1.1 Layered Tasks:
Consider two friends who communicate through mail
Fig (i): Tasks involved in sending a letter
In the above fig (i), we have a sender, a receiver & a carrier that transports the letter. There is a
hierarchy of tasks.
At the sender site:
The sender writes the letter, inserts the letter in an envelope, writes the sender & receiver
addresses, and drops the letter in a mail box.
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 recipients 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 & delivered to the recipients mailbox.
Higher Layer: The receiver picks up the letter, opens the envelope, and reads it.