OSI Stands for open system interconnection model.  This is reference or virtual model, you cannot see it , touch it ,but its exists conceptually just like electron, protons etc.

OSI model is invented by ISO(International standard organization). ISO is non profit organization that defines several  standards. It is just like other standard e.g ISI mark, IEEE, CE etc that you must have come across.These standard work for the welfare of society for setting different standard in different areas like food , electronic networks etc.

OSI model have seven layer describe as below.

1)Physical layer

2)Data link layer

3)Network layer

4)Transport layer

5)Session layer

6)Presentation layer

7)Application layer

OSI is one of the most important reference model in networking , if you want to learn networking you must be aware about this reference model.

At each layer there are some sets of task performed and header attached  and forwarded to next layer. Data sending always start from Application layer whereas data receiving always start at physical layer.

So If someone ask the layers of OSI model start from physical layer, but if someone ask OSI layer from sender point of view always start from Application layer

As I told you every layer  perform its task and whatever task perform by that layer it attach header to data defining the task that layer has done with data. Header is nothing but the information or task detail done by each layer.

You can consider every layer as a technician who is contributing to make a Car.Every Technician will perform some sort of task , e.g someone assemble the parts and attach header , that he assembled the parts, someone paint and attached header that he painted the car, someone drive a test drive and attach his header that he has tested the car drive. At final steps before delivering the car to showroom , all the headers will be removed one by one and at the end you will get the actual car or say data in terms of OSI model.

To understand the working of OSI model , we need to understand the OSI layers functions. So Let’s start the OSI model layer step by step.

Application Layer.

Application layer is responsible for application related dealing like, word, excel ,paint ,websites, DOS etc these are all application that you create for sending it from one machine to another.It is also known as user interaction layer.

At application layer you prepare one application and sent it to other computer. There are some protocols used for sending , sharing that application.

Protocols at application layer are Telnet, HTTP, HTTPS, SMTP, POP etc.

Here at application layer sender has created one data file with name CAT, application layer attach its header , H7 and send the data to next layer i.e presentation layer.

Presentation layer.

Presentation layer deal with syntax and semantic of data and it encapsulate the data at sender end.

At presentation layer we encapsulate the data into some other format so that no one can see my data. Presentation layer convert your data into some other encrypted format e.g ASCII code, Binary code, EBCDIC code etc. You can consider Winrar , Winzip etc which encrypt your data.

Finally after encapsulation  presentation layer attached one header to data. Let’s call data header as H6. After attaching header presentation layer send the data to next layer for processing ,i.e Session layer.

Here presentation layer has encapsulation data CAT into some other format as @$# that you can see in figure above.

Session layer.

Its is basically responsible for two things , Dialog control & synchronization.

  • Dialog Control

Dialog control the name itself define that this features allows the system to have dialogs or communication with each other. But how the dialog will be whether it will simplx, half duplex , or full duplex. Dialog control also called as Transmission mode which is of three types viz simplex, half duplex or full duplex.

Simplex: In simplex mode only one machine can send and other can only listen. Example is Radio or Television where Radio and television is always the transmitter and we are the receiver.

Half Duplex: In this mode both the machine can send/receive but not at the same time .Example Walkie-talkies , Alpha calling beta in movie border you must be remember.

Full duplex: In this mode both the machine can send and receive at the same time. Example Mobile phone.

  • Synchronization

Session layer is also responsible for adding checkpoints or synchronization point/bits to data. This is done to minimize the loose of sending whole data incase if session reset.

Example Imagine if a system is sending data of let’s say 1000 pages, it is good to send the data in bunch of 100 by adding checkpoint for every 100 pages , so that if system reset at 910 only the last 100 pages need to be send again , while the remaining 900 is already being sent.

Session layer is responsible for Creating session , Checkpoints and synchronization. Session layer also attached the header to the data. Let’s call the header as H5.

Transport layer.

The data or PDU at transport layer is known as segments.

Transport layer is one of the most import layer in reference model and its provides many services.

  • Service-point or port or socket addressing

Service-point or port or socket addressing is one and the same thing , one of the primary responsibility of transport layer is of program delivery from one computer to another. To identify the program transport layer adds port information to the program.

For example if your data is for web service , transport layer may add port 80 or 443 , if the data is for telnet it will add port 23 and so on.

