OSI/AS Programming Manual

OSI Upper-Layer Concepts
APS Procedures Programming Concepts
056783 Tandem Computers Incorporated 2–17
of BER, see “Encoding ACSE and Presentation User Data,” later in this section. Other
transfer syntaxes have been proposed, including compression, encryption, and the
Japanese INTAP-1. Transfer syntaxes for each proposed abstract syntax are negotiated
at presentation connect time.
Data transfer between open systems requires the identification of:
The abstract syntax being used
A transfer syntax capable of representing the data values that can be generated
using the specified abstract syntax
Presentation Services as Defined by the OSI Standards
According to the OSI standards, the services provided by the Presentation Layer
include the transformation—that is, the encoding and decoding—of user data in the
form of an abstract syntax in and out of a transfer syntax. The Presentation Layer does
not interpret the meaning (semantics) of the information; that task belongs to the user
of presentation services (for instance, ACSE, FTAM, or X.400). For example, ACSE has
an AARQ element in its abstract syntax. The ACSE service knows that this element is
an A-ASSOCIATE request, and knows what to do with it.
A user of presentation services must understand all the proposed abstract syntaxes; if
presented with an abstract syntax it does not recognize, the application must reject the
connection, or at least reject the use of that abstract syntax.
As described on the following pages, the OSI/AS implementation of the Presentation
Layer makes some modifications to the way user data encoding works.
Establishing a Presentation or ACSE Connection with OSI/AS
When using OSI/AS on a presentation or ACSE connection, you include in your
connect request a presentation context definition list or a presentation default context
name to identify how user data is to be encoded. You can specify one or both of these,
but you must specify at least one of them.
A presentation context definition list consists of up to 16 (15 for ACSE) presentation
context definitions, each of which is identified by a presentation context identifier
(PCID). Each presentation context definition contains a PCID, an abstract syntax
name, and one to three transfer syntax names. The PCID must be an odd integer in the
range 1 through 9999. The presentation context definition list is set in a presentation
or ACSE connect request through a call to APS_ASSOC_CONNECTREQ_. It can be
retrieved in a presentation or ACSE connect indication with a call to
APS_ASSOC_GETPARAM_.
A presentation default context name identifies a default presentation context (abstract
syntax and transfer syntax pair) to be used if no presentation context definition list is
given. The presentation default context name is set and retrieved in the same way as
the presentation context definition list.