Fiber Latency
Convert the distance of a fiber run into propagation delay: one-way and round-trip (RTT) in µs and ms. Light in fiber travels at c/n (≈4.90 µs/km in single-mode), slower than in vacuum. Everything in your browser.
The delay in all units
| Magnitude | one-way | round-trip |
|---|
Fiber vs vacuum (radio / line of sight)
| Medium | n | µs/km | one-way |
|---|
In vacuum light travels at c (≈3.336 µs/km);
in fiber it drops to c/n. The penalty is the factor n.
How it is calculated · speed of light and index
1. In vacuum light travels at
c = 299,792.458 km/s, that is
≈3.336 µs/km (1e6 / c).
2. In a medium with refractive index
n the velocity drops to v = c / n. Single-mode
fiber has n ≈ 1.4682.
3. One-way delay:
t = distance · n / c. In µs:
distance · n / c · 1e6 ≈ 4.90 µs/km for
n = 1.4682.
4. The round-trip (RTT) is there and back:
RTT = 2 · one-way. It is the link's minimum physical
latency; it does not include processing, queuing or serialization.
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