March 8, 2019

What is the OSI networking model?

The Open Systems Interconnection (OSI) model is a conceptual framework for networking or telecommunication systems. It provides a visual description of the networking stack from “top” (end-user focused) to “bottom” (physical machine layout).

Reading: The OSI model explained.

OSI model diagram by Vikram Kumar

The seven layers are:

  • Application - closest to end-user, eg., web browsers, VoIP and email clients, and other connected applications
  • Presentation - prepares or translates application data formats from/to network formats, eg., encryption/decryption
  • Session - handles setup, coordination, and termination of the communication context, eg., SIP in telephony
  • Transport - coordinate data transfer between machines, eg., TCP/IP and UDP
  • Network - reponsible for packet forwarding and routing of IP addresses
  • Data Link - provides node-to-node data transfer, error correction.
    • two submodels in MAC and LLC
  • Physical - cables, radio frequencies, layout of pins, voltage, etc.

The article from NetworkWorld quotes Vikram Kumar, answering why the OSI model is still useful today:

The OSI model structure helps frame discussions of protocols and contrast various technologies.

Sounds like a great reason to at least understand the basics!

Some followup questions:

  • What is a MAC address and how is it used?
  • What is Logical Link Control (LLC) and how is it used?
  • What are the four layers to the TCP/IP stack?

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