The data applied to the OSI model is entirely different on the beginning when sent to the Application layer and at the end when transformed by all seven OSI layers. Each layer is adding its own piece of information to the initial data. All these pieces are addressed to the correlating layers at the receiver node. In order to explain which layer is adding what data, we would use an example of the loading web-page from the web-browser. First user types the address of the web page into the browser and press enter. browser sends data to the Application layer with the request of the web-page.
Now the magic happens:
Application layer is adding application header in front of the data. The header includes information about the browser requirements, so the correct version of the web-page would be loaded. The data is being forwarded further down the OSI in a form of protocol data unit (PDU).
Presentation layerdetects if the encryption is required by the Application layer. If the web page is secured and requires encryption, the presentation header of PDU would have information about that. All the information about graphics and formatted text would be in presentation header.
Session layer is adding session header with information about computer communication with the network. The PDU is being forwarded further in the OSI.
Transport layer is breaking down all the data that it receives to smaller segments. Segments size is defined by the type of network transmission method. The sequence identifiers for the segments are added to the transport header. Also transport header consists segments checksum, flow control data, and acknowledgement data. The segments are forwarded to the Network layer, one by one.
Network layer is adding addressing info to each single segment, so every node in the network would know who is sender and where is receiver. Segments with logical addresses are called packets and are being forwarded to Data Link layer.
Data Link layer is the last layer that is adding some information to data. It summarizes packets by adding header and trailer at the beginning and the end of PDU respectively. It is now called frame, and it has identified beginning and the end of it. Identifying those ensures that there was no data loss during transmission and it is not damaged. After data integrity is acknowledged it is ready to be sent.
The process is happening in reverse at the server side after signal has been received. It starts by electrical pulses in Physical layer and ends after Application layer as understandable by the web-server command.
Figure 1: Graphical representation of OSI layers involved in communication between 2 systems
