OSI Definition of Presentation Layer

The purpose of the presentation layer is to assure that end systems may successfully communicate even if they use different representations. It does this by providing a common representation to be used in communication and by converting from the local representation to this common representation.

Here is a list of the services provided by the presentation layer.

Transformation of syntax.
Selection of syntax.

There are three syntactic versions of information exchanged between Application entities:

The syntax used by the Application entity that requested the connection.
The syntax used by the receiving Application entity.
The syntax used between Presentation entities. This is known as the Transfer Syntax.

The presentation layer translates the representation of the information between the transfer syntax and each of the other two syntaxes.The transfer syntax to be used on a presentation connection is decided upon between the two presentation entities. Each side trys to choose a transfer syntax so that it can make a transformation to and from its user's syntax. The syntax used may change during the lifetime of the connection.

Each presentation connection is mapped one to one onto a session connection. The presentation protocol is only concerned with the transfer syntax, syntaxes used by other layers such as the Application layer are not of interest.

Here is a list of the functions of the presentation layer.

Session establishment request.
Data transfer.
Negotiation and renegotiation of syntax.
Transformation of syntax including data transformation, formatting and special purpose transformations.
Session termination request.