Time domain reflectometry or TDR is a cable testing technique for finding breaks or shorts in a cable.
A time domain reflectometer is a device that sends a pulse onto a cable and measures the time that it takes for the reflection to return from a short or break in the cable. (This is analogous to the use of sonar to determine the depth of a sea.)
The time interval between transmission and reception of the signal is called the signal delay; this delay can be used to determine the location of the short or break, typically within a few centimeters, even though the break might be hidden within the cable’s jacket and not visible. You can also use the reflectometer to determine the length of an undamaged cable and identify cables running through walls and false ceilings in a cabling installation.
Most high-quality cable testers can perform time domain reflectometry (TDR) tests in addition to their other functions. Time domain reflectometers are available for testing both copper cabling and fiber-optic cabling.