The Seven Layer Model

The solution adopted is to decompose the big problem into several smaller problems and in 1983 a layered standard was proposed. Or, to be more precise, a reference model was proposed, the ISO Open Systems Interconnection (OSI) Reference Model. This is commonly known as the OSI Seven Layer Model. It describes several principles you should OSI think about when approaching a standard for a network. It doesn't actually give a standard for a network itself (though there was one directly based on it as a separate standard).

The principles involved were:

• A layer should be created where a different level of abstraction is needed;
• Each layer should perform a well-defined function;
• The function of each layer should be chosen with an eye towards defining internationally standardized protocols;
• The layer boundaries should be chosen to minimize the information flow across the interfaces;
• The number of layers should be large enough that distinct functions need not be thrown together out of necessity and small enough that the architecture does not become unwieldy.

The magic number of layers was decided to be seven: this was felt to be just the right number. Here we describe the seven layers with their classical properties, though you should note that not everyone sticks hard and fast to this kind of division of behaviours.