Presentation Layer

Layer 6 of the OSI model is the presentation layer. This layer works right below the application layer and is primarily responsible for putting data given to it by the application layer into a format that other computers using the OSI model can understand. It provides a common way for systems to display and use information despite what application the user may be using on their system. For instance, suppose that Jack creates a document on a Windows machine using Microsoft Word. He then wants to send this file to Jane who uses uses Open Office instead of Word. To achieve this, the application layer passes the file on to the presentation layer which decides the proper way to encode it; in this case, it chooses American Standard Code for Information Interchange. The message continues to move down through the other layers, across the wire, and up through the model on the end system. When the presentation layer on Jane's machine receives the message it looks at the headers placed by Jack's system. It sees that the information is in ASCII format and tells the application layer which decides what program is appropriate for opening this type of file.

The presentation layer is focused solely on the syntax and format of the data and pays no attention to the meaning of any of the data. It translates the format that an application uses to a standard format that is used when transferring information over a network. When a program saves a file, such as an image, a format must be specified, such as GIF, or JPEG. The presentation layer adds information to the file that tells the computer how to display the file and process it. This way if the file is later sent, the receiving computer will also have instructions on how to properly display the file.

The presentation layer is also responsible for compression and encryption of files. If a compressed or encrypted file is to be sent over a network, the presentation layer will write information about how it is compressed of encrypted to the header of the packet. Again, this tells the receiving system's presentation layer what process was used to compress or encrypt the file and it can pass that information on to the application layer. In the event that the receiving system doesn't know that compression algorithm or file format, the file will be displayed with an unassociated icon.