OSI Reference Model

As the name it self says it's a reference model and it is not in use. The Open Systems Interconnection (OSI) model is a reference tool for understanding data communication between any two network systems. you may have a doubt that OSI is not in use but why should we study about it. OSI model is developed in order to connect open systems together for sharing of information and form a network. But it failed practically. This OSI model was introduced by International organization of standardization in the year 1984 in order to make it a standardized network model. The network model we are using today is TCP/IP which is developed in 1970s.  Then do think there is no use of OSI reference model? No. OSI model became to be studied by many computer professionals to understand network communication in a better way and develop better digital communication products ( hardware & software). 

There are seven layers we study in OSI model. They are

1. Application layer
2. Presentation layer
3. Session layer
4. Transport layer
5. Network layer
6. Datalink layer
7. Physical layer

1. Application Layer :: ( layer 7 )

Application layer is the top most layer and it contains various protocols implemented for user interaction.

Everything at this layer is application specific. This layer provides application services for file transfer, email and other network software services.

Network protocols used in this layer are HTTP, SMTP, FTP, NFS, Telnet etc

Hardware used : Application Gateway

2. Presentation Layer :: ( layer 6 )

Presentation layer is mainly focuses on syntax and semantics of the information transmitted.

This layer at sender's end changes the information from sender dependent format into globally accepted format and at receiver's end this layer convert globally accepted format into receiver's dependent format.

This layer is responsible for protocol conversion, data encryption at sender's end and data decryption at receiver's end (This is done for securely transferring the data)

This layer also compresses the data.

The other name for this layer is syntax layer.

Protocols used in this layer are ASCII, EBCDIC, X.25 PAP

3. Session Layer :: ( layer 5)

This layer establishes, manages and terminates connections between applications.

The session layer setups, coordinates and terminates conversations, exchange files and exchange dialogues between the application at each end. 

Protocols used in this layer are SMPP, ZIP, ISO-IP, PAP, SQL, NFS etc

4. Transport Layer :: ( layer 4) 

The transport layer is responsible for source to destination ( end-to-end ) delivery of entire message. 

Transport layer's main function is to accept data from the session's layer, divide it into small pieces called segments and pass this segments to network layer and make sure that data reached the other end correctly. 

Each data segment is assigned a sequence number so that  data is passed serially.

Protocols used are  TCP ( Transmission Control Protocol) for connection oriented approach, UDP ( User Datagram Protocol) for connectionless approach. 

5. Network Layer ::  ( layer 3)

It establish route from source to destination by using routing algorithms. 

It converts logical memory address into physical address ( MAC address )

This layer provides transfer of data in the form of packets across the communication networks.

It controls congestion ( network traffic )

It establishes connections with other heterogenous networks ( different networks ) 

Protocols used are IP, IPX, NAT

Hardware used : Routers 

6. Data link layer :: ( layer 2)

Datalink layer's objective is to transform the raw  transmission into error-free transmission from network layer.

This layer breaks the packets received from network layer into frames.

It maintains a duplicate set of frames, if any case frames are damage or lost during transmission it sends again. 

Datalink layer is used by Bridges and switches for their operations.

protocols used in this layer are PPP, HDLC, SDLC, NDP etc

7. Physical layer :: ( layer 1)

Physical layer is responsible for transmitting raw bits (0,1 s) over the communication channel (physical cable).

Physical layer is the interface between devices and transmission media. 

Hardware used : Repeater and Hub