The OSI Application Layer :
If you are reading this tutorial then without a doubt
you have a browser running on which this page is being
displayed. The browser, I think I am repeating myself
but I want to make it very clear, is the application
and belongs completely to the category called application
layer.
It is the user interface the product of all
the hula bulla which goes on behind the scenes is
displayed on this browser for the user to see. Without
a user friendly user interface Internet would be
for the geek's and nerds of the world and a normal
user would make no sense of it at all. Viewing websites
can be accomplished through the use of utilities
like Netscape and Internet Explorer, you can also
have access to email gateway like mail.yahoo.com
through you browser but many users prefer using utilities
such as Outlook express which makes life a lot easier
when you are inundated by mails day in and day
out. Outlook express of Microsoft would be an example
of a Email gateway, again emails work on smtp protocol
hence the front end is only used to display the
mails and the graphics, the connection and the rest
would need to be taken care of by the layers below
the Session layer.
Well so there it is the Application
layer is nothing but the front end of any web based
application. It might sometimes be required the
the application layer checks for the presence of
all the resources available before the application
can kick start. For example , when you fire an outlook
express application , it checks to determine if
the email server (like MSN or Yahoo) is reachable.
The Presentation Layer :
Let us take the example of an Image. We view several
images on the web and I am sure you are aware that
image follow several compression standards. Without
compressing an Image it would take forever to download
it to you browser which might account for increase
in blood pressure or early greying of the hair.
In the case of images the most popular are tiff
and jpeg. Now imagine a web site which contains
an image, when the web site is displayed on your
browser the image has to be downloaded from the
server where the site is hosted. If the compression
standard followed in JPEG in that case the application
has to be smart enough to make sense of the compressed
data and display it one the screen. This is where
the presentation layer of the application comes
into picture.
Any application developed should take
care to cater to all the standards presently existent
if it wants to be popular among the masses. An internet
explorer application would have an interface which
reads any image data and converts it into a sensible
format (a format which makes sense to the front
end). The image can be in TIFF format or JPEG format,
its upto the application to handle it and the application
written to handle it is the property of Presentation
layer. Makes sense doesn't it. So OSI has protocol
standards which define how standard data should
be formatted. Tasks such as encryption,decryption,compression
and decompression belong to the presentation layer.
Without the existence of proper standards for the
above tasks (like JPEG for images,MPEG for videos
are standards of data representation) the life of
a software programmer would be one holy nightmare.
The Session LayerIf you are an ardent user of the Yahoo Messenger
or any messenger for that matter and if you are
popular among you friends, I am sure you would have
faced the crisis of replying to several message
windows popping up on your screen. Imagine the complexity
of the software (yahoo messenger for ex), it has
to cater to serveral connections at the same time.
Each window is a connection, the front end (the
blank space where you type your message) is the
application layer, the smiley's and italics that
you conjure up are displayed on the window at the
receiving end through the presentation layer and
each session is maintained by the session layer.
Every time you open a new chat window the session
layer sets up the connection to the yahoo server
and any message received for the window is forwarded
to the appropriate window or in other words session
layer manages the connections and when the window
is closed it tears down the connection. If the messenger
has a buggy session layer application I shudder
to think about the plight of the messenger user,
messages meant for one person might end up in the
window of another( I hope you got the picture).
Continue to: OSI Transport Layer
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