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The OSI Application Layer

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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

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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 Layer

If 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