It represents the total optical power lost when a fiber cable, connector, or assembly is inserted into a transmission link. Excessive insertion loss can lead to weak signals, increased bit errors, and even complete link failure. Engineers consider insertion loss a cornerstone measurement when calculating link budgets, testing fiber installations, and selecting. If an optical device is inserted into a setup, some of the optical power may be lost in the device or at optical interfaces. Some of the optical. Insertion loss is usually shortened to IL, and the unit of measurement for insertion loss is dBm.