Time-domain reflectometry

Time-domain reflectometry (TDR) is like a game of sound ping-pong. We start off with a special machine that creates sound waves by sending a pulse of energy (like electricity) down a wire. When the pulse of energy hits something on the wire, it bounces back (like an echo). The special machine then listens or 'hears" the echo coming back. By measuring how long it takes for the echo to come back and how loud the echo was, we can figure out how far away the thing that it hit was and how long the wire was. This is useful for finding problems in long wires like telephone and cable lines.