Layer 6. The Presentation Layer

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Sometimes called the Syntax Layer, this layer is usually that part of an operating system that is concerned with the representation (syntax) of messages' data associated with an application during the transfer between two application processes. Applications routing data are simply routing binary streams, which has no meaning without a definition as to how it is to be formatted. A raw binary representation alone isn't good enough, since two computers communicating with each other may have totally different configurations: One could be using a 16 bit word size, the other 32; a PC could be using an ASCII character set, while an IBM mainframe could be using EBCDIC, etc.

The Presentation Layer must therefore do its part to provide transparent communications services by masking the differences of varying data formats between dissimilar systems and, in general, converting incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window).

This layer is also concerned with methods of data encryption and data security, and compression algorithms that may have also changed the data format.

Here's how the Presentation Layer works: The Presentation Layer in one computer will attempt to establish a "transfer syntax" with the Presentation Layer in another computer by negotiating a common syntax that both applications can use. Failure to do so results in a non-connection. A widely used standard for the Presentation Layer is ISO 8824 and 8825

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