Figure 
shows the parameters of A-ASSOCIATE. []
Mode is an
opportunity to provide compatibility for the 1984 CCITT recommendation
on Message Handling Systems. The Application Context Name identifies
the particular collection of ASEs that comprise the application entity.
The calling and called application entity information parameters
identify particular application processes and the application
entity within that process if there are more than one. These are
provided to permit the underlying services to determine which active
process should receive any incoming communications over this
association. They are optional because the necessary information can be
obtained from the presentation address of the association (the PSAP -
presentation service access point - through which the Application
Service elements access the presentation layer, and thus all the lower
layer services).
Figure: Parameters of A-ASSOCIATE
The Presentation Context Definition List is a link between each
Application Service Element and the services of the Presentation Layer.
In particular, the Presentation context associates a transfer syntax
with the abstract syntax associated with each ASE
in the application entity. (Refer to Section for a
discussion of abstract syntax, transfer syntax, and the relationship
between the application view of information and the Presentation Layer
view of data.) Each ASE must have an abstract
syntax description of the information it will exchange over the
association. That abstract syntax must be mapped to a transfer syntax
for communication purposes. This mapping is the Presentation Context.
User data, if present, contains initialization information for the user of the ACSE - for example, the PDU initiating an association between File Access, Transfer, and Management (FTAM) Service Elements that are using this ACSE. The quality of service parameter carries information concerning the type of connection expected from the lower layer services. We will consider the content of this parameter more fully when we look at the service choices provided by the lower layers.
Parameters particularly associated with the Response and Confirm primitives of A-ASSOCIATE include the responding AE information, which is most often the same as the called AE information. The mandatory result field specifies whether or not the association is accepted by the responder. If the association is rejected, then the value carried in the result parameter specifies whether the rejection is permanent or transient. The diagnostic field clarifies a rejection reason. The result source, which appears only in the A-ASSOCIATE.confirm, identifies the source of the result. This is the responding ACSE service user if the association is accepted or if the application entity rejected the association; otherwise the value of this parameter is presentation service provider. This last case would occur if the presentation service is unable to establish a connection with the remote ACSE service provider, for example.
Figure: Parameters of A-P-ABORT
