The 7 layers of OSI MODEL | What is OSI Model?


Table of Contents
The OSI (Open Systems Interconnection) model is a framework used to understand and design the way different computer systems communicate over a network. It divides the communication process into seven distinct layers, each with specific functions. Here's a simple breakdown of each layer:

1. User Data at Application Layer (Layer 7):

   - The process begins with user data at the highest layer, the Application Layer.
   - This layer interacts directly with the user's applications, such as web browsers, email clients, or file transfer programs.

2. Encapsulation at Presentation Layer (Layer 6):

   - The Presentation Layer is responsible for data translation, encryption, and compression.
   - The data from the Application Layer is formatted and encrypted if necessary.

3. Encapsulation at Session Layer (Layer 5):

   - The Session Layer manages the establishment, maintenance, and termination of sessions (connections).
   - It adds control information for managing dialogues between systems.

4. Encapsulation at Transport Layer (Layer 4):

   - The Transport Layer ensures end-to-end communication and provides reliability.
   - Data from the Session Layer is segmented, and transport layer headers are added to each segment.

5. Encapsulation at Network Layer (Layer 3):

   - The Network Layer is responsible for logical addressing, routing, and forwarding of data packets.
   - The Transport Layer segment is encapsulated into packets, and network layer headers are added.

6. Encapsulation at Data Link Layer (Layer 2):

   - The Data Link Layer handles MAC addressing, framing, and error detection.
   - The packet from the Network Layer is encapsulated into frames, including Data Link Layer headers and trailers.

7. Encapsulation at Physical Layer (Layer 1):

   - The Physical Layer deals with the hardware, electrical, and mechanical aspects.
   - The frames from the Data Link Layer are converted into electrical signals or other physical media for transmission.

8. Transmission Across Physical Medium:

   - The physical transmission of the frames occurs over the network medium, such as copper cables, fiber optics, or wireless signals.

The data then travels through the network to the destination.

Decapsulation Process at Receiver:

Upon reaching the destination, the process is reversed:

1. Physical Layer (Layer 1):

   - The received signals are converted back into frames at the Physical Layer.

2. Data Link Layer (Layer 2):

   - Frames are checked for errors and stripped of Data Link Layer headers and trailers.

3. Network Layer (Layer 3):

   - Packets are extracted by removing the Network Layer headers.

4. Transport Layer (Layer 4):

   - Segments are obtained by removing the Transport Layer headers.

5. Session Layer (Layer 5):

   - Control information for session management is processed.

6. Presentation Layer (Layer 6):

   - Decryption and translation of data occur.

7. Application Layer (Layer 7):

   - Finally, the original user data is delivered to the destination application.

This process of encapsulation and decapsulation allows for the reliable and efficient communication of data across different layers of a network. Each layer adds its own specific headers or trailers to the data, contributing to the overall functionality of the OSI model.

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