The primary purpose of the Presentation Layer is to provide
independence to application processes from differences in data
representation, i.e., syntax. The Presentation Layer protocol allows
the user to select a “Presentation Context” The Presentation
Context may be specific to an application such as a library
protocol or virtual terminal, to a type of hardware such as a
particular machine representation or to some standard or canonical
representation. Thus a user of OS1 wanting to develop an OS1
application protocol defines an application protocol using the
relevant parts of the Common Application Service Elements and
a Presentation Context which defines the representation of the
data to be transferred. The OS1 user may use an existing context
or define his own and register it with ISO. Draft Proposed
Standards for Presentation Layer services and protocols are expected
in early 1984.