In the study of computer networks it is essential to study the way our networks work. Computer networks are operated by network models; most prominently the OSIRM and the TCP/ IP Model. This Topic gives the understanding of the OSI reference model.
CONCEPT OF LAYERED TASK
i. The main objective of a computer network is to be able to transfer the data from sender to receiver. This task can be
done by breaking it into small sub tasks, each of which are well defined.
ii. Each subtask will have its own process or processes to do and will take specific inputs and give specific outputs to the
subtask before or after it. In more technical terms we can call these sub tasks as layers.
iii. In general, every task or job can be done by dividing it into sub task or layers. Consider the example of sending a letter
where the sender is in City A and receiver is in city B
v. The process of sending letter is shown below:
v. The above figure shows
a. Sender, Receiver & Carrier
b. Hierarchy of layers
vi. At the sender site, the activities take place in the following descending order:
a. Higher Layer: The sender writes the letter along with thesender and receivers address and put it in an envelope
and drop it in the mailbox.
b. Middle Layer: The letter is picked up by the post man and delivered to the post office
c. Lower Layer: The letters at the post office are sorted and are ready to be transported through a carrier.
vii. During transition the letter may be carried by truck, plane or ship or a combination of transport modes before it reaches
the destination post office.
viii. At the Receiver site, the activities take place in the following ascending order:
a. Lower Layer: The carrier delivers the letter to the destination post office
b. Middle Layer: After sorting, the letter is delivered to the receivers mail box
c. Higher Layer: The receiver picks up the letter, opens the envelope and reads it.
ix. Hierarchy of layers: The activities in the entire task are organized into three layers. Each activity at the sender or
receiver side occurs in a particular order at the hierarchy.
x. The important and complex activities are organized into the Higher Layer and the simpler ones into middle and lower