computer network

OSI Model

The Open System Interconnection (OSI) model is a conceptual model developed by the International Standards Organization (ISO) in 1984. The OSI model provides a standard for communication between different/diverse computer systems.

The OSI model has seven layers in which each layer has a specific set of functions and communicates with the layer above and below itself.

Layers of OSI model

Physical Layer:

The physical layer is the lowest layer of the OSI model. It is responsible for transmitting message bits over a medium and it also takes care of mechanical, electrical, procedural, and functional specifications for communication.

Functions:

  • Transmission mode: It defines a transmission mode from Simplex, half-duplex, and full-duplex.
  • Network Topology: It specifies the arrangement of devices in a network.
  • Physical characteristics of the transmission medium
  • Line Configuration: It selects from either point-to-point or multipoint line configuration.
  • Data Rate: The physical layer defines the number of bits transmitted per unit of time.

Data Link Layer(DLL):

The data link layer breaks data packets received from the network layer into smaller pieces called frames and is responsible for the error-free transmission of these frames from one node to another using its MAC address.

The DLL is divided into two sublayers:

  • LLC(Logical Link Control): It deals with functions like flow control and error control.
  • MAC(Media Access Control): It controls the physical addressing and framing functions of the data link layer.

Functions:

  • Flow Control: It makes sure that the transmitting speed and the amount of data sent to match the capacity and speed of the receiver so that no data gets corrupted.
  • Framing: DLL adds certain bits at the beginning(called header which contains the source and destination addresses) and at the end(called trailer which contains error correction and detection bits) to the message frame. 
  • Error Control: DLL uses CRC(cyclic redundancy check) to check if any error occurred during transmission.
  • Physical Addressing: DLL adds the physical address(MAC address) of the destination and source in the header of each frame.
  • Access Control: Determines which device has control over the link if the same communication channel is shared by multiple devices.

Network Layer:

The network layer is responsible for finding the best path for the delivery of data packets from the source host to the destination host. 

Functions:

  • Host-to-host connectivity.
  • Fragmentation: The network layer divides the received data into smaller fragments called packets.
  • Routing: The network layer is responsible for finding the best possible path for a data packet to reach its destination, this is known as routing.
  • Logical Addressing: The network layer adds the source and destination’s IP address(which is used to identify a device uniquely universally) in the header of the frame.

Transport Layer:

The purpose of the transport layer is to provide a mechanism for the delivery of a message from one to another process and ensure that the data units are delivered sequentially, without error, and without loss or duplication.

Functions

  • Service Point Addressing: To ensure that the message is sent to the appropriate process the transport layer header includes an address type known as the service point address or port address.
  • Segmentation and Reassembly: The transport layer receives a message from the session layer, divides it into smaller pieces called segments, and assigns each segment a sequence number that uniquely identifies that segment, and at the destination transport layer reassembles the message based on their sequence numbers.
  • Flow Control: The transport layer ensures the speed at which data is sent corresponds to the speed at which data is received.
  • Error Control: The transport layer performs error control to ensure that data reached its destination without any error.

Session Layer:

This is the layer in charge of initiating and terminating communication between the two devices. The session is the period between the start and end of the communication.

Functions

  • Dialog Control: It means communication between two processes can take place in either a half-duplex or full-duplex way.
  • Synchronization: The session layer allows adding some checkpoints while transmitting data so that if a failure of some kind occurs between checkpoints, all data can be retransmitted from the most recent checkpoint.

6. Presentation Layer:

The presentation layer converts the information from the application layer into a suitable format for network transmission.

Functions 

  • Translation: Convert the information sent by the sender into a common format so that it can be understood by the receiver.
  • Encryption/Decryption: The process of converting an original message into another format for secure transmission is called encryption and obtaining the original message from the encrypted message is called decryption. Key values ​​are used to encrypt and decrypt data.
  • Compression: Reducing the number of bits in the message to be transmitted.

 Application Layer:

The application layer allows users to access network resources. The application layer is the OSI layer closest to the end-user.

Functions

  • File transfer, access, and management (FTAM): It allows a user to access, manage and retrieve files on a remote computer.
  • Mail Services
  • Directory Services: This layer provides access to global information for various services.