The OSI (Open Systems Interconnection) model is a conceptual framework used to understand how different networking protocols interact in layers to create a network. It was developed by the International Organization for Standardization (ISO) in the late 1970s and early 1980s. The OSI model divides the networking process into seven separate layers, each with a specific function:
Layer 7: Application Layer
- Closest to the end-user.
- Provides network services directly to applications such as web browsers and email clients.
- Protocols & Services: HTTP, FTP, SMTP, POP3, DNS.
Layer 6: Presentation Layer
- Transforms data into a format that can be consumed by the application layer above it.
- Handles data compression, encryption, and translation services.
- Examples: SSL/TLS, ASCII, EBCDIC, MPEG.
Layer 5: Session Layer
- Establishes, maintains, and terminates user connections.
- Manages sessions or connections between applications on different devices.
- Protocols: RPC, PPTP, SMB.
Layer 4: Transport Layer
- Ensures end-to-end communication, flow control, and error recovery.
- Segments data from the session layer and adds source and destination port numbers.
- Protocols: TCP, UDP, SCTP.
Layer 3: Network Layer
- Responsible for determining the best path to route data packets from the source to the destination.
- Deals with logical addressing (like IP addresses) and routing.
- Protocols: IP (both IPv4 and IPv6), ICMP, OSPF, BGP.
Layer 2: Data Link Layer
- Provides a means for data to be transferred from one device to another on a local network or a point-to-point connection.
- Concerned with physical addressing (MAC addresses), error detection, and framing.
- Sub-layers: Logical Link Control (LLC) and Media Access Control (MAC).
- Protocols & Technologies: Ethernet, Wi-Fi, PPP, ARP.
Layer 1: Physical Layer
- Defines the physical medium (like cables) and the characteristics of the transmission (like voltage levels).
- Concerned with bit-level transmission, signaling, and the physical connectors.
- Technologies: Ethernet cables (Cat5e, Cat6, etc.), optical fiber, USB, Bluetooth.
It’s important to note that the OSI model is largely theoretical. The TCP/IP model, which is a more practical framework and is the foundation of the internet, condenses the OSI model into four layers. However, the OSI model remains a valuable tool for understanding and teaching network operations.