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Open System Interconnection (OSI) Model

Open System Interconnection (OSI) Model

Question: What is OSI model? Elaborate your 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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