An abstract syntax of the presentation layer of the OSI reference model describes the set and type of data types that are passed from the application layer in the form of an Application Protocol Data Unit( APDU) to the presentation layer so that the latter forwards the APDUs to the corresponding end system.
The ISO standard 8824 describes a notation with which it is possible to define abstract syntaxes. This language is called Abstract Syntax Notation One (ASN.1). ASN.1 can be used to describe relatively simple hierarchically structured data types. ASN.1 offers a communication possibility of two communication partners in the context of the representation layer. During data transfer, each communication partner converts its local representation into a common one.