What is Packet Switching?
The applications, like Google Chrome, Skype, Outlook, that need the Internet to function are called network applications. In a network application, the end systems or hosts need to send data to each other over the Internet. The data can be an image, audio, video, or text file. Instead of sending the whole file at once, it is divided into small data units at the source, say 1 MB, and are called packets. These packets, then, move over a network of packet switches to reach the destination host. Packet switches can be routers or link-layer switches. This method of data transmission over a network of packet switches in the form of small data units or packets is called packet switching.
What is Store-and-Forward Transmission?
Consider two hosts connected to each other via a network router. Host A wants to send an image to host B. So host A divides the image file into three packets each having L-bits and starts sending a packet at a rate of R bits/sec. Therefore, the total time required to transmit L-bits or one packet is L/R seconds. So at the time, less than L/R second, the router has received only a portion of the first packet P1. By this time, the router cannot forward the received bits of packet P1 to host B. It first needs to store all bits of the packet before it can begin the packet transmission. It is called Store-and-Forward transmission.
What is Propagation Delay?
Let us assume that host A sends a bit to the router at time, t=0 sec. It reaches the router after 3 ms. So, the delay in receiving the bits at the router is 3 ms. It is called propagation delay.