OSI 7-Layer Model (Open Systems Interconnection 7-Layer Model)
The OSI 7-Layer Model is a conceptual framework standardized by the International Organization for Standardization (ISO) that divides the process of network communication into seven distinct, independent layers. It is extremely useful for understanding and troubleshooting complex network interactions.
1. OSI 7-Layer Structure and Roles
Communication starts at Layer 7 (Application), is transmitted through Layer 1 (Physical), and then reconstituted by moving back up from Layer 1 to Layer 7 at the receiving end.
| Layer | Name | Data Unit | Primary Role and Function | Associated Devices & Protocols |
|---|---|---|---|---|
| 7 | Application | Data | Provides network services to the user. (The end-user interface layer). | HTTP, SMTP, DNS, FTP, IMAP |
| 6 | Presentation | Data | Data formatting, encryption/decryption, and compression/decompression. | JPEG, MPEG, SSL/TLS |
| 5 | Session | Data | Establishes, manages, and terminates communication sessions between applications. | NetBIOS, RPC |
| 4 | Transport | Segment | Ensures reliable end-to-end data transfer and error recovery. | TCP, UDP |
| 3 | Network | Packet | Routing data across networks using IP addresses and logical addressing. | IP, ICMP, Router |
| 2 | Data Link | Frame | Error-free data transfer and flow control between adjacent devices using MAC addresses. | Ethernet, Wi-Fi, Switch, Bridge |
| 1 | Physical | Bit | Defines the electrical and mechanical characteristics for transmission over a physical medium (cables, wireless). | Cables, Hubs, Repeater |
2. Communication Process: Encapsulation and Decapsulation
① Sending: Encapsulation
As data moves down from Layer 7 to Layer 1, each layer performs its function and adds its own Header to the data.
- Example: At Layer 4 (Transport), adding the TCP header creates a 'Segment.' At Layer 3 (Network), adding the IP header creates a 'Packet.'
② Receiving: Decapsulation
On the receiving side, as the data moves up from Layer 1 to Layer 7, each layer removes its corresponding header and passes the remaining data up to the next layer.