A technique used to isolate faults in the digital circuitry of a transmission system.

Technique for testing the digital processing circuitry of a communications device. It can be initiated locally, or remotely, via a telecommunications circuit; the device being tested will echo back a received test message, after first decoding and then reencoding it, the results of which are compared with the original message. Compare with Analog loopback.

A loopback in either direction that is associated with the DTE port of a DCE.