OSI Model and Protocols!

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OSI model (Open Systems Interconnection Model )is a conceptual framework used to describe the functions of a networking system.

It is an Open System Interconnection conceptual model created by International Organization for standard which enables for working using communication protocols. If you don’t understand about protocol I had to explain my previous blog as the HTTP protocol.

The OSI model having seven layers. These layers are a target for doing several functions used with protocols. Each layer of the OSI Model handles a specific job and communicates with the layers above and below itself.

In the easiest way to understand the OSI model is when you are sending information/ message via data packets to another computer, these data packets are created and go through several layers and work with a special function of each layer to make changes to send another computer with the use of the internet.

Example of data packet send through each layer

Now we have to understand the OSI model layers. There are seven main layers in the model. Each of the layers is doing several jobs.

Seven layers:
Application Layer
Presentation layer
Session Layer
Transport layer
Network Layer
Data Link Layer
Physical layer

OSI model layers

Application Layer

This is the 7th layer / End-user layer in the OSI model. This is the only layer that directly interacts with data from the user. It receives information directly from users and displays incoming data to the user.

There are several functionalities handle in this layer. Those are,
● This layer provides various e-mail services(Email forwarding and storing).
● Receiving and handling webpage requests from users.
● Sending data and files.
● Allows users to log on to a remote host

in order to do these functionalities, there are several protocols are using to do such things. HTTP, HTTPS, SMTP, DNS are mostly using protocols but HTTP( HyperText Transfer Protocol) is the most widely used protocol.

Presentation Layer

This is a 6th layer on the OSI model. This is also called the Syntax layer. This layer ensures whether the data is in the usable format and is where data encryption occurs. The layer “presents” data for the application or the network. Example: encryption and decryption of data for secure transmission.

The main responsibility of this layer is to prepare data so it can be used for the application layer. The presentation layer is responsible for the translation, encryption, and compression of data.

It ensures that the information sent by an application layer of one system is readable by the application layer of a system. File Transfer Protocol is most used here.

Functions of the presentation layer are:
● Character code translation from ASCII to EBCDIC.
● Data compression: This allows to reduce the number of bits that need to be transmitted on the network.
● Data encryption: Helps you to encrypt data for security purposes. example, password encryption.
● Provides a user interface and support for services like email and file transfer.

Session Layer

This is the 5th layer of the OSI model. This is mainly to control the dialogues between computers which means that opening-closing communication between computers. This layer synchronized data with the checkpoint. For example, if a 100-megabyte file is being transferred, the session layer could set a checkpoint every 5 megabytes.

It provides a mechanism for managing sessions for example establish, maintain, and synchronize.

The functions are in the session layer is:
● It establishes, maintains, and ends a session.
● It enables two systems to enter into a dialog.
● It also allows a process to add a checkpoint to the stream of data.

Transport layer

This is the 4th layer of the OSI model. The responsible for this layer is end-to-end communication between two devices. For that data taking into the session layer to make it into chunks(packets). Segmentation is data or information is divided into small parts and then send it. This is also responsible for flow control and error control. The transport layer uses protocols such as TCP, UDP, DCCP, etc

TCP- Transmission Control Protocol

● Connection-oriented(exchange message between devices).
● Slower than UDP

UDP- User Datagram Protocol

● Connectionless protocol
● Lightweight protocol
● Faster than TCP

Functionalities in the Transport layer is:
● It divides the message received from the session layer into segments and numbers them to make a sequence.
● It also makes sure that the entire message arrives without any error else it should be retransmitted.

Network Layer

This is the 3rd layer of the OSI model. Responsible for facilitating data transfer between two different networks. The network layer breaks up segments from the transport layer into smaller units, called packets, on the sender’s device, and reassembling these packets on the receiving device.The network layer may use protocols like DDP, IPsec, IPX, etc

Responsibilities for the Network layer is:
● Routing protocol
● network-layer address assignment.

Data Link Layer

This is a 2nd layer of the OSI model. This layer gets the packets from the network layer and transforms them into frames by padding error connection bits. The layer helps to implement the routing of packets through a network.

It’s divided into two sublayers one is Media Access Control (MAC) layer other is the logical link control layer.

Functions of the Data link layer is:
● Adds Logical addresses of the sender and receivers
● Provide a mechanism to transmit data over independent networks which are linked together.

Physical Layer

This is the 1st layer of the OSI model. this layer helps to define the electrical and physical specifications of the data connection. This is not concerned with any protocol. For that physical specifications are cables and switches.



Software Engineer at Virtusa (Pvt) Ltd.