OSI Model and it's Layers in Computer Network's

The OSI model, or Open Systems Interconnection model, is a conceptual model that was developed by the International Organization for Standardization (ISO) to help standardize communication protocols between different computer systems. It is composed of seven layers, each with its own specific functions and responsibilities.

Here is a breakdown of each layer:

1. Physical Layer: This layer is responsible for transmitting raw bits over a communication channel. It deals with the physical characteristics of the connection, such as the electrical signals, cables, and connectors. Examples of devices at this layer include network interface cards (NICs), hubs, and repeaters.

2. Data Link Layer: This layer provides reliable and error-free transmission of data over the physical layer. It divides the data into frames and adds header and trailer information to each frame. Examples of devices at this layer include switches and bridges.

3. Network Layer: This layer is responsible for routing and forwarding data between different networks. It determines the best path for the data to travel and handles addressing and logical connections. Examples of devices at this layer include routers and Layer 3 switches.

4. Transport Layer: This layer provides end-to-end communication between applications on different hosts. It ensures that data is transmitted correctly, with no loss or duplication, and that it arrives in the correct order. Examples of protocols at this layer include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

5. Session Layer: This layer establishes, maintains, and terminates connections between applications. It manages the dialogues between them and ensures that data is exchanged smoothly. Examples of protocols at this layer include NetBIOS and RPC (Remote Procedure Call).

6. Presentation Layer: This layer provides a common format for data exchange between applications. It handles data encryption, compression, and conversion between different formats. Examples of protocols at this layer include JPEG (Joint Photographic Experts Group) and MPEG (Moving Picture Experts Group).

7. Application Layer: This layer provides services to user applications. It includes protocols for email, file transfer, and remote login, among others. Examples of protocols at this layer include HTTP (Hypertext Transfer Protocol), SMTP (Simple Mail Transfer Protocol), and FTP (File Transfer Protocol).

Each layer of the OSI model interacts with the layer above and below it. Data is transmitted from the top layer down to the physical layer, where it is transmitted over the network, and then back up the layers on the receiving end. This process is known as encapsulation.

For example, let's say you're sending an email from your computer to a friend's computer across the internet. The email message starts at the Application Layer, where it is formatted and prepared for transmission. It then moves down through the layers, with each layer adding its own information and encapsulating the data. The Transport Layer breaks the message into packets and adds a header with information about the source and destination addresses. The Network Layer determines the best route for the packets to take and adds its own header with the IP addresses of the source and destination hosts. The Data Link Layer adds its own header and trailer, including the MAC addresses of the source and destination NICs. Finally, the Physical Layer transmits the packets over the network using electrical or optical signals.

When the packets arrive at the recipient's computer, the process is reversed. The Physical Layer receives the signals and passes them up the layers, with each layer stripping off its own header and passing the data up to the next layer. Finally, the Application Layer receives the complete email message and delivers it to the user.

Overall, the OSI model provides a framework for understanding how communication protocols work and how different devices on a network interact with each other. It has helped to standardize communication protocols and make it easier for different computer systems to communicate with each other.