In the OSI (Open Systems Interconnection) reference model, the communications between a computing system are split into seven different abstraction layers: Physical, Data Link, Network, Transport, Session, Presentation, and Application.
One popular mnemonic, starting with Layer 7, is All People Seem To Need Data Processing.
| Layer | Description | Comment | Examples |
|---|---|---|---|
| 1 | Physical | Actual hardware sits at this layer. It transmits signals over media. | Cable, RJ45 |
| 2 | Datalink | Translates binary (or BITs) into signals and allows upper layers to access media. | MAC, Switches |
| 3 | Network | This layer determines how data is sent to the receiving device. It’s responsible for packet forwarding, routing, and addressing. | IP, Routers |
| 4 | Transport | This layer coordinates data transfer between system and hosts, including error-checking and data recovery. | TCP, UDP, Port numbers |
| 5 | Session | This layer establishes and terminates connections between devices. It also determines which packets belong to which text and image files. | Syn/Ack |
| 6 | Presentation | This layer converts data to and from the Application layer. In other words, it translates application formatting to network formatting and vice versa. This allows the different layers to understand each other. | Encryption, ASCII, PNG, MIDI |
| 7 | Application | Most of what the user actually interacts with is at this layer. Web browsers and other internet-connected applications (like Skype or Outlook) use Layer 7 application protocols. | NMP, HTTP, FTP |