# How to calculate propagation speed?

Two routers are connected via ethernet, to find the propagation delay between them, we calculate `Distance/Propagation_Speed`. How do I find the propagation speed between the two routers?

• homework? study question? ...those are off topic here. Please delete your Q if this is the case. – Craig Constantine Jan 12 '16 at 1:41

## 1 Answer

Two routers are connected via ethernet, to find the propagation delay between them, we calculate Distance/Propagation_Speed. How do I find the propagation speed between the two routers?

According to Wikipedia:

Propagation delay (Wikipedia) is equal to d / s where d is the distance and s is the wave propagation speed. In wireless communication, s=c, i.e. the speed of light. In copper wire, the speed s generally ranges from .59c to .77c.

You need to find both Distance and Speed for the electromagnetic waves for the specific transmission medium.

For instance, for fiber:

Optical cables transfer data at the speed of light in glass. This is the speed of light in vacuum divided by the refractive index of the glass used, typically around 180,000 to 200,000 km/s, resulting in 5.0 to 5.5 microseconds of latency per km. Thus the round-trip delay time for 1000 km is around 11 milliseconds.