Telecommunications
In telecommunications, loopback (short loop) is a hardware or software method which feeds a received signal or data back to the sender. It is used as an aid in debugging physical connection problems. As a test, many data communication devices can be configured to send specific patterns (such as all ones) on an interface and can detect the reception of this signal on the same port. This is called a loopback test and can be performed within a modem or transceiver by connecting its output to its own input. A circuit between two points in different locations may be tested by applying a test signal on the circuit in one location, and having the network device at the other location send a signal back through the circuit. If this device receives its own signal back, this proves that the circuit is functioning.
A hardware loop is a simple device that physically connects the receive channel to the transmit channel. In the case of a network termination connector such as X.21, this is typically done by simply connecting the pins together in the connector. Media such as optical fiber or coaxial cable, which have separate transmit and receive connectors, can simply be looped together with a single strand of the appropriate medium.
A modem can be configured to loop incoming signals from either the remote modem or the local terminal. This is referred to as loopback or software loop.
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