The Open Systems Interconnection (OSI) model is an abstract structure that displays the telecommunication and network functions in network systems. The OSI model was the first one that all major computer companies (including penetration testing companies) adopted in 1983. In 1984, it became a global standard. In general, it aims to allow interoperability between computer systems with the help of network protocols. Nowadays, the present internet does not use the OSI model. But still, it is broadly used because of its seven layers that visualise the flow of data.
The Seven Layers of the OSI Model
1. Physical Layer
The Physical Layer is responsible for the exchange of raw data. It breaks the frames received from the data link layer into bits (0s and 1s). Then, it transmits the bits through a wireless or cable connection.
2. Data Link Layer
It is responsible for breaking the data packets received from the network layer into frames. It transmits the data frames from the source node to the destination node after performing error checking.
3. Network Layer
The Network Layer finds the best route for data transmission. It breaks the data segments received from the transport layer into data packets. These data packets are forwarded to the destination with the best routing techniques.
4. Transport Layer
It allows for successful data flow and error control. It breaks the data received from the session layer into segments, processes it, and transmits it to the network layer. In case of an error, it retransmits the data.
5. Session Layer
It is responsible for the opening and closing of the channels for data exchange between the systems. Hence, it keeps the sessions/channels open when systems are communicating and close when the communication ends.
6. Presentation Layer
It creates a context for the data received from the application layer. The received data can be in a different format, so we need the presentation layer to make it meaningful. Thus, it provides freedom in data representation.
7. Application Layer
The Application Layer is the top layer of the OSI model that deals with users directly. Its rules allow information exchange between users. Hypertext Transfer Protocol (HTTP) is the most commonly used protocol in the Application Layer.
The OSI model has divided the process of data flow into seven particular layers. Each layer has a specific purpose, from the high-level distribution of data to the physical implementation of bits transmissions. Thus, every layer has a goal that serves the data received from the upper layer, prepares it, and moves it to the next layer. Overall, it gives a precise structure to data flow between networks.