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CS Theory / Computer Networks
03.04 · Lesson

OSI Model

It is a network architecture model based on the ISO standards. It is called the OSI model as it deals with connecting the systems that are open for communication with other systems. The OSI model has seven layers.

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OSI (Open System Interconnections)

It is a network architecture model based on the ISO standards. It is called the OSI model as it deals with connecting the systems that are open for communication with other systems. The OSI model has seven layers.

  • Create a new layer if a different abstraction is needed.
  • Each layer should have a well-defined function.
  • The function of each layer is chosen based on internationally standardized protocols.

Physical Layer

It is the lowest layer of the OSI reference model.

  • It is used for the transmission of an unstructured raw bit stream over a physical medium.
  • Physical layer transmits the data either in the form of electrical/optical or mechanical form.
  • The physical layer is mainly used for the physical connection between the devices, and such physical connection can be made by using twisted-pair cable, fibre-optic or wireless transmission media.

It is used for transferring the data from one node to another node.

  • It receives the data from the network layer and converts the data into data frames and then attaches the physical address to these frames which are sent to the physical layer.
  • It enables the error-free transfer of data from one node to another node.
  • Frame synchronization: Data-link layer converts the data into frames, and it ensures that the destination must recognize the starting and ending of each frame.
  • Flow control: Data-link layer controls the data flow within the network.
  • Error control: It detects and corrects the error occurred during the transmission from source to destination.
  • Addressing: Data-link layers attach the physical address with the data frames so that the individual machines can be easily identified.
  • Link management: Data-link layer manages the initiation, maintenance and termination of the link between the source and destination for the effective exchange of data.

Network Layer

Network layer converts the logical address into the physical address. The routing concept means it determines the best route for the packet to travel from source to the destination.

  • Routing: The network layer determines the best route from source to destination. This function is known as routing.
  • Logical addressing: The network layer defines the addressing scheme to identify each device uniquely.
  • Packetizing: The network layer receives the data from the upper layer and converts the data into packets. This process is known as packetizing.
  • Internetworking: The network layer provides the logical connection between the different types of networks for forming a bigger network.
  • Fragmentation: It is a process of dividing the packets into fragments.

Transport Layer

It delivers the message through the network and provides error checking so that no error occurs during the transfer of data.

  • Connection-oriented transmission: In this transmission, the receiver sends the acknowledgement to the sender after the packet has been received.
  • Connectionless transmission: In this transmission, the receiver does not send the acknowledgement to the sender.

Session Layer

The main responsibility of the session layer is beginning, maintaining and ending the communication between the devices.

  • Session layer also reports the error coming from the upper layers.
  • Session layer establishes and maintains the session between the two users.

Presentation Layer

The presentation layer is also known as a Translation layer as it translates the data from one format to another format.

  • At the sender side, this layer translates the data format used by the application layer to the common format and at the receiver side, this layer translates the common format into a format used by the application layer.
  • Character code translation
  • Data conversion
  • Data compression
  • Data encryption

Application Layer

Application layer enables the user to access the network. It is the topmost layer of the OSI reference model.

  • Application layer protocols are file transfer protocol, simple mail transfer protocol, domain name system, etc.
  • The most widely used application protocol is HTTP(Hypertext transfer protocol ). A user sends the request for the web page using HTTP.
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Item 19 of 28 · updated 2026-10-12