Insertion loss tells you how much signal arrives at the receiver; return loss tells you how much signal bounces backward toward the transmitter. They represent distinct aspects of signal transmission and differ for both media types. Here we explain the key differences between these two parameters, why. In the test report for a fiber cable, you may often see some data related to fiber insertion loss (IL) and return loss (RL), but do you know what insertion loss and return loss actually mean? How do the values of IL and RL impact the quality of the fiber cable? Are higher values better, or lower. Insertion Loss (IL) is the amount of optical power lost as the signal travels from one point to another in a fiber optic link, usually across connectors or splices. Formula for. Ever connected a fiber optic cable only to find your signal dropping like a bad cell call in a basement? You're not alone—poor fiber performance metrics like insertion loss and return loss plague even seasoned network pros, costing time, money, and sanity.