What are transmission and propagation delay?

Network delay is defined as how much time will take a bit of data to travel from one node to another node.

Network delay can be divided into following ways −

  • Transmission delay
  • Propagation delay

Let us learn about both of them in detail.

Transmission delay

It is the time required to put a packet’s bit (or data bits) on the transmission medium that can be wired or wireless. Transmission delay depends on the length of the packet and bandwidth of the network.

Transmission delay can be calculated as follows −

Transmission delay = Packet size / bandwidth
      Data line ( 1 bit per second)


We have 12,000 bits Ethernet packet being sent out on a 100mbps = 100 * 106 bps link. So the transmission delay can be calculated as − 12000/100*106 = 0.12 milliseconds


Consider, bandwidth of data line = 1 bit per second

Length of package = 10 bit

Transmission delay = 10/1= 10 seconds.

Propagation delay

It is the time required for bits to reach its destination from the start point. A propagation delay depends on the distance and propagation speed.

Consider a sender S and a receiver D, it is not necessary that the receiver receives data as soon as the sender has finished sending, therefore when the sender sends some data it reaches the receiver only after a certain amount of time and that time is called propagation delay.

   Propagation delay

Propagation delay depends on some factors which are mentioned below −

  • distance(d) between sender and receiver(if both of them are far apart then propagation delay is high)

  • speed of data line(v)

Propagation delay can be calculated as follows −

Propagation delay = distance / transmission speed


We have a copper wires and optical fibres media to travel, these media have a speed about 2/3 the speed of light (i.e. speed of light = 3 * 108 m/s so speed of media = 2 * 108 m/s). We have a single wire around 5000km i.e. 5 * 106 meters. So the propagation delay can be calculated as: 5 * 106 / 2 * 108 = 0.25 milliseconds.


Consider an optical fibre network where data needs to be carried along a distance of 2.1 km.

Here the speed is not mentioned, but we know the speed of data transfer in an optical fibre is 70% of the speed of light hence

Speed= velocity of light * 70%
Speed= ( 3*10^8 )*70%= 2.1 * 10^8
Propagation delay = distance/speed=(2.1*10^3)/(2.1*10^8)
[Note 10^3 is used to convert km to m]
Propagation delay= 10^-5 sec