Loopback is a testing procedure in telecommunications in which a test signal is sent from a service provider’s central office (CO) to the customer premises and is returned or echoed by the customer premises equipment (CPE) back to the service provider.

Loopback tests are used to check line integrity and the proper functioning of customer premises equipment and to diagnose and troubleshoot communication problems. Loopback tests can be performed by wide area network (WAN) access devices such as CSU/DSUs (Channel Service Unit/Data Service Units) and routers to place calls to themselves over a WAN to test the integrity of the WAN link.

If the loopback signal fails to return, the WAN link is down and must be repaired. If the loopback signal returns, the device compares the original signal with the returned one; any discrepancies found can be used to troubleshoot communication problems.


For example, if you are a subscriber using an Integrated Services Digital Network (ISDN) line to connect to your service provider, you can usually perform a loopback test yourself. If the Service Profile Identifiers (SPIDs) and ISDN directory numbers have been configured for your ISDN interface, a loopback test will determine

  • Whether you can connect with your provider’s ISDN exchange. If not, you might have a cable or interface problem.
  • Whether your ISDN numbers are correctly assigned.
  • Whether you have caller ID or other advanced ISDN services on your line.

Another type of loopback test is the local loopback test, which a WAN access device uses to test networking connectivity with locally attached network devices. You can also implement a local loopback test by having network application software place a call to the WAN access equipment and having the equipment return an echo to the application.

See also:


Articles posted after being checked by editors.

Recent Posts