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Activities that take place at the sender site:
- Higher layer : The sender writes the letter, inserts the letter in the envelope, writes the sender and receiver addresses, and drops the letter in the 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 and 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.
Activities 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 recipients mailbox.
- Higher layer : The receiver picks up the letter, Opens the envelope and reads it.
Analysis Based on hierarchy:
- 3 activities at sender site
- 3 activities at receiver site
The task of transporting the letter between the sender and receiver is done by the carrier.
We notice the tasks must be done in the order given in the hierarchy.
Each layer at the sending site uses the services of the layer immediately below it.
- The sender at the higher layer uses the services of the middle layer.
- The sender at the middle layer uses the services of lower layer.
- The sender at the lower layer uses the services of the carrier.
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