Open System Interconnection (OSI) Model

Question: What is OSI model? Elaborate your answer.

Answer:  

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:

1. 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.
2. Lower Layer ( Data Transport):
• 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.

Figure: Layers of OSI reference model

1. 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
• connection
• Topology
• Transfer rate
• Repeaters & Hub are used in physical lyaer.
2. Date Link Layer (DLL)
• The data unit on this layer is called frame (Group of bits )
• This layer divided into two sub layers:
1. 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.
2. Logical Link Control ( LLC )
• The LLC layer controls frame synchronization  flow control and error checking.
• In summarize DLL is responsible for hop to hop delivery:
• Framing
• physical Addressing
• Error Control
• "Bridge" uses in DLL.
3. Network Layer
• 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:
• Packet
• Logical Addressing
• Routing
4. Transport Layer
• 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
• Segmentation and Reassembly
• Connection Control
• Flow Control
5. Session Layer
• The session layer decided when to turn communication ON and OFF between two computers.
• The session layer responsible for following:
• Dialog Controller
• Synchronization
• Translation
• Encryption
6. Presentation Layer
• In this layer data packet unit called is "Presentation Protocol Data Unit".
• It perform code conversion and data reformatting.
• Compression
7. Application Layer
• 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.

In summarize OSI model is as: ( source: http://en.wikipedia.org/wiki/OSI_model )

Figure: OSI model & Functionlity