What Is the OSI Model?

The OSI Model, which stands for Open Systems Interconnection Model, is a conceptual framework used to understand and standardize the functions of a telecommunication or computing system without regard to its underlying internal structure and technology.

Developed by the International Organization for Standardization (ISO) in 1978, the model is divided into seven layers, each specifying particular network functions. The purpose of the OSI model is to guide product developers and promote interoperability of networking products and software through a common set of guidelines.

Here are the seven layers of the OSI Model, from the lowest layer to the highest:

1. Physical Layer (Layer 1): This layer deals with the physical connection between devices and the transmission and reception of raw bit streams over a physical medium. It includes specifications for cables, connectors, and electrical signals.

   
   

2. Data Link Layer (Layer 2): This layer is responsible for node-to-node data transfer—a link between two directly connected nodes. It also handles error correction from the physical layer, frame synchronization, and flow control. Ethernet and PPP are examples of data link layer protocols.

   
   

3. Network Layer (Layer 3): The network layer is responsible for packet forwarding including routing through intermediate routers, as it provides the means of transferring variable-length network packets from a source to a destination through one or more networks. IP (Internet Protocol) is an example of a network layer protocol.

   
   

4. Transport Layer (Layer 4): This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) are examples of transport layer protocols.

   
   

5. Session Layer (Layer 5): The session layer establishes, manages, and terminates connections between applications. It sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

   
   

6. Presentation Layer (Layer 6): This layer translates data between the application layer and the network. It is responsible for translation, compression, and encryption/decryption. It ensures that data is in a usable format and is where data encryption occurs.

   
   

7. Application Layer (Layer 7): The application layer is the closest layer to the end user, and it interacts with the software application that implements a communicating component. Thus, both the application layer and the user interact directly only with software applications. This layer provides network services to end-users and provides protocols that applications use to exchange data, such as HTTP for web browsing, SMTP for email, and FTP for file transfer.