What is the difference between link speed and propagation speed of a network link?

Question

In one assignment, the direct link transmission rate is given as 20 Mbps and the propagation speed over the link is 25x 10^7 meter/sec. This link connects host A to B.

How is there a difference between the link speed and propagation speed? How are they calculated?

Answer 1

Link Transmission Rate and propagation speed are measuring different things.

Link Transmission Rate is the amount of information that can be pushed through an interface at one time, and measures the volume of data that can be moved in a period of time. (If looking at an Internet connection, think Advertised speed of the connection)

Propagation speed is the speed with which the information travels (if looking at an Internet connection think ping time between the 2 points). Propagation speed would be the influenced by the speed at which the signals can be switched and the distance between the switches - for example it takes at least 60ms from a signal to travel from LA to New Zealand due to the speed of light (pings are > 120ms, because a ping goes there and back again)

Larger Link Transmission Rates can increase propagation speed by allowing data to not queue up at the exiting interface.

Answer 2

Concepts:

Link transmission rate (Data Transmission speed) is the number of bits per second that can be transmitted.

Propagation speed is the speed at which a bit (signal) moves through a medium. It generally is within the range of 2x10^8m/s to less than the speed of light (3x10^8m/s).

So... Data transmission rate depends on the switch/router that has to push the packet bit-wise onto the link, while the speed at which the bit travels is called the propagation speed.

The time taken by the bit to propagate/travel from one router to the next is called propagation delay and can be calculated by:

(Length of the medium) / (Propagation speed over the link) 
=  Time spent in propagating from end to end.