Transmission and Propagation Delay

In page Processing and Queuing Delay, you have learnt that delays can happen inside a router. After the packets finish queuing to be transmitted to the link, packets still experience delays. Delays can happen when the packets are being transmitted or propagated. In this page, you will learn about transmission delay, propagation delay, comparison between both delays, and their examples.

Transmission Delay

Transmission delay is a delay that occurs when some packets are waiting for other packets that have arrived before them to be completely transmitted. It can be estimated if the length of the packet and the transmission rate of the link between two routers are known. If you have a packet with length L bits and transmission rate from router A to router B by R bits/s, the transmission delay is L/R. For example, if a packets with length by 1 Mb is transmitted using a 10 Mbps ethernet link, the transmission delay is 0.1 s. Transmission delay is the amount of time to transmit all parts of the packets into the link. According to Kurose & Ross (2017), transmission delay normally happens on the order of microseconds to miliseconds.

Propagation delay is the amount of time it takes for the head of the signal to travel from the sender to the receiver. This of course depends on the link length and the wave propagation speed over a specific medium.

The propagation delay through the network can be calculated by dividing the maximum length (d) by the speed (s).

In wireless communication, the speed is the speed of light (c), just below 300 million meters per second. In copper wire, it generally ranges from .59c (176,877,550 m/s) to .77c (230,840,193 m/s).

For 10BaseT wisted pair ethernet cables, which has a propagation delay of .59c (176,877,550 m/s). 176,877,550 m/s divided by 10 million bits per second is 17.7 meters, which is the size of a single bit on a 10BaseT network.

For 10Base2 thin coax network ethernet cables, this is 185 meters divided by 194,865,098 m/s, or about 949 nanoseconds. If the actual propagation delay from one end of the network to the other is greater than 950 nanoseconds, late collisions may occur.

Reference:
Kurose, J and Ross, K. (2017). Computer Networking: A Top-down Approach (7th ed). Pearson.