The launch cable is added in front of tested fiber link and the normal length is more than 100 meters. When the tested fiber link is short,it may be within OTDR deadzone or test results are not stable. At this moment,
if using the launch cable, it will increase the link length and decrease the fluctuation and ensure test accuracy.
When using launch cable, the beginning point could be the end point of launch cable. In this way,OTDR could test insertion loss at 0m.