In practice, a typical mini OTDR has enough sensitivity to measure a 0.5 dB fibre splice at a distance of 100 km. The concept of OTDR operation is simple in that the unit shoots a pulse of laser light into the end of a fibre and then looks for reflections. The return time and the nature of the reflections characterises the fibre and locates any discontinuities along its length. Reflections from general light backscatter determine the underlying attenuation along the fibre. While the concept is simple and one-button operation produces quick results, fibre testing remains fraught with complexity.
Installations vary between local networks and long haul routes.
Source pulse width settings on a typical mini OTDR range from 10 ns to 20 ms, which in turn changes the power density of the light source and has a dramatic effect on overall performance. While OTDRs generally have no problems in identifying totally cut fibres, you'll also need to detect more subtle fibre manufacturing or installation defects. Even the refractive index of the fibre itself has an appreciable tolerance and this plays a large part in setting overall measurement accuracy. No mini OTDR attempts to handle all these variables within a single instrument. Instead, all vendors build mini OTDRs as mainframes that accept a number of plug-in modules.