The presentation layer is concerned with the representation (syntax) of data during transfer between two communicating application processes.
To achieve true open systems interconnection, a number of common abstract data syntax forms have been defined. ISO defined a general abstract syntax suitable for the definition of data types associated with most distributed applications, known as abstract syntax notation number one or ASN.1. The data types associated with ASN.1 are abstract data types and an associated transfer syntax has been defined.
At present, only one transfer syntax notation exists in OSI - the Basic Encoding Rules (BER) of ASN.1. As experience with applications using BER grows, new transfer syntax notations, such as those including data compression or encryption, will no doubt be defined.
Syntax conversion may be the major role of the presentation layer, because it performs processing operations on all data prior to transfer and on all data that is received. It is also a good idea to perform data encryption and if required data compression in the presentation layer.
David Smith / dpsmith@tcd.ie
The Presentation Layer