Explain the OSI model and its functions with a neat diagram.

 

1. Introduction to OSI Model

  • Definition

  • Purpose

  • ISO involvement

  • Layered architecture

2. OSI Layer Structure

  • Seven-layer model

  • Peer-to-peer communication

  • Service, Interface, Protocol

3. OSI Layers (Top to Bottom)

7. Application Layer

  • Network services

  • FTAM

  • Email services

  • Directory services

6. Presentation Layer

  • Translation

  • Encryption/Decryption

  • Compression/Decompression

5. Session Layer

  • Session establishment

  • Session maintenance

  • Dialog control

  • Synchronization

4. Transport Layer

  • Process-to-process delivery

  • Port addressing

  • Segmentation/Reassembly

  • Flow control

  • Error control

  • Connection control

3. Network Layer

  • Logical addressing (IP)

  • Routing

  • Packet forwarding

  • Fragmentation

2. Data Link Layer

  • Framing

  • Physical addressing

  • Error control

  • Flow control

  • Medium Access Control (MAC)

1. Physical Layer

  • Physical characteristics

  • Bit representation

  • Data rate

  • Bit synchronization

  • Line configuration

  • Physical topology

  • Transmission modes

4. Additional OSI Concepts

  • Layer independence

  • Encapsulation/Decapsulation

  • Protocol stack

  • Interoperability


OSI (Open Systems Interconnection) MODEL

The OSI model is a seven-layer network reference model developed by the International Organization for Standardization (ISO).
This model standardizes the way different systems communicate over a network.
It also provides layer-by-layer responsibilities, clear separation of functions, and ensures interoperability between heterogeneous devices and networks.

The OSI model divides the entire communication process into 7 independent layers, where each layer performs a specific set of tasks, communicates with its corresponding peer layer, and provides services to the layer above it.

The OSI model is only a reference architecture, meaning it does not define actual protocols but guides protocol designers on how communication tasks must be handled.


NEAT DIAGRAM OF OSI MODEL

        ------------------------------
        |        7. Application       |
        ------------------------------
        |       6. Presentation       |
        ------------------------------
        |         5. Session          |
        ------------------------------
        |        4. Transport         |
        ------------------------------
        |         3. Network          |
        ------------------------------
        |        2. Data Link         |
        ------------------------------
        |        1. Physical          |
        ------------------------------

DETAILED FUNCTIONS OF OSI LAYERS

1. PHYSICAL LAYER

This is the lowest layer and deals with the physical transmission of raw bits over a medium.

Functions:

  • Defines physical characteristics of network interfaces and media.

  • Specifies encoding of bits into signals (electrical/optical/radio).

  • Defines data rate (bps).

  • Ensures bit synchronization between sender and receiver.

  • Defines line configuration:
    point-to-point or multipoint.

  • Specifies physical topologies: bus, ring, star, mesh.

  • Defines transmission modes:
    simplex, half-duplex, full-duplex.

2. DATA LINK LAYER

Provides node-to-node reliable communication.

Functions:

  • Framing: Converts raw bits into frames.

  • Physical addressing: Adds MAC addresses of sender & receiver.

  • Error control: Detects & corrects errors using trailer (CRC).

  • Flow control: Avoids overflow at receiver.

  • Medium Access Control (MAC): Controls channel access in shared media.

  • Ensures hop-to-hop delivery.

3. NETWORK LAYER

Responsible for source-to-destination delivery across multiple networks.

Functions:

  • Logical addressing: Assigns IP addresses to identify devices.

  • Routing: Determines the optimal path for packet travel.

  • Packet forwarding: Sends packets to next-hop router.

  • Handles fragmentation and reassembly when network MTUs differ.

This layer ensures inter-networking and handles multiple routes.

4. TRANSPORT LAYER

Provides end-to-end or process-to-process delivery of the entire message.

Functions:

  • Port/Service-point addressing: Identifies specific application processes.

  • Segmentation & Reassembly: Splits message into segments; reorders them.

  • Connection control:

    • Connection-oriented (reliable)

    • Connectionless (fast, no reliability)

  • Flow control: Regulates data rate between end systems.

  • Error control: Ensures error-free delivery using acknowledgments.

This layer ensures complete, accurate, and reliable data transfer.

5. SESSION LAYER

Responsible for establishing and managing sessions between applications.

Functions:

  • Session establishment, management, and termination.

  • Dialog control:

    • Manages communication: half-duplex or full-duplex.

  • Synchronization:

    • Inserts checkpoints during long data transfers to enable recovery after failure.

6. PRESENTATION LAYER

Deals with syntax and semantics of transmitted information.

Functions:

  • Translation: Converts data between application and network formats.

  • Encryption & Decryption: Ensures data security.

  • Compression & Decompression: Reduces data size for efficient transmission.

This layer acts as the “translator” between application and network.

7. APPLICATION LAYER

Closest to end users; provides network services directly to applications.

Functions:

  • Supports File Transfer, Access, and Management (FTAM).

  • Provides email services (SMTP).

  • Provides directory services.

  • Supports network transparency and resource allocation.

  • Enables remote login and resource sharing.

Why OSI Model is Important?

  • Provides standardization for network communication.

  • Enables interoperability between different vendors & systems.

  • Helps in understanding, designing, and troubleshooting networks.

  • Each layer is independent, simplifying upgrades and maintenance.