Layered OSI Model

With OSI (Open System Interconnection) the International Organization for Standardization (ISO) defined a model for a standardized network that would replace TCP/IP, DECNet, and other protocols, as the primary network protocol used in the Internet. However, because of the complexity of the OSI protocol, not many implementations were built and put to use. TCP/IP was much simpler, and thus can now be found everywhere. But many new ideas from the OSI protocol can be found in the next version of IP, IPv6.

While the OSI protocol didn't catch on, the OSI seven layer model was very successful, and it is now used as a reference model to describe different network protocols and their functionality.

The layers of the OSI model separate out the basic tasks that network protocols must accomplish, and describe how network applications can communicate. Each layer has a specific purpose and is connected to the layers immediately above and below it. The seven layers defined by OSI are shown here:

• The application layer defines a programming interface to the network for user applications.
• The presentation layer is responsible for encoding data from the application layer ready for transmission over the network, and vice versa.
• The session layer creates a virtual connection between applications.
• The transport layer allows reliable communication of data.
• The network layer makes it possible to access nodes in a LAN using logical addressing.
• The data link layer accesses the physical network with physical addresses.
• Finally, the physical layer includes the connectors, cables, and so on.