Fiber Optic Latency and Propagation Delay Calculator

How to use the calculator

Enter the cable route length, the fiber group index and the latency of the equipment at each end. The result shows delay per kilometre, one-way fiber delay, round-trip time and the longest route that fits your round-trip budget. Use real route length from your as-built records because cables are longer than the straight line between sites by 20 to 50 percent in many metro and long-haul routes.

• Group index differs from the refractive index slightly; use the group figure
• Equipment latency of coherent optics can be tens of microseconds
• Dispersion compensating fiber adds its own length of delay

Worked example

For 80 km of fiber at a group index of 1.4682, one-way delay is 391.8 microseconds. With 20 microseconds of equipment latency at each end the round trip is 2 x 391.8 + 4 x 20 = 863.6 microseconds, or 0.864 ms. A 1 ms budget therefore supports a route of about 94 km before the budget is used, which is the kind of figure used when checking synchronous replication or high-frequency trading link limits.