9 µs Rule: Standard telecom fiber (SMF-28) introduces approximately 4. 9 microseconds of latency per kilometer of distance. Index defines speed: The higher the refractive index (n) of the fiber core, the slower the optical signal travels. In free space, light travels at 299,792,458 meters per second. It measures both one-way latency and round-trip time (RTT), factoring in the speed of light in fiber and delays from network equipment such as routers and switches. Once the true velocity (v). Consider a cable 100 km long with an optical fiber refractive index of 1. This tool calculates theoretical minimum. Fiber optic cables revolutionized global communications, enabling high-speed data transfer over long distances with minimal signal loss.