Layered Tasks

We use the concept of layers in our daily life. As an example, 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 Figure shows the steps in this task.

In Figure we have a sender, a receiver, and a carrier that transports the letter. There is a hierarchy of tasks.

At the Sender Site

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

0n 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.