DEMYSTIFYING THE OSI MODEL

The OSI (Open Systems Interconnection) model is a way to understand how computers and devices communicate with each other over a network. It breaks down the communication process into different layers, each with its own specific tasks and responsibilities.

The purpose of the OSI model is to promote standardization, modularity, and troubleshooting. It serves as a foundation for the development, implementation, and management of computer networks.

There are 7 layers in this model.

1. Physical Layer: This layer deals with the actual transmission of bits and signals over the network medium, such as fiber optic cables, ethernet cables or wireless connections. Here the stream of bits - zeroes and ones - are converted into electrical signals for copper cables, or light signals for fiber optic cables.
2. Data Link Layer: Here, the focus is on organizing the bits into meaningful chunks called frames. It ensures a reliable and error-free transmission between directly connected devices, just like how the postal system ensures the letter is properly packaged and labeled.
3. Network Layer: This layer is responsible for addressing and routing. It assigns unique addresses, called IP addresses, to devices on the network and determines the best path for data to travel. It is similar to how you write the recipient's address on the envelope and the postal service determines the route for the letter to reach its destination.
4. Transport Layer: This layer manages the communication between applications on different devices. It breaks down large chunks of data received from the upper layers into smaller segments and ensures they are delivered reliably and in the correct order. The data segments from different applications at the sending machine will be attached with sequence numbers and source & destination port numbers. Sequence number is used for reassembling the segments, and port numbers are used to handover the data to the intended application at the receiving machine. Note: each application running on the network opens a dedicated port for it to perform.
5. Session Layer: This layer establishes, manages, and terminates communication sessions between applications. It sets up a connection, allows data exchange, and ensures both sides agree on when to start and end communication.
6. Presentation Layer: This layer takes care of data formatting and representation. It translates data into a format that the receiving application can understand. ASCII to Binary while sending and and vice-versa while receiving. It may also handle tasks like encryption/decryption (SSL) and compression to secure and optimize data transmission.
7. Application Layer: This is the layer interacts with the user, with the network applications such as web browsers, email clients, IM clients etc. Set of protocols such as HTTP/s, IMAP, SMTP, FTP, etc. are employed here.