Loopback testing is a common technique to test a communication link prior to normal operation or to determine the source of transmission errors. Typically, a loopback test is used to test both the communication medium, e.g., optical fiber, and the physical layers of the communication devices at both ends of a link (i.e., the items connecting communication devices). Various amounts of the communication devices can be tested based upon where the loopback traffic is initiated in a sending device (i.e., the test initiating device, or initiator) and where the traffic is sent back in the receiving device (i.e., the device in which the test traffic is looped; the loopback device).
Typically, loopback testing is performed by manually initiating a tester to send a test signal to the loopback device that sends loopback traffic back to the initiating device. The initiating device receives the returned loopback traffic and compares the returned loopback traffic with the transmitted traffic to determine the working condition of the communication link and/or parts of the initiating device and/or loopback device. For such tests, the loopback device is an active device able to receive control commands to enter a loopback test mode. In loopback test mode, normal data transmission and reception is disabled. If the loopback device is nonresponsive (e.g., due to a cut fiber connection, a device failure, or a loss of power), then no loopback test can be performed to check the link between the initiating device and the loopback device. If the loopback device is an unattended remote device or is located on a customer's premises, then a technician is sent to check the loopback device and the link between the initiating and loopback devices to determine if the problem is due to a link failure, or something else. Sending a technician to diagnose problems costs money and time.
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