propagation delay

Shout across a canyon and there is a pause before your voice reaches the far wall — that pause is just the time sound needs to travel the distance. Propagation delay is the same idea for a signal on a wire or radio link: the time it takes for a single bit, once it has left the sender, to physically travel down the link to the receiver. It depends only on how far the signal goes and how fast it travels in the medium.

The formula is simply distance divided by propagation speed. Signals travel near the speed of light, but not at it — roughly 2 times 10^8 meters per second in copper and fiber (about two-thirds of light in vacuum), and close to 3 times 10^8 m/s through air or space. So a 3000 km link of fiber has a propagation delay of about 3,000,000 m divided by 2 times 10^8 m/s, which is about 15 ms each way. This delay does not change with how many bits you send or how fast your link is — it is set purely by geography.

This is the part of delay you cannot engineer away, and it is the reason 'just buy more bandwidth' does not fix latency. A satellite at geostationary orbit is about 36,000 km up, so even at light speed the round trip is over 250 ms no matter how fat the link — which is why such links feel laggy for interactive use. Propagation delay is one of the four delay components, alongside transmission, queuing, and processing delay; it tends to dominate on long-distance paths.