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The Concept of OSI - Explained

The first topic you would be covering while preparing for CCNA would be about the OSI model. Lot of people who read about the OSI wonder about the usefulness of a such a hypothetical model. Why have a model in the first place and where is it actually implemented? Is it implemented while writing a software? Is it implemented while building a network using routers and switches? Can it be coded up using C++(I remember one of my friends asking me about that). How can a model be of use practically? Well 'OSI model' in one word is the "backbone" of the internet. Without a model in place there would ensue caos of the worst kind among vendors of different web based applications. Well don't you agree that in the end internet serves the purpose of helping an application user connect to a server or another client and hence transfer/receive data.

Whatever you do on the internet is nothing but transfer of data; for example say you want to visit the site yahoo.com, you go about opening you internet explorer or mozilla depending upon your patriotism to microsoft or linux, and you would type in the url name which is http//:www.yahoo.com and voila the page is displayed. Well like what they say in showbiz, what happens behind the scenes?


The answer is "an execution of a complete series of data transfer which follow a definite protocol at every stage". The above statement requires a lot of explanation. For one what is a protocol, when in a layman's language is a set of rules. Society would be in utter chaos if anarchy prevails, wouldn't you agree the same goes for Internet? You need a government which sets up rules and regulation which you have to be obey to be a part of the society. In the case of Internet there are several authoritative bodies who have had to do the gruesome job of setting up protocol which would need to be followed by the application in order to transfer data between the requestor and the provider(a client and a server in most cases). Getting back to the problem of understanding how a site ,which resides in some server belonging to a hosting company in United States or any other country for that matter, can be accessed by any computer connected to the internet (provided the computer has a browser installed).

In the above mentioned case browser is the application. Lets say you are using internet explorer (no offence to linux users), in that case what is shown on the screen is a front end developed by the hardworking engineers at Microsoft. The front is the interface for users. The required data, which in our case is the url is typed into the appropriate place. Then like I said above "voila the page appears". Now the entire process can be distributed into several phases of the OSI model. The OSI model consists of the following layers ( which I am sure you as a serious CCNA preper are already aware of)

1.) Application layer
2.) Presentation layer
3.) Session layer
4.) Transport layer
5.) Network layer
6.) Data link layer
7.) Physical layer

Like I mentioned before "Internet Explorer is an application". The first four layers that is the application, presentation, session layer and transport layer belong to and are present in any web based application. An application developer has to concentrate on all the four layers in order to develop a properly functioning web based application. Let us understand why a 4 layer approach is required to develop a web based application.

Here I provide a bried explanation about all the layers mentioned above



Continue to: OSI Application Layer