What is OSI Model?
First you should understand why we need to learn OSI layer.There is a doubt in your mind I know. So don’t worry . If you read entire article I will assure that you will gain thorough understanding of the OSI layer.
If you want to communicate two or more computers with each other .The OSI layer will help computers to communicate with them. Can we see the OSI layer ??? NO, this just a framework.The Open Systems Interconnection(OSI) model describes seven layers that computer systems use to communicate over a network. Each layer has its own set of protocols
7.Application layer
Web browsers and other internet connected apps such as skype outlook are used to communicate the user to the network..what service( email ,file transfer)we will do ,it will decide in this application layer, Application layer interacts with the user directly.
Ok, i think you get confused, let's see some example, typing a “Hello” message to a friend in WhatsApp. Where do you type ? using whatspp screen. This is application layer.
6.Presentation layer
Ok let’s Imaagine.This is a small (Kutty) story.You love foreign girl.But she doesn't know your mother toungue . You have written bundle of love letters in your mother tongue . You contact your friend to submit your love letters. Friend knows your lover's mother tongue. So he translated letters to her mother tounge . You gave your friend a bundle of letters. So He decided to put all of the letters in a small box.When submitting letters, anyone can read them, such as the postman or her mother, so your friend creates a language so that only your lover can read the letter.
Your friend is the Presentation layer. Your lover is computer which you want to communicate.Computer only knows binary. Your friend is making language is called encryption , Putting all letters in a small box, that process is compressing.
This message reaches the presentation layer. Your messages are encoded, encrypted, and compressed here. if you send an image or text we cant send it original form. we send it as raw data so it can be converted to 01(Data translation ) then encryped .It means to prepare the data that are presented in the application in a format it understands.Are you confused ok simply say the Presentation layer presents data for the network
5.Session layer
After completing the presentation layer ,data will move to the session layer. This layer determines how long a system will wait for another application to respond. Ok, let's look at some example . If you want to see the bank balance , you will go to netbank and login and check the balance. If you are not doing anything after login,the session will be automatically terminated, So this layer helps to setup coordination and termination between the application at each of end of the session.
4.Transport layer
The transport layer is the hero of OSI model. After the session is estabilished the data passes to the transport layer.The layer is divided into three sections.1.Segments 2.flow control 3. Error control . and also 2 protocols such as TCP and UDP.In TCP, You can get acknowledgment when you send “hello” message to your friend. but in UDP protocol, you cant get acknowledgment.In this layer, the data is converted into small segments. In the segment, you can notice port number. Itdepends on services. For example when we communicate webserver (HTTP) port number is 80.
You download something from the server. However, you can download 1GBps but in your phone 100MBps is acceptable. This speed control is managed by transport layer.So this layer coordinates how much data to send , how fast it should be sent, where it should go etc
Error control means checking whether the data was received or not.If they dont send then it will send request to send again.This is also managed by Transport layer.
Further more to read click this article
https://medium.com/hacking-hunter/the-simple-way-to-osi-model-904ee373cac0
