Using the index of refraction supplied by the user, the OTDR produces a fiber span indicating the total length and attenuation associated with that length of fiber. The Optical Time Domain Reflectometer (OTDR) is useful for testing the integrity of fiber optic cables. It can verify splice loss, measure length and find faults. Later, comparisons can be made. The OTDR measures the time it takes for the light to return, which helps determine the fibre length and the loss over its span. Tier 1 testing ensures that the fiber link is within the maximum allowable loss budget for the application.