Also see loopback test.

In telephone systems, a loopback is a test signal sent to a network destination that is returned as received to the originator. The returned signal may help diagnose a problem. Sending a loopback test to each telephone system piece of equipment in succession, one at a time, is a technique for isolating a problem. (The loopback can be compared to the Internet's ping utility, which lets you send a message out to a host computer on the Internet. The ping echo tells you whether or not the host computer is available and the time the signal took to return.)

If you are an ISDN user with more than one B-channel, you can do a loopback test from your computer.

This was last updated in August 2006

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