Voice and data testing instruments analyze voice, data, and video copper and fiber optic cables to ensure they are in good working order and to identify faults. Copper cable testing instruments diagnose lines and test for open, shorts, and continuity. Fiber optic cable testing instruments ensure optic signals are at full strength and identify faults in the line. Network testing instruments analyze cables and determine if they are compatible with specific networking speeds. Telephone line test sets are often carried by field technicians and test existing phone lines for signal strength and service. Test equipment accessories pair with compatible instruments to enhance testing capabilities. Time domain reflectometers (TDR) send pulses through cables and measure the signals reflected back to troubleshoot lines and diagnose problems. Tone generators and speaker probes and kits use audible signals to map out and identify cables and find the exact location of faults.