OSI means Open Systems Interconnection. It is a conceptual framework on how application communicate over a network. Note that we use the TCP/IP model in practice but the OSI model is still used for planning, marketing, explaining, and troubleshooting. The OSI model still influences how network engineers think and talk about networks.
| Layers | Description |
|---|---|
| 7 | Application |
| 6 | Presentation |
| 5 | Session |
| 4 | Transport |
| 3 | Network |
| 2 | Data Link |
| 1 | Physical |
Layers
- Application
- Closest to the end user
- Interacts with software applications like your web browser
- HTTP/HTTPS are layer 7 protocols
- Presentation
- Data needs to be translated into a format it can be transferred over the network
- Layer 6 job is to translate between application and network formats
- Session
- Controls sessions between communicating hosts
- Establishes, manages, and terminates connections between the local and remote applications
Application developers work with Layers 5, 6, and 7 of the OSI model to connect their applications over the network.
- Transport
- Segments and reassembles data for communication between end hosts
- Breaks large pieces of data into smaller segments which can be more easily send over the network
- Provides host-to-host communication
- Network
- Provides connectivity between hosts on different networks
- Provides logical addressing (IP addresses)
- Provides path selection between source and destination
- Routers are layer 3 devices
- Data Link
- Provides node-to-node connectivity and data transfer (i.e. PC to switch, switch to router, router to router)
- Detects and corrects physical layer errors
- Uses MAC addresses instead of IP addresses (layer 3)
- Switches are generally layer 2 devices
- Physical
- Voltage levels, maximum transmission distances, physical connectors, cable specifications, etc.
- Digital bits are converted into electrical (for wired connections) or radio (for wireless connections) signals
Acronyms to help you remember the OSI model
- All People Seem To Need Data Processing
- Please Do Not Teach Students Pointless Acronyms