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How a Network Functions

by vindhyaprocess
osi model

How a Network Functions

The OSI Model

  • International Standards Organization (ISO) specifications for network architecture.
  • Called the Open Systems Interconnect or OSI model.
  • Seven layered model, higher layers have more complex tasks.
  • Each layer provides services for the next higher layer.
  • Each layer communicates logically with its associated layer on the other computer.
  • Packets are sent from one layer to another in the order of the layers, from top to bottom on the sending computer and then in reverse order on the receiving computer.

OSI Layers

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical


Application Layer

  • Serves as a window for applications to access network services.
  • Handles general network access, flow control and error recovery.

Presentation Layer

  • Determines the format used to exchange data among the networked computers.
  • Translates data from a format from the Application layer into an intermediate format.
  • Responsible for protocol conversion, data translation, data encryption, data compression, character conversion, and graphics expansion.
  • Redirector operates at this level.

Session Layer

  • Allows two applications running on different computers to establish use and end a connection called a Session.
  • Performs name recognition and security.
  • Provides synchronization by placing checkpoints in the data stream.
  • Implements dialog control between communicating processes.

Transport Layer

  • Responsible for packet creation.
  • Provides an additional connection level beneath the Session layer.
  • Ensures that packets are delivered error free, in sequence with no losses or duplications.
  • Unpacks, reassembles and sends receipt of messages at the receiving end.
  • Provides flow control, error handling, and solves transmission problems.

Network Layer

  • Responsible for addressing messages and translating logical addresses and names into physical addresses.
  • Determines the route from the source to the destination computer.
  • Manages traffic such as packet switching, routing and controlling the congestion of data.

Data Link Layer

  • Sends data frames from the Network layer to the Physical layer.
  • Packages raw bits into frames for the Network layer at the receiving end.
  • Responsible for providing error free transmission of frames through the Physical layer.

Physical Layer

  • Transmits the unstructured raw bit stream over a physical medium.
  • Relates the electrical, optical mechanical and functional interfaces to the cable.
  • Defines how the cable is attached to the network adapter card.
  • Defines data encoding and bit synchronization.

The 802 Project Model

  • Defines Standards for the Data Link and Physical Layers.
  • Network Adapter Cards
  • WAN components
  • Components used to create twisted-pair and coaxial cable networks.
  • A crazy mnemonic for this table, but it works 🙂

OSI Model Enhancements

The bottom two layers – Data Link and Physical – define how multiple computers can simultaneously use the network without interfering with each other.

  • Divides the Data-link layer in to the Logical Link Control and Media Access Control sublayers.
  • Logical Link Control
    • manages error and flow control and
    • defines logical interface points called Service Access Points (SAP’s). These SAP’s are used to transfer information to upper layers
  • Media Access Control
    • communicates directly with the network adapter card and
    • is responsible for delivering error-free data between two computers.
    • Categories
      • 802.3
      • 802.4
      • 802.5 and
      • 802.12 define standards for both this sublayer and the Physical layer

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