Propagation Delay
Time for a signal to travel a distance at propagation speed.
Formula
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Example
d=3000 km, v=2×10⁸: t = 15 ms
How it works
Propagation delay is the travel time of a signal along a medium such as a cable, fibre, or air path. It comes from the basic relation speed = distance/time, rearranged to time = distance/speed, so the longer the path or the slower the medium, the longer the delay. In networking, this is the pure physical travel time and does not include queuing, processing, or transmission delays.
When to use it
Use this formula when you need to estimate how long it takes a packet, pulse, or electromagnetic wave to move from one point to another over a known distance. It is common in telecom links, satellite paths, fibre-optic cables, and any latency calculation where the travel speed is known.
Worked example, step by step
A signal travels through an optical fibre with distance d = 1200 km and propagation speed v = 2.0 × 10^8 m/s. Convert distance to metres: 1200 km = 1.2 × 10^6 m Use the formula: t = d / v Substitute values: t = (1.2 × 10^6 m) / (2.0 × 10^8 m/s) Calculate: t = 6.0 × 10^-3 s Final answer: t = 0.006 s = 6 ms
Common mistakes to avoid
- Forgetting to convert distance into metres before calculating
- Using the signal frequency instead of propagation speed
- Confusing propagation delay with total network delay, which also includes processing and queueing time
Did You Know?
Even at light speed, a signal takes about 120 ms to cross the globe — a floor no upgrade can remove.
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