The Open Systems Interconnection (OSI) model is a conceptual framework used to understand network interactions in a layered manner. It was developed by the International Organization for Standardization (ISO) to standardize network protocols and allow different systems and networks to communicate with each other. The OSI model divides the networking process into seven distinct layers, each with a specific function:
Physical Layer:
- Deals with the physical connection between devices.
- Concerned with electrical, mechanical, and timing interfaces.
- Transmits raw bits over a communication channel (e.g., copper cables, fiber optics).
- Examples: Ethernet, USB.
Data Link Layer:
- Responsible for creating a reliable link between two directly connected nodes.
- Handles error detection and correction, as well as flow control.
- It’s divided into two sub-layers: Logical Link Control (LLC) and Media Access Control (MAC).
- Examples: Ethernet MAC, PPP.
Network Layer:
- Determines the best path to route data from the source to the destination.
- Handles packet forwarding and routing.
- Concerned with logical addressing (like IP addresses).
- Examples: IP, ICMP.
Transport Layer:
- Ensures end-to-end communication, reliability, and data integrity.
- Establishes, maintains, and terminates connections.
- Responsible for flow control, error correction, and segmenting data for transmission.
- Examples: TCP, UDP.
Session Layer:
- Establishes, maintains, and ends communication sessions.
- Ensures that data from the upper layers is properly synchronized and organized.
- Examples: RPC, PPTP.
Presentation Layer:
- Transforms data from one format to another.
- Deals with data compression, encryption, and translation services.
- Ensures that data is in a readable format for the application layer.
- Examples: SSL/TLS, JPEG.
Application Layer:
- Closest to the user.
- Provides network services directly to applications.
- Handles user authentication, file transfers, email, and other end-user services.
- Examples: HTTP, FTP, SMTP, DNS.
It’s important to note that the OSI model is largely theoretical, and the actual protocols used in modern networks don’t always fit neatly into the seven layers. The Internet Protocol Suite (commonly known as the TCP/IP model), which is the basis for the internet, uses a more simplified four-layer approach. However, the OSI model remains a useful tool for understanding and teaching networking concepts.