OSI Model
Overview
The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes how different network systems communicate with each other. It divides communication into 7 layers, each with a specific role. The model is vendor-neutral and serves as a reference for understanding how protocols interact.
The 7 Layers
| # | Name | Key Responsibility |
|---|---|---|
| 7 | Application | User-facing protocols and services |
| 6 | Presentation | Data formatting, encoding, encryption |
| 5 | Session | Session management between applications |
| 4 | Transport | End-to-end delivery between processes |
| 3 | Network | Logical addressing and routing |
| 2 | Data Link | Frame delivery within a local network |
| 1 | Physical | Raw bit transmission over a medium |
Layer Details
Layer 1 – Physical
Transmits raw bits over a physical medium (cables, radio, fiber). Defines voltage levels, pin layouts, and bit timing. Has no concept of addressing.
Examples: Ethernet cables, Wi-Fi radio signals, fiber optics, hubs, repeaters
Layer 2 – Data Link
Packages bits into frames and handles delivery within a single network segment using MAC addresses. Also detects transmission errors via CRC.
Sublayers: LLC (Logical Link Control) and MAC (Media Access Control)
Examples: Ethernet, Wi-Fi (802.11), ARP, switches
Layer 3 – Network
Handles logical addressing (IP) and routing of packets across multiple networks. Determines the best path from source to destination.
Examples: IP (IPv4, IPv6), ICMP, routers
Layer 4 – Transport
Provides end-to-end communication between processes. Manages segmentation, reassembly, flow control, and error recovery.
- TCP: connection-oriented, reliable, ordered delivery
- UDP: connectionless, faster, no delivery guarantee
Examples: TCP, UDP, port numbers
Layer 5 – Session
Establishes, manages, and terminates sessions (logical connections) between applications. Supports synchronization and checkpointing for long data transfers.
Examples: NetBIOS, RPC, session tokens
Layer 6 – Presentation
Translates data between the application and network format. Handles encoding, serialization, compression, and encryption.
Examples: TLS/SSL (encryption), JSON, XML, JPEG, MPEG
Layer 7 – Application
The layer closest to the user. Provides network services directly to applications. Does not refer to the applications themselves, but to the protocols they use.
Examples: HTTP, HTTPS, FTP, SMTP, DNS, SSH
Mnemonic
To remember the layers from bottom (1) to top (7):
Please Do Not Throw Sausage Pizza Away`
Physical, Data Link, Network, Transport, Session, Presentation, Application