CCNA English-02: OSI Model

What is the OSI model?

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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 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 source to destination. These rules are called protocols. And the model that has been created by combining these protocols is called the OSI model. This model is determined by ISO.

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

Applications
Presentation
Sessions
Transport
Network
Datalink
Physical

7. Application layer:

Protocol FTP TFTP Telnet DHCP DNS Pop IMAP SMTP HTTP

Port Address 20/21 69 23 7/8 53 110 143 25 60

This is the seventh layer of the OSI model. The application layer provides the user interface and processes the network data. The port addresses of some of the application layer protocols are given

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. This layer does the work of 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, to control that connection, and to 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 method, when the data flow from one side is over, the data from the other side is flowing to the other side.
Full duplex: In a full-duplex system, 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 other devices. Uses two types of transmission to transmit data to this layer:

Connection-Oriented

Before sending data to Connection-Oriented, the sender makes a connection with the customer via an acoustic signal. This is the case with TCT.

Connectionless

The sender does not make a connection with the receiver's medium before sending data to the 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 the OSI model. The function of the data link layer is to transmit data from one device to another through the physical layer without error. 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. In this layer, data is transferred bit by bit. The devices used in this layer are hubs, switches, etc.

Let's talk briefly from the bottom layer to the top layer,

In the physical layer, the signals are transferred between the cables in the form of bits. These bits are converted into frames in the Datalink layer, and since the frames pass through the router, the frames are converted into packets. Now let's see how these packets go in the connection-oriented state or connectionless state this decision is taken by the transport layer. After deciding the transport layer, the session layer decides in which mode the segments will be transferred (simplex, half-duplex, full-duplex). The presentation layer then determines the format in which these data will be presented (.jpg, .mpeg, etc). Finally, the application layer creates an interface with the user.

Many times the question arises of how many layers of the application layer or network layer OSI model can easily remember this sentence.

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