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CCNA Introduction - Lecture 2 - OSI Model

 What is OSI model?

OSI Model
OSI Model 

The main purpose of one computer communicating with another computer is to share information.

Suppose two computers are located in different places and these two computers want to exchange

information. Then when a computer sends the data, the data will reach the destination computer

through many media. There are some rules to ensure that the data can reach the destination without

any problem while moving from the source to the destination. These rules are called protocols. And the

model that has been created by combining these protocols is called OSI model. This model is

determined by ISO.

The OSI model is divided into seven layers. The levels are:








. Application layer:

This is the seventh layer of the OSI model. The application layer provides the user interface and

processes the network data. The functions that the application layer does are resource sharing, remote

file access, directory service, etc. The port addresses of some of the application layer protocols are given


Port Address 20/21 69 23 67/68 53 110 143 25 80

Try to remember the port numbers. Because CCNA tests usually have such questions, such as what is the

port number of HTTP?

6. Presentation layer:

This layer acts as a data translator for network services. The functions that this layer does are data

conversion, data compression, decryption, etc. The data formats used in this layer are .jpg, .mpeg etc.

5. Session Layer:

The function of the session layer is to establish a connection between the source and the destination

device, control that connection, and disconnect at the end as needed. 3 types of controls are used to

send data.

Simplex: In Simplex, data flows in one direction.

Half Duplex: In the Half Duplex system, when the data flow on one side ends, the data on the other

side flows.

Full duplex: In full duplex method data can flow in both directions simultaneously.

4. Transport layer:

The fourth layer of the OSI model is the transport layer. The function of this layer is to ensure that the

data received from the session layer is reliably transmitted to another device. Two types of transmission

are used to reach this layer of data:

Connection Oriented

Before sending data to Connection Oriented, the sender makes a medium connection with the

subscriber signal. This happens in the case of TCT.


The sender does not make a medium connection of the signal with the receiver before sending the data

in connectionless oriented. This happens in the case of UDP.

3. Network layer:

The job of the network layer is addressing and packet delivery. This layer adds the network address to

the data packet through encapsulation. In this layer the router is used and the routing table is created.

2. Datalink layer:

This is the 2nd layer of OSI model. The function of the datalink layer is to transmit data from one device

to another through the physical layer without any errors. This layer creates a logical link between the

two devices. This layer converts the data into frames.

1. Physical layer:

The lowest layer of the OSI model is the physical layer. This layer determines in what way the signal will

be transmitted from one device to another, what will be the electric signage or data bit format, etc. Data

is transferred bit by bit in this layer. The devices used in this layer are hubs, switches, etc.

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