Open Systems Interconnection Reference Model
The Open Systems Interconnection (OSI) model is a seven-layer conceptual framework used to understand and describe how different network systems communicate. It breaks down the complex process of data transmission into manageable layers, helping IT professionals troubleshoot issues and understand interoperability between products.
In this architecture, each layer serves the layer above it and is served by the layer below it. Data flows down the stack at the sending device (encapsulation) and up the stack at the receiving device (decapsulation).
| # | Layer Name | Function | Examples / Protocols |
|---|---|---|---|
| 7 | Application | Human-computer interaction layer; where applications access network services. | HTTP, DNS, FTP, SMTP, SSH |
| 6 | Presentation | Ensures data is usable; handles encryption, compression, and translation/formatting. | SSL/TLS, JPEG, GIF, MPEG |
| 5 | Session | Maintains connections (sessions) between computers; controls ports and sessions. | NetBIOS, PPTP, RPC |
| 4 | Transport | Transmits data using transmission protocols (TCP/UDP); handles error recovery and flow control. | TCP, UDP |
| 3 | Network | Decides physical path for data; logical addressing (IP) and routing packets. | IP, ICMP, IPSec, Router |
| 2 | Data Link | Defines format of data on the network; physical addressing (MAC) & switching frames. | Ethernet, Switch, MAC Address, VLAN |
| 1 | Physical | Transmits raw bit stream over the physical medium (cables, wireless). | Hub, Cables (Cat6, Fiber), Wi-Fi |
The Application Layer is the human-to-computer interaction layer. It enables the user (human or software) to interact with the application or network whenever they elect to read messages, transfer files, or perform other network-related tasks. This layer provides the interface between user applications and the underlying network services.
The Presentation Layer translates or formats data for the application layer based on the semantics or syntax the application accepts. It handles encryption and decryption required by the application, ensuring different systems can read the data. Data compression also happens here to ensure efficient transmission.
The Session Layer controls the connections, or "sessions," between computers and applications. It sets up, coordinates, and terminates conversations to ensure data is exchanged efficiently. It also manages authentication and reconnection after an interruption, using checkpoints to resume transfers without data loss.
The Transport Layer is responsible for transporting data between devices. It breaks data into "segments" and ensures complete data transfer with error detection and correction. The two main protocols here are TCP (reliable, connection-oriented) and UDP (fast, connectionless).
The Network Layer moves data between different networks. It receives segments from Layer 4 and breaks them into packets. Its primary job is routing—finding the best physical path for data to reach its destination using logical addressing (IP addresses). Routers operate at this layer.
The Data Link Layer handles data transfer between devices on the same network node. It packages packets into frames. It uses Physical Addressing (MAC addresses) to ensure data reaches the correct device. Switches typically operate at this layer. It is often divided into two sublayers: LLC (Logical Link Control) and MAC (Media Access Control).
The Physical Layer consists of the physical hardware and transmission medium (cables, radio waves). It transmits raw bits (1s and 0s) over the network. It defines voltages, speeds, pinouts, and physical connector shapes. Hubs and repeaters operate at this layer.