The OSI stand for "Open System Interconnection".
It was first introduced in the late 1970s by the ISO (International Standard Organization).
An OSI is a set of protocol that allow any two different system to communicate regardless of their underlying architecture.
The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and inter-operable.
It consists of seven separate but related layers, each of which defines a part of the process of moving information across network.
These seven layers of OSI model can be divided into two categories:
- Upper Layer (Application Layer):
- This part consist three top layer as: Application layer, Presentation layer, Session layer.
- These layers provide the application services required for the exchange of information.
- These layer consist remaining four layers as: Physical layer, Data link, Network layer, Transport layer.
- These layers handle the data transport issues.
- In other word these layers provides the end-to-end service necessary for the transfer of data between two system.
- Physical Layer
- The data units on this layer are called bits.
- This is also concerned with the following:
- Interface between the device and the transmission medium.
- Types of transmission media
- Signals may be electrical or optical
- Data rate
- Transfer rate
- Repeaters & Hub are used in physical lyaer.
- The data unit on this layer is called frame (Group of bits )
- This layer divided into two sub layers:
- Media Access Control ( MAC )
- The MAC sub-layer controls how a computer on the network gains access to link resources and grant permission to transmit it.
- The LLC layer controls frame synchronization flow control and error checking.
- physical Addressing
- Error Control
- The unit of data at network layer is called packet or Data-gram.
- The network layer establish a route between sending & receiving stations.
- "Router" device used in network layer.
- Network layer is responsible for the source-to-destination delivery:
- Logical Addressing
- In this layer data unit called segment.
- The transport layer is responsible for overall end-to-end validity and integrity of the transmission.
- "Gateway" devices used in transport layer.
- Transport layer responsible for process-to-process delivery as:
- Port Addressing
- Segmentation and Reassembly
- Connection Control
- Flow Control
- The session layer decided when to turn communication ON and OFF between two computers.
- The session layer responsible for following:
- Dialog Controller
- In this layer data packet unit called is "Presentation Protocol Data Unit".
- It perform code conversion and data reformatting.
- The application layer is the top layer of OSI model.
- It defines the language and syntax that programs uses to communicate with other programs.
- The application layer enables the user, whether human or software, to access the network and services.
|Table: OSI model & Functionlity|