OSI model
The OSI (Open Systems Interconnection) model was developed by the International Organization for Standardization (ISO) in the 1980s. It was created with the intent to help developers standardize software and hardware in networking, making protocol development more rational and interoperable.
| № | OSI layer | Description | Example protocol |
|---|---|---|---|
| 7 | Application | Application layer | HTTP, FTP, SMTP |
| 6 | Presentation | Data representation | SSL/TLS, JPEG, ASCII |
| 5 | Session | Interhost communication | NetBIOS, PPTP, RPC |
| 4 | Transport | End-to-end connections | TCP, UDP |
| 3 | Network | Path determination | IP, ICMP, IPSec |
| 2 | Data Link | Physical addressing | Ethernet, Wi-Fi (802.11) |
| 1 | Physical | Media, signal, and binary transmission | Ethernet (physical layer), DSL, USB |
On the physical layer, data is sent as bits; on the data link layer, it is sent as frames; and on the network layer, it is sent as datagrams. PDU (Protocol Data Unit) refers to packets, which are the basic units of communication used in networking to transfer data across different layers of the OSI model.
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