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.)
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