OSI/AS Programming Manual

Overview of OSI/AS Presentation Layer Support
Introduction
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Overview of OSI/AS
Presentation Layer
Support
The procedures described in this manual also provide access to the functions described
in the Presentation Layer of the OSI Reference Model. The formal definitions of the
presentation service and protocol are given in the OSI standards ISO 8822 and ISO
8823. In addition, ISO 8824 (Abstract Syntax Notation-1, or ASN.1) and ISO 8825
(Basic Encoding Rules for ASN.1) are related to the Presentation Layer. OSI/AS
supports version 1 of the presentation service and protocol.
The information in the presentation, ASN.1, and BER standards is not duplicated here,
except where it is needed to provide context. The remainder of this subsection briefly
outlines the functions of the Presentation Layer and the Presentation Layer support
provided by OSI/AS.
What Are the Functions of
the Presentation Layer?
The Presentation Layer is Layer 6 of the OSI Reference Model. This layer provides for
a common syntactical representation of the user data values transferred between OSI
applications. The Presentation Layer is concerned just with the syntax of the data; the
semantics (meaning) of the data is known only at the Application Layer.
OSI/AS provides ASN.1 encoding and decoding for the following transfer syntax:
{ joint-iso-ccitt asn1 (1) basic-encoding (1) }
Any other ASN.1 encoding and decoding, including the encoding and decoding of
SEQUENCE OF EXTERNALs, must be provided by the application. For details, see
“User Data Encoding for Presentation and ACSE Connections” in Section 2.
Which Functional Units Are
Supported?
Of the presentation functional units, OSI/AS supports only the kernel functional unit.
The context management and context restoration functional units are not supported.
Of the session functional units available through the Presentation Layer, OSI/AS
supports all except the expedited data functional unit.