How Data Flows:

Sending Text (e.g., "Hi")

When you send a short text message, it travels down the OSI layers. The Application layer takes your text, Presentation encrypts it, and Session manages the connection. Transport breaks it into small, manageable segments. Network adds routing information (IP addresses), and Data Link adds local hardware addresses (MAC addresses). Finally, the Physical layer converts it all into a signal (like radio waves) to be sent over the network.

Uploading a File

Uploading a file is similar but on a larger scale. The file is broken into many more segments by the Transport layer. Each segment is individually numbered so they can be reassembled correctly at the destination. The process of adding headers at each layer (encapsulation) happens for every single segment, creating a large stream of data packets that are sent to the server.

Downloading a File

Downloading is the reverse process. The server sends file segments down its OSI stack. Your computer receives these signals at the Physical layer and moves them up. Each layer strips off its corresponding header, checks for errors, and passes the data up. The Transport layer is crucial here, as it reassembles all the segments in the correct order to perfectly reconstruct the original file. The Application layer then presents this complete file to you.