your landline.

2 Connecting only the FRITZ!Box to the ISDN NT

The following steps are only necessary if other ISDN devices are connected to the ISDN NT in addition to the FRITZ!Box:

  1. Disconnect all ISDN hubs, all ISDN devices including their cables, and any other cables from the ISDN NT.
  2. Use the original cable to connect the FRITZ!Box to the ISDN NT. Do not connect any additional devices.
  3. Check whether the issue persists.
    • If the FRITZ!Box functions properly when no other devices are connected, the issue is not caused by the FRITZ!Box. The error can have various causes:
      • An ISDN cable being used is too long or it is defective (cables may not exceed a length of 10 m).
      • Another ISDN device is defective.
      • You are using an ISDN hub that does not function in accordance with the corresponding specifications.
      • An S0 bus that is not terminated was installed.
      • The topology of the S0 bus is incorrect (no clear interface, for example due to the presence of branches that are not allowed).
        • Usually the following measures can eliminate the cause of the disturbance:
          • Do not connect ISDN devices using ISDN cables that exceed a length of 10 m.
          • If the issue occurs when using an ISDN hub: Connect a pre-assembled resistor to a port on the ISDN hub if the ISDN hub does not already have one, or if the ISDN hub is not terminated.
          • Use an ISDN cable that is as short as possible to connect all ISDN devices directly to the ISDN hub. Do not connect any further S0 bus extensions to the ISDN hub.
          • If an S0 bus was laid: Remove all branches from the S0 bus that do not directly lead to an ISDN device.
          • Make sure that two 100 Ohm resistors are used to terminate the end of the S0 bus and that termination is turned on in the ISDN NT. If the ISDN NT is located in the middle of your S0 bus installation, you must have termination resistors on both bus ends. In this case you must turn off the termination in the ISDN NT.
          • Remove all ISDN cables from the ISDN-NT and/or S0 bus that do not have ISDN devices connected to them.
          • Do not use more than 12 ISDN jacks. Do not connect more than eight ISDN devices to each ISDN jack.

3 Replacing the DSL splitter and the ISDN NT

  • Replace the DSL splitter and the ISDN NT. Try to avoid using the same device types.

    Note:Your DSL provider can supply you with a new DSL splitter and ISDN NT.