  • Segmentation and reassemble

Segmentation is nothing but a processes of diving a data into smaller parts called chunks. After segmenting the data into chunks it number the checks called reassembly.

The same chunks will combine together and desegment at receiver transport layer.

  • Connection Control

The Transport layer support connection control by using either connection oriented or connection less communication. Two protocols are used viz , TCP & UDP for connection oriented services & connection less services respectively.

In connection oriented user will get receipt when the data is delivered or fail. While in connection less user will not get any receipt when data delivered or fail.

  • Reliability/Unreliability

Transport layer possess two protocol viz TCP & UDP for reliable and unreliable communication

TCP is reliable as its send ACK for every communication. UDP is unreliable as it don not send ACK for any Communication. TCP is slower whereas UDP is faster.

  • Flow control

Transport layer is also responsible for flow control, flow control is imposes when the user data absorb capability is less that the user data sending capability. In this case transport layer impose flow control and send small amount of data as the receiver capacity is low.

  • Error control

Transport layer is also responsible for error control. Transport layer make sure that the data send to receiver is error free( loss , damage, duplication).If there is any error transport layer retransmit the data again to receiver.

Here transport layer has divided data @#$ into three parts known as chunks and number the data and attached its header H4 information. Transport layer will further pass this data to its next layer i.e Network layer for processing.

Network Layer.

The PDU or data at network layer is known as packet or IP packet. The Network layer is responsible for two thing as below.

  • Logical addressing

One of the responsibility of network layer is to add the local address to the data. The logical addressing mean the Ip address information to data so that data can travel or send to other computer or networks.

  • Routing

Routing is a process of where two different network communicate with each other.


Here Network layer will add IP detail in each chunks of transport layer , now known as ip packet as network layer. Finally network layer attached its header H3 information and send the data to data link layer for processing.

Data Link Layer.

The PDU or Data at Data link layer is known as Frame. Following are the responsibility of data link layer.

  • Framing

The data link layer divides the data received from network layer into frame by add physical addressing to the data.

  • Physical addressing

Data link is also responsible for adding physical address information to the data. Physical address is a mac address of a system which is hard-coded on NIC(network interface card).

  • Flow control

Same as Transport layer it also support flow control.

  • Error control

Data link layer provide error control for lost or damaged frame to physical layer.

Here data link will add physical addressing know as mac address to the packet received from network layer. It will add its header H2 and Trailer T2 and send the data further.

Note: Data link will add header as well as Trailer to the data , Trailer denotes termination of data processing , means the data is ready to send and no more processing required.

Physical layer.

Physical layer is responsible for the following.

  • Physical characteristic of interface and media

Physical layer defines the characteristics of the interface between the devices and transmission medium. It defines the types of transmission medium(e.g wired , wireless)

  • Representation of bits

The physical layer converts the data bits ( 0 or 1) into stream of signals to be transmitted.

  • Line configuration

Line configuration defines how the devices are connected to the media. Line configuration is of two type viz point to point or multipoint.

  • Physical topology

One of the responsibility of physical layer is to define the topology of the networks. The various network typologies are start, tree, mesh, hybrid etc

At the receiving end data first interact with physical layer as shown in figure.At receiving end each layer will process data, strip its own header and send it further to upper layer. Finally at application layer you will get the actual data that was send.

Layers Protocols Devices used PDU Responsibilities
Application HTTP,HTTPS,TELNET,SMTP,POP,etc Users, Computers etc DATA Provides services to users
Presentation EBCDIC, ASCII,ANSI etc No Specific devices DATA Syntax and symantic
Session NETBIOS No Specific devices DATA Dialog control and Checkpoints
Transport TCP,UDP,SCTP Layer 4 Switch Segment Segmentation/Reassembly,Error control,Flow control
Network IP,IPX,ARP,RARP,APPLETALK etc Router,Layer 3 SW,Firewall etc IP packet Logical addressing, Routing
Data HDLC,SDLC,ATM,FRAME-RELAY Layer 2 SW,ATM SW etc Frame Physical addressing, Error control, Flow control
Physical No Specific Protocols Cable and connectors DATA Line configuration, Topology,Physical characterstic of interfaceasand media


mnemonic phrases to remember OSI layer.

Please Do Not Throw Salami Pizza Away

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