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Open System Interconnection (OSI) Model

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.

Layers of OSI reference model
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 )

OSI model & Functionlity
Table: OSI model & Functionlity

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