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Osi Layers:


These are used in olden days and now it is replaced by TCP/IP model which contains 4 layers.


TCP/IP layers are used because it is flexible and it can be altered based on needs.


OSI is a reference model. It is a blueprint of how communications should take place.


This topic is not only useful for colleges, I have designed in such a way that it is useful for CCNA, N+ exams to. It is asked in maximum government exams like SBI, Indian oil, Bsnl etc.


OSI Layer Functions:


OSI Layer IllustrationOSI Layer Illustration


Application Layer:


It is communication between applications. For example consider Whatsapp. You can communicate with your friend who is using Whatsapp, because the app is designed in a way that both using the same application can communicate with each other.


If you and your friend are using Hike, the application is designed in such a way that both communicates.


Can Whatsapp user send text to an Hike user?


It is not possible because, the application is not designed in such a way.


The application layer works as an interface between actual application programs ie between skype, yahoo messenger for example.


Using browser application you can communicate with the application layer of the server.


Kindly note that your application like browser or whatsapp or hike doesn't reside on Application layer. They interface with Application layer protocols like http.


Application layer protocols:






   Used between mail servers. For example if you send an email from your Gmail to hotmail user, the gmail's server uses SMTP protocol to communicate with hotmail server so that your email is sent.

SMTP is used between servers to exchange email from one server to another server for example from Gmail to yahoo mail serverSMTP is used between servers to exchange email from one server to another server for example from Gmail to yahoo mail server



3. FTP:

   File Transfer Protocol is used to transfer files from one system to another. For ex: Uploading a website.



Presentation Layer:

    It takes care of encryption and decryption. This layer defines “how the data is presented”.


Session Layer:


The session layer manages a session by initiating the opening and closing of sessions between end-user application processes.


Connection setup and teardown occurs in this layer.


There are three different modes : simplex, half duplex (happens in hub) and full duplex (as in switches and routers).


Hub is half duplex as only one port has to transmit and other ports has to listen.


Protocols  : Netbios, Session Announcement protocol, Real time Transport protocol.


The session layer divides session into sub sessions for avoiding re transmission of entire messages by adding 'check point' features.


In video games, you have a check point. ie., when you fail at some level, the game would continue in the checkpoint last completed.


Example of a checkpoint. Similar to a checkpoint, the session layer also creates a check point.Example of a checkpoint. Similar to a checkpoint, the session layer also creates a check point.

If you want to send a 1000 pages document to another node, I have started sending, then after 106 pages, connection between two nodes failed for some reason. After connection establishes, no need to send from page 1, it will start at page 107.


This is because, the session layer creates sub sessions.


Ref :


Transport Layer:














Transport layer is responsible for taking data from one place to another.


For example if you browse, transport layer is responsible for taking data from your mobile or laptop to Cyfuture server at Noida.


Consider for example the transport like bus, it is responsible to transport you from place to place.


Protocols : TCP, UDP


Network Layer:


Network layer is responsible for taking data from one point to another in the quickest way possible.


Protocols like RIP finds the shortest way possible. This protocol is now absolete.


In current scenario, protocols like OSPF,  Intermediate system to Intermediate System finds the path which can be faster.


For example: sometimes shortcuts may be of short distance but sometimes National highway is preferred because though the distance may be long, you can reach in short time.


The same thing is done by routers. Though the route is short, it will not take the route if it is congested.


Traceroute command shows how the traffic is routedTraceroute command shows how the traffic is routed


You can see how the data flows from my modem to my website.


The route is taken in such a way that you enjoy the fast browsing experience.


Physical layer:


Physical layerPhysical layer

Physical layers may be optical fibers, microwave dishes etc where data is routed.