OSI Model
The OSI Model is a theoretical, seven-layered model of how networks work. The Open Systems Interconnect Model (the OSI Model) is a theoretical model of networking that organizes network functions into seven layers (physical layer, datalink layer, network layer, transport layer, session layer, presentation layer and application layer) and specifies the communication interfaces between the OSI Model's layers and between network endpoints utilizing an OSI Model-based protocol suite.
  • The OSI Model is a way of thinking about how networks 'work'
  • THEORY: The OSI Model is a theoretical model
    • The OSI Model is not a technology.
    • The OSI Model is not a protocol.
    • The OSI Model is not a program or software.
    • The OSI Model sorts network communication functions into seven layers
    • Layer functions are not specified and should be transparent to other layers.
    • Layer Communications (how layers communcate with each other) ARE specified in the model
    • Layer behavior should be invisible other layers

The OSI Model was developed as the ISO designed their own suite of networking protocols (the ISO OSI Protocol Suite). The OSI Model is always in the middle of any discussions of networking and network and sometimes computer troubleshooting. The OSI model is used so commonly as a point of reference that you really should learn the OSI Model's seven layers and learn how the OSI Model explains this layered approach to networking. Engineers working on a network problem will frequently say "Well, it's a data link layer problem", clearly referring to the data link layer of the OSI Model and more specifically, the physical network connection, not the networking software or the applications. The OSI Model's layers are so well known that they are referred to by number:  "Well, looks like we have a layer 2 problem here". If you don't know the OSI Model, you will have great difficulties as a network engineer, or even talking to one.

The International Standards Organization (ISO) developed a theoretical model of how networks should behave and how they are put together. This model is called the Open Standards Interconnect (OSI) Model. The "ISO OSI Model" was developed because it appeared that IBM was going to patent the design of their SNA networks so that no one else could use IBM's design model for networking. The ISO OSI model is used throughout the network, internet and telecom industries today to describe various networking issues. The OSI model is also of use in a learning or training environment where a novice can use it as a point of reference to learn how various technologies interact, where they reside, what functions they perform and how each protocol communicates with other protocols.

Don't worry about memorizing the OSI Model just yet, you'll run into the OSI Model of networking over and over again in all the tutorials on this site (including this OSI Model tutorial)--all of which link back to this OSI Model tutorial whenever they use these terms, you're going to see it so often, you may end up memorizing it without trying...

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