Knowledgebase
How to do an Isolation test?
Posted by Gabriel Yu on 14 March 2008 05:06 AM
Often the problem with an ADSL connection is within your premesis; for instance it could be a bad joint in the wire, phone line running alongside power cables, a faulty ADSL filter, or a low-quality phone handset.

The best procedure for identifying any problem device is an Isolation test, and is essentially the same procedure that your telephone company will ask you to do for a bad telephone connection.
  1. Before starting the test, check the current SNR Margin, Loop Attenuation, Up and Down speeds; as described in How can I know if I have a good connection?.
  2. Unplug all phones, fax machines, modems, etc. from the phone line used for your ADSL connection. Now plug in ONLY the ADSL modem and test the connection. Check the SNR Margin, Loop Attenuation, Up and Down speeds; as described in How can I know if I have a good connection?.
  3. If there is NO appreciable difference, then none of the devices which you have temporily disconnected from your phone line are having a negative effect. There is no point continuing with the other steps.
  4. If there is a difference between these values and your normal values, then connect one other phone device to the phone line, and check the SNR etc.
  5. Do the same for each phone device you have. This should indicate which extension/device is causing a problem.
  6. Note that loose cables and faulty connections can also cause problems, as can faulty ADSL filters – so you might like to try different combinations of device / extension socket / cable to determine which of these is the culprit.
Note that the Isolation test only tests the devices connected to your wall sockets. If Telstra test your phone line, they will test at the point it enters the building - that leaves the wiring inside your house, office or appartment building which hasn't been tested.
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