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.

2Data Link


  • 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