OSI Model layers functions

What you will learn here about OSI model layers:

  • Why we use OSI model
  • OSI model Layers
  • Core Functions of Application layer
  • Core Functions of Presentation layer
  • Core Functions of Session layer
  • Core Functions of Transport layer
  • Core Functions of Network layer
  • Core Functions of Data link layer
  • Core Functions of Physical layer
  • OSI model protocols

OSI model stands for Open System Interconnection model. OSI model is a reference model and used for communicating with systems that are open for communication with other systems.

Why we use OSI model?

In old days what use to happen, if you have a laptop of X company, then you need to buy peripherals of the same company. For example if you buy a laptop or computer from the X company then you need to buy printer of the same company. This is because packet structure or data structure was known to that particular company only. Since each company has it’s own packet structure or data structure, peripherals of different companies were not working with one other. So to avoid that International Organization for Standardization (ISO) decided that data has to go in particular manner from one place to another place so others can understand. This is where reference model i.e. OSI model came in picture. ISO decided that, when data goes from one place to another place it has to go through the layers of the reference model so that others can also understand it. That’s why we use OSI model.

OSI model layers

OSI model has seven layers and each layer is responsible for performing different operations. The OSI model layers is shown below.

osi model

OSI Model explanation

OSI model layers function is given below.

APPLICATION LAYER:

The application layer is the last layer of the OSI model. This layer works at the client or user side which is shown below. This layer is available in the form of software on a laptop, computer, mobile, etc.

Functions of Application layer

The core functions of Application layer is given below.
1)The application layer is responsible for receiving webpage request from user or client using the most widely used protocol namely HTTP protocol and displaying response back in the browser.

2)It is also responsible for sending data or files from one place to another place.

3)It is used to send Emails

4)It is responsible for converting a domain name i.e website name into IP address i.e DNS.

5)Application layer uses HTTP, HTTPS, DNS, FTP, SMTP etc protocols

Functions of Application layer

Example of Application layer

Example of Application Layer receiving web page request and displaying response back

PRESENTATION LAYER

The presentation layer is the middle layer between the Application layer and the Session layer which is shown below. It receives data from the application layer.

Functions of Presentation layer

The core functions of Presentation layer is given below.
1)We all know in computer or laptop or mobile, data or information is stored in the form of binary format. Each computer or laptop or mobile manufacturer uses different character coding technique (i.e ASCII or EBCDIC etc) to represent the character in the form of binary format. Means on each computer or laptop or mobile, representation of character i.e data representation depends on which character coding technique manufacturer is using. The job of the Presentation layer is to convert or represent data in common format that all types of computers or laptops or mobiles etc can understand. In short, the presentation layer acts as a data translator. Presentation Layer is also called a syntax layer.

2)The presentation layer is also responsible for data encryption or decryption.

3)It is also responsible for data compression or decompression.

4)The presentation layer uses ASCII, EBCDIC, etc as Coding techniques.

Functions of presentation layer

SESSION LAYER

The session layer is the middle layer between the Presentation layer and the Transport layer which is shown below.

Functions of Session layer

The core functions of Session layer is given below.
1)The session layer is responsible for creating a session between client and server. Session means any activity performed within start time and stop time is called session.

2)In the OSI model, the Session layer is responsible for establishing a connection, managing the connection and terminating the connection between the client and the server. In the TCP/IP model, this functionality is provided by Transport Layer.

3)The session layer is also responsible dialog control means for keeping the track of whose turn it is to transmit.

4)It is also responsible for token management i.e preventing two parties from attempting the same critical operation simultaneously.

A practical example of a session:

Below pratical example of session is given.

On Facebook, we can not see or access our profile or other activities directly without sign in on Facebook. So whenever we make a sign in request on Facebook one session gets created and whenever we make a sign out request session gets expired.

Functions of session layer




TRANSPORT LAYER

The transport layer is the middle layer between the Session layer and the Network layer. It receives data or information from the Session Layer. If we send entire data or information together and if we lose that data while sending then we will lose the entire data or information. so it is better to divide that data or information into small parts or units and then send it. So even we lose one to two parts of data while sending and the rest parts will be delivered. This job of dividing data or information into small parts or units is done by Transport Layer. This process of dividing data or information into small parts or units is called Segmentation.

Functions of Transport layer

The core functions of Transport layer is given below.

1)Transport layer does segmentation.

2)Transport layer uses TCP and UDP protocols. When transport layer uses the TCP protocol, it gives the gaurantee of transmission. When transport layer uses the UDP protocol, it does not give the gaurantee of transmission.

3)Transport layer is also responsible for retransmission of data, if data transmission is fail when transport layer uses TCP protocol.

4)Transport layer sends acknowledment for successful and unsuccessful message delivery.

5)Transport layer uses TCP, UDP, SCTP, DCCP, etc protocols.

NETWORK LAYER

The Network layer is the middle layer between the Transport layer and the Data link layer.

Functions of Network layer

The core functions of Network layer is given below.
1)Network layer converts received segment into packets.

2)Network layer Adds Source and destination IP address to each packet.

3)Responsible for routing of packets.

4)Handles network traffic i.e network conjunction.

5)Network layer adds sequence number to each packet.

6)Network layer uses IP, ICMP, IGMP, IPsec.

Functions of network layer

DATA LINK LAYER

The Data link layer is the middle layer between the Network layer and the Physical layer. It receives the Packets from the Network layer and converts them into frames by padding error correction bits.

The data link layer is divided into two sublayers:

1)Media access control: Responsible for controlling how devices in a network gain access to a medium

2)Logical link control: Responsible error checking and frame synchronization

Functions of Data link layer

The core functions of Data link layer is given below.

1)Data link layer converts Packets into frames.

2)Adds physical address of sender of sender in frames.

3)Data link layer is reponsible for Error detection and correction.

4)Responsible for  Error control.

5)Responsible for Flow Control.

6)Data link layer uses ARP, RARP, PPP, Ethernet, WiFi, etc protocols

Functions of data link layer

PHYSICAL LAYER

It is the first layer of the OSI model. It is related to sending data bits over the communication medium or channel.

Functions of Physical layer

The core functions of Physical layer is given below.

1)The physical layer is responsible for sending data bits over the communication medium or media.

2)It is also responsible for defining data rates, voltage levels.

3)It decides the communication mode i.e simple or half duplex or full duplex.

Functions of physical layer




Practicle example of OSI model:

Below practicle example of osi model is given.

1)The user is making a request from a browser which is shown below.

2)Whenever a user makes a request from a browser it goes through the Application layer to the Physical layer of the OSI model which is shown below.

Example of osi model

3)The response received by a user is shown below.

4)Whenever a user receives a response, it comes from the Physical layer to the Application layer which is shown below.

OSI model protocols

The OSI model protocols list is given below.
OSI model Protocols

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