Distance between routers
ft
Propagation speed
m/s
Propagation delay
ms

The propagation delay calculator will allow you to calculate the time it takes for a signal to travel from its source to its final destination. Propagation delay is a ratio of the propagation speed to the distance between the source and destination. In this text we will be focusing primarily on propagation delay in computer networks and the delay between two routers; however, the same propagation delay formula can apply to digital circuits and many topics in physics. The information below will explain how to calculate the propagation delay using the calculator on your left.

In the case of your at-home internet system, your download or transfer time for files may be affected more by your bandwidth and internet speed.

What is propagation delay?

Propagation delay is how long it takes a signal to travel over a network from its sender to its receiver. As computers send signals between one another, there are delays based on the distance between the two routers. The delay between a network that uses a satellite will take hundreds of milliseconds, as the signal has to travel from Earth to the satellite and back again. The delay between two computers within the same building will be much less due to the shorter distance, but the propagation speed may have a greater effect.

Propagation speed is usually close to the speed of light (2.998 x 108 m/s) when a signal is travelling through the air, such as a wireless signal or a signal traveling to a satellite. However, if the signal propagates through a cable rather than air, the speed decreases. The type of dielectric material, or the insulating material within a cable, affects the speed of propagation due to the material's impedance. The rate at which the signal propagates through that medium is known as the velocity factor. In general, a signal traveling through a cable is slowed down by as much as 2/3, as the dielectric materials in cables usually have velocity factors betwen 0.65 to 0.9.

How do you calculate propagation delay?

Now that you know what propagation delay is, let's discuss the components of the propagation delay formula and how to calculate propagation delay.

Propagation delay formula

Propagation delay = distance between routers / propagation speed

The distance between routers is the distance between the the sending and receiving router.

The propagation speed is equal to the speed of light x velocity factor

The speed of light is 2.998 x 108 m/s and the velocity factor ranges between 0-1 depending on the medium through which the signal is propagating. This calculator assumes propagation through air, which has a velocity factor of 1 and therefore has a propagation speed equal to the speed of light.

If you are calculating the propagation delay for a signal sent through a cable, you may want to use the advanced mode, found at the bottom of the calculator. Here you can find values for the velocity factors of several dielectric materials, or you can input a custom value for the velocity factor of your cable or material. If you are unsure of the type of cable you have, the best conservative estimate is to assume a standard (cat 5 and cat 6) ethernet cable made of polyethylene (PE), which has a velocity factor of 0.65.

Propagation delay calculator example

Say you are living in New York and wirelessly send an image file to a friend living in London, which is a distance of 5,567 km.

Propagation delay = distance between routers / propagation speed

Propagation delay = 5,567 km / 2.998 x 108 m/s

The propagation delay is 18.57 ms.

Let's look at a second example using the propagation delay formula where a signal sent via an ethernet cable.

Your computer is connected to a router by a polyethylene ethernet cable that is 3 m long. The advanced mode tells us that the polyethylene ethernet cable has a velocity factor of 0.65.

Propagation speed = speed of light * velocity factor

Propagation speed = 2.998 x 108 m/s * 0.65

The resulting propagation speed is 1.949 x 108 m/s.

We now need to calculate the propagation delay:

Propagation delay = 3 m / 1.949 x 108 m/s

The propagation delay is only 15.4 nanoseconds - not too shabby!

Why does propagation delay matter?

Why would I need to use a propagation delay calculator?

In the financial industry, time is money, and even a millisecond can affect the bottom line when the exact time a stock offer is made can affect whether it is accepted. In general, propagation delay matters most when sets of computers need to coordinate with one another.

In the case of most of us at home, propagation delay may affect the lag in playing a game with friends, attending a Zoom meeting, or just add to the never-ending time you spend on emails. While a delay may not cost you millions of dollars, it can be a bit annoying.

There is not much you can do about propagation delay, as distance is the primary contributing factor, but it is still important to understand it. Other technology terms you may hear related to sending data across networks - such as latency, lag, ping, and round-trip time - all incorporate the propagation delay but also take into account additional factors such as the packet size or the time it takes for the signal to return to the original sender.

Carolyn Kovacs
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