let us consider two friends who communicate through postal maiL The process of sending a letter to a friend would be complex if there were no services available from the post office.Below the figure shows the steps in this task.
Tasks involved in sending a letter:-
Sender, Receiver, and Carrier :- In Figure we have a sender, a receiver, and a carrier that transports the letter. There is a hierarchy of tasks.
At the Sellder Site Let us first describe, in order, the activities that take place at the sender site.
Higher layer: The sender writes the letter, inserts the letter in an envelope, writes the sender and receiver addresses, and drops the letter in a mailbox.
Middle layer:- The letter is picked up by a letter carrier and delivered to the post office.
Lower layer:- The letter is sorted at the post office; a carrier transports the letter.
on the Way:- The letter is then on its way to the recipient. On the way to the recipient's local post office, the letter may actually go through a central office. In addition, it may be transported by truck, train, airplane, boat, or a combination of these.
At the Receiver Site
Lower layer:- The carrier transports the letter to the post office.
Middle layer:- The letter is sorted and delivered to the recipient's mailbox.
Higher layer:- The receiver picks up the letter, opens the envelope, and reads it.