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The ISO (International Organization for Standardization) is the mother of the OSI model. Back in 1980 they began work on a set of protocol that would promote open networking environments, which is how we ended up with multivendor computer systems communicating with one another by using internationally accepted communication protocols.
After a lot of research and decisions, birth was finally given to the OSI model. As you probably already know, the OSI model defines a layered architecture, which means certain actions, functions and services take place at every layer.
Keep in mind that the OSI model is a reference model and does not define any standard protocols. Software vendors use it to produce products that are guaranteed to work over a variety of computer platforms and operating systems.
The OSI model as you correctly mentioned, does have 7 layers and the reason is simple… The ISO decided that 7 layers was adequate to create the reference model they wanted! If the OSI model had more or less layers, it wouldn't mean that the protocols or software created would have extra or less functionality of what they have today, because as we already said, this is a reference model.
You might also be aware of the DoD (Department Of Defense) model, which is somewhat similar to the OSI, but only has 4 layers! Logically, multiple OSI layers would map to one layer of the DoD layer, so here we have 4 layers (DoD) describing the same functions, services as the the 7 layers (OSI).
There is plenty of information on the OSI model on the Internet, a simple search will reveal enough material to keep you hooked up to the computer for days!
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