Explain the OSI Model with neat diagram and functions of each layer.


page no 15

  • defn
  • dia
  • func of osi model

OSI MODEL

  • OSI stands for Open System Interconnection.

  • It is a reference model that describes how information from a software application in one computer moves through a physical medium to the software application in another computer.

  • OSI consists of seven layers, and each layer performs a particular network function.

  • OSI model was developed by the International Organization for Standardization (ISO) in 1984, and it is now considered as an architectural model for the inter-computer communications.

  • OSI model divides the whole task into seven smaller and manageable tasks. Each layer is assigned a particular task.

  • Each layer is self-contained, so that task assigned to each layer can be performed independently.









FUNCTIONS OF THE OSI LAYERS 

1. PHYSICAL LAYER(Puppy runs, slides down; squirrel licks peanut trail.)
The physical layer coordinates the functions required to transmit a bit stream over a physical medium.
  • Physical Characteristics of Interfaces and Media :The physical layer defines the characteristics of the interface between devices and the transmission medium.

  • Representation of Bits: To transmit bits, it must be encoded to signals; the physical layer defines the type of encoding.

  • Signals: Determines the type of signal used for transmitting information.

  • Data Rate or Transmission Rate: Number of bits sent per second is defined by the physical layer.

  • Synchronization of Bits: Sender and receiver must be synchronized at the bit level; their clocks must match.

  • Line Configuration:

    • Point-to-point: Two devices connected via a dedicated link.

    • Multipoint: A link shared among several devices.

  • Physical Topology: Defines how devices are connected — mesh, bus, star, or ring.

  • Transmission Mode :The physical layer defines the direction of transmission between two devices: simplex, half-duplex, or full-duplex.

2. Data Link Layer (Frogs play; foxes eat mangoes.)

Responsible for transmitting frames from one node to the next.

  • Framing: Divides the bit stream into data units called frames.

  • Physical Addressing: Adds a header to define sender and receiver for proper frame delivery.

  • Flow Control: Ensures receiver isn’t overwhelmed if it processes data slower than sender.

  • Error Control: Detects, retransmits lost/damaged frames, and prevents duplication using a trailer.

  • Medium Access Control: Determines which device controls the link at a given time.

3. Network Layer(Lion roars.) –
Responsible for the delivery of packets from source to destination, determining the best path based on network conditions, service priority, etc.

  • Logical Addressing: Used when packets cross networks, assigning source and destination logical addresses for identification on the internet.

  • Routing: Determines the best path among multiple options from source to destination.

4. Transport Layer (Penguins swim, cats fly everywhere)–
Responsible for process-to-process (end-to-end) delivery of the entire message, ensuring ordered arrival.

  • Port/Service Point Addressing: Adds a port address to deliver the message to the correct process.

  • Segmentation and Reassembly: Divides the message into segments with sequence numbers; reorders them on arrival.

  • Connection Control:

    • Connectionless: Delivers each segment independently.

    • Connection-oriented: Establishes a connection before delivery and terminates after.

  • Flow Control: Manages flow end-to-end, not just across one link.

  • Error Control: Detects and handles errors end-to-end.

5. Session Layer(Dog sleeps) –
Establishes, manages, and terminates connections between applications.

  • Dialog Control: Acts as a dialog controller, allowing communication between two processes (half-duplex or full-duplex).

  • Synchronization: Adds checkpoints during data transmission; if errors occur, retransmission starts from the last checkpoint (synchronization and recovery).



6. Presentation Layer(Tiger eats cake) –
Concerned with the syntax and semantics of exchanged information.

  • Translation: Ensures interoperability by converting data into a common format.

  • Encryption/Decryption: Transforms data for secure transmission and converts it back.

  • Compression/Expansion: Reduces or restores data size, especially for text, audio, and video.

7. Application Layer –

Enables user access to the network, handling network transparency and resource allocation.

  • FTAM: Allows users to access files on remote hosts.

  • Mail Services: Supports email forwarding and storage.

  • Directory Services: Provides access to database sources about various resources and objects.