Fiber optic loss is calculated in two parts: cable loss and connector loss. Cable loss (dB) = cable length (km) × attenuation coefficient (dB/km). 2 dB/km for single-mode fiber at 1550nm and 0. To be able to judge whether a fiber optic cable plant is good, one does a insertion loss test with a light source and power meter and compares that to an estimate of what is a reasonable loss for that cable plant. Contractors often install, terminate, and certify cabling without knowing the client's specific requirements. Therefore. Fiber optic loss is one of the most fundamental parameters in optical network engineering, yet it is often misunderstood as a purely theoretical value used only during design calculations.