The OSI Reference model is based on a proposal that was developed by the  ISO (International Standards Organization). That is why it is also called as ISO OSI (Open Systems Interconnection) Reference Model as it deals with connecting and communicating the open systems (i.e. systems that are open for communication with other open systems). The OSI model is based on Layered Network Architecture. Unlike TCP/IP model OSI reference model has 7 layers. These layers are explained as follows:

 

The Physical Layer:

This layer of OSI model performs following functions:

• To activate, maintain and deactivate the physical connection.
• To define data rates and voltage that are needed for transmission.
  
• To convert the digital bits into electrical signals.
• The physical layer of OSI model does not perform the detection and correction of errors. Below the physical layer lies the transmission medium.

Data link layer:

This layer of OSI model performs following functions:

• Framing: It divides the stream of bits received from the network layer into smaller and manageable data units called frames.
• Physical addressing: Data link layer adds a header to the frames to define the sender and receiver of the frame. i.e. it adds up MAC address.
  
• Flow control: when the rate at which the data are sent by the sender is greater than the rate at which data are received by the receiver or vice-versa, the data link layer uses some  flow control mechanism so that the receiver is not overwhelmed.
• Error control: it has mechanism to detect and re-transmit damaged or lost frames.
• Access control: whenever there are two or more devices that are connected to the same network link, data link layer protocols are needed to determine which device (host) has control over the network link at any given time.

Network Layer:

This layer of OSI Reference Model performs following functions:

• To provide optimal routes for the signals to reach other end.
• To act as network controller by deciding which route data should take.
• It works in logical address. i.e. IP addresses are added with the data segment received from higher layer and form a packet.
• To break the outgoing messages into smaller packets and to assemble incoming packets into larger messages for the higher levels of OSI Model.

Transport layer:

This layer of OSI model performs following functions:

• It accepts data from above it, splits it up into smaller units called segments and passes them to network layer. • It ensures that the pieces all arrive correctly at the other end.
• It guarantees the transmission of data from one end to the another.
• It also provides the facility of broadcasting the messages to multiple destinations.

The session layer:

This layer of OSI Reference Model performs following functions:

• This layer manages and synchronizes the conversation between two different applications. At this level, the user will establish system to system connection.
• It include dialog control i.e. keeping track of whose turn it is to transmit data
• The session layer allows a process to add checkpoints or synchronization points to a stream of data, so that it can pick up from where they left off in the event of a crash and subsequent recovery.

The Presentation Layer:

OSI model’s presentation layer performs following functions:

• This layer receives the data from the application that is running in the application layer. It makes sure that the information is delivered in such a form that the receiving system will understand and use it. i.e. it converts the data into machine format( binary format).
• It also performs the compression and encryption of the data so that there is integrity in data.

Application Layer:

The application layer of OSI contains a variety of protocols that are commonly needed by the users. One of the widely used application layer protocol is HTTP (Hyper Text Transfer Protocol), which is the basis for the WWW. Whenever  a browser wants a Web page or resource, it will send a HTTP request for that page to the server. The server then sends the page back as a response. Other application protocols are used for file transfer (FTP), electronic mail (SMTP), and network news.

 

References:

A.S. Tanenbaum, “Computer Networks”, 3rd Edition, Prentice Hall India

W. Stallings, “Data and computer Commnication”, Macmillan Press.

Kurose Ross, “Computer Networking: A top down approach”, pearson education

Larry L. Peterson, Bruce S. Davie, “Computer Networks: A System Approach”, Morgan Kaufmann Publishers