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How to perform an Isolation Test

The way equipment is connected in your home, including phone cables and 'filtering' on devices attached to your phone line, are among the most common causes of slow internet speeds and drop-outs.


You can check for these problems by doing what we call an 'isolation' test. It might sound a bit technical but it's really very straightforward – basically, it's just checking all the devices like phones, faxes and answering machines, alarms and so on that plug into your phone line aren't introducing any interference which may impact the quality of your service.





  1. Disconnect every device (including any ADSL filters) from every telephone socket in your home and turn off any devices connected by Wi-Fi.

  2. Plug your modem into the first phone socket without a filter using the shortest cable you have (less than 1 metre is ideal), connect one computer using an Ethernet cable and run a speed test or monitor your connection to see if dropouts continue. Note the results.

  3. Try this also in any other phone sockets in your home or office and check the results to see if it makes a difference.

  4. If this seems to have improved the quality or speed of your service, reconnect the other devices one at a time starting with any ADSL filters. Run a speed test or monitor for dropouts each time to determine if the issue resumes – if it does, the problem is probably with the device you've just connected.

    If this process doesn't help to pinpoint a problem, the cause may be elsewhere – something in your home or office environment like a faulty modem or wiring, or possibly an issue in the network and it's time to seek help from our Technical Support team. You can contact them on 133 933.


Please remember to be careful when removing devices off the phone line and when especially when dealing with electrical items – if in doubt, consult a qualified electrician.





Use our Smart Troubleshooting Tool to help fix common ADSL and Cable broadband connection, Wi-Fi and email issues.

Smart Troubleshooting Tool >




An isolation test is when you 'isolate' the modem on your phone line. All other devices that connect to or use your phone line are removed. These include ADSL filters, phones, faxes, answering machines, Foxtel Digital (or any other Digital PayTV) set top box, EFTPOS machines, back-to-base alarm systems, dialup modems, medical alerting systems etc – anything that plugs into the phone line. Some devices can be 'hard wired' into your home, i.e. wires are unable to be removed, this can include back-to-base alarm systems and medical alerting systems. When it comes to hard wired items please consult a licensed electrician.

The purpose of removing all devices from your phone line, other than your modem, is to eliminate any potential source of interference caused by equipment in your house. 

After all devices have been removed:


  1. Plug in your telephone handset and test for dial tone.
    • If you have dial tone and can use the phone to make and receive calls continue on to the next steps. 

    • If you don't have dial tone, or you notice noise/static on your line please report it to our faults department on 132203 or use the troubleshooting tool above.

  2. Remove the telephone handset and plug your modem directly into the first phone socket into the house, bypassing any filters or splitters(just to eliminate these as a possible problem).
    • The first socket is generally the one in the common area of the house, or the one nearest the road. It can vary depending on the setup of your house. If you aren't sure which is your "first" socket, simply test all sockets in your house.

    • Always make sure when performing an isolation test that you use the shortest possible phone cable, (In Telstra's eyes any cable longer than 2 meters is considered an extension cable). In some cases, it might be a good idea to try a brand new cable, however you don't need to do this to start with.

  3. Monitor the Internet connection for the difficulty you were experiencing to see if it clears.
    • If the difficulty you were experiencing clears, then start reconnecting devices back onto the line ONE at a time, starting with your ADSL filter. Wait and watch for the difficulty to reoccur as each device is returned to the line. If the difficulty reappears, then logic dictates that the problem is most likely with that device.

    • If this process doesn't help pinpoint a problem, the cause may be elsewhere – something in your home or office environment like a faulty modem or wiring, or possibly an issue in the network and it's time to seek help from our Technical Support team. Contact us >




If you are having issues with things like your connection dropping out (modem losing ADSL sync), noise/static on your phone line, slower speeds than usual, then this test can help isolate the issue.


The above issues are often caused by some form of interference induced onto line, which could be a result of bad filters, extension cables, corroded cabling and/or sockets, malfunctioning telephony equipment, or even nearby electrical devices.


Being able to identify and resolve an issue with your equipment will enable you to get your service back online quickly. As it is standard process for our tech support consultants to ask you to perform this test, if you already have done these steps it will save you time and frustration.



  • We recommend that you do not use phone extension cables if you have ADSL. As the signal degrades the longer the cable is and that the longer cables can act as antenna for electromagnetic interference induction as mentioned above. 
  • Ensure that if you have ADSL and a B2B or Back to Base alarm that connects to your phone line that you have a central filter installed. This can be done by Telstra or a private contractor.
  • Original detailed explanation supplied by Whirlpool and modified for CrowdSupport.


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