OSI/AS Programming Manual

Sending and Retrieving User Data
APS Procedures Programming Concepts
2–42 056783 Tandem Computers Incorporated
Figure 2-12. One Single ASN.1 and One Octet-Aligned User Data Element With the Same PCID
User Data Header
Number of Elements = 2
User Data Element Header
PCID = px
Type = Single ASN.1 Type (SAT)
Length = lx
User Data Element Value
PDV (lx)
APS User Data
PDV list
PCID: px
Type: SAT
Length: lx
PDV list
PCID: px
Type: OCTET
Length: ly
PDV (lx)
PDVs (ly)
User Data from PPDU
User Data Element Header
PCID = px
Type = Octet-aligned (OCTET)
Length = ly
User Data Element Value
PDVs (ly)
070
If a user data element includes an ASN.1 EXTERNAL or SEQUENCE OF EXTERNAL
(indicating a change of PCID and presentation context), the user of the OSI/AS
presentation service must encode the EXTERNAL. The PCID specified in the data-
element header must be the PCID of the EXTERNAL or SEQUENCE OF EXTERNAL.
For primitives issued on ACSE connections, the OSI/AS implementation of ACSE
embeds each user data element you supply in a SEQUENCE OF EXTERNAL. You
must encode the EXTERNAL(s).
OSI/AS encodes the ACSE protocol control information for ACSE connections, and
only the Presentation Layer protocol control information for presentation connections.
OSI/AS encodes PPDUs and APDUs in BER as specified in ISO 8823 and ISO 8650.
OSI/AS encodes all PPDUs (except TD-PPDUs and TTD-PPDUs) and APDUs using
definite length encoding. If APS_DATA_REQ_ or APS_TYPEDDATA_REQ_ is called
with more-data specified or with user-data larger than 30,000, OSI/AS encodes
the user data portion of the PPDU with indefinite length encoding. For these cases, the
transfer syntax must be BER to allow the remote presentation entity to decode the user
data. If incoming PPDUs are encoded using indefinite-length encoding, the transfer
syntax must be in BER format.