WHAT IS OSI REFERENCE MODEL?
OSI model is based on a proposal developed by the International Standards Organization (ISO) as a first step toward international standardization of the protocols used in the various layers (Day and Zimmermann, 1983). It was revised in 1995 (Day, 1995). The model is called the ISO OSI (Open Systems Interconnection) Reference Model because it deals with connecting open systems—that is, systems that are open for communication with other systems. We will just call it the OSI model for short. This is reference model it helps network engineers to develop a computer network. It is also called as ISO –OSI reference model because it is designed to deal with open system i.e. the System which is open for communication with other systems. We can develop compatible system which can communicate with each other. In short it helps us to understand basics of computer network. In the OSI model, data communication starts with the top layer at the sending side, travels down the OSI model stack to the bottom layer, then traverses the network connection.
FUNCTIONS OF DIFFERENT LAYERS
THE PHYSICAL LAYER
- To activate ,maintain and deactivate the physical layer
- To define physical parameters like voltage, current etc.
- To convert digital signal into analog signal
- To decide transmission mode
THE DATA LINK LAYER
- It performs synchronization and error control
- It enables error detection; it adds error detection bits like parity bit etc.
- It performs the function of framing of outgoing data.
- It ensures reliable transmission of data
- Examples of data link protocols are HDLC, SDLC and X.25.
THE NETWORK LAYER
- To route the packet using devices like router
- It check the address of packet then transmits to destination
- It divides message into packets
- Another important function performed by this layer is conjunction control.
- Example of network layer protocol is IP
THE TRANSPORT LAYER
- This layer deals with process to process delivery
- Socket operates at this layer
- Transport layer protocols are either connection oriented or connection less.
- Transport layer also performs flow control and error control.
- Examples of transport layer protocols are UDP, TCP.
THE SESSION LAYER PROTOCOL
- This layer manages and synchronizes conversation between two different applications.
- It controls logging on and off, user identification, billing and session management.
- During transmission of data stream of data is marked so they can be reassembled at the destination.
THE PRESENTATION LAYER
- The presentation layer ensures that information delivered in such a form that receiving system can interpret that information.
- For example it translates ASCII to EBCDIC and vice versa.
THE APPLICATION LAYER
- The application layer is at the top of all layers. it provides different services such as manipulation of information in various ways, re-transmission the file of information
- It provides directory services it allows user to LOGIN
- It provides services to human it allow user to access computer network
- Example of application layer protocol is DNS
Benefits of the OSI model:
- OSI is not dependent on the operating system used.
- Any errors that occur are handled in each layer.
- The different layers can operate automatically.