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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
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