OSI/AS Programming Manual

2 APS Procedures
Programming Concepts
056783 Tandem Computers Incorporated 2–1
This section introduces you to some concepts you need to know to use the APS
procedures. It discusses the following topics:
Services
Naming conventions for the APS procedures and data definitions
OSI upper-layer concepts
The connection endpoint identifier (CEPI)
Wait and nowait mode
Fault tolerance
Preparations needed before running your application program
Calling an APS procedure
Initializing the API
Establishing a connection
Sending and retrieving user data
Basic concatenation
Releasing a connection
Some parts of this section are illustrated with sequences of procedure calls. In these
sequences, the procedures in square brackets can sometimes be omitted, depending on
the context. Procedures not enclosed in brackets are essential to the sequence.
Services In the context of OSI, the term service can can be used in two different ways.
First, the service for a particular OSI layer (or a particular service element, such as
ACSE, within a layer) is the set of functions provided by that layer or service element
to the layer above it, or to applications using that layer or service element. (In contrast,
the protocol for a layer or functionality set is the set of rules governing the
communication between that layer and its remote peer at the same layer.) The service
provider for a given service is the software or hardware that furnishes that service.
For example, OSI/AS is the service provider for Tandem applications using the
session service, the presentation service, and/or the ACSE service. Some of the APS
procedures include a service identifier, or service ID, parameter that identifies which
of these services is being used.
The term “service” is also sometimes used to refer to a particular function provided as
part of a given service. For example, the resynchronize service and the orderly release
service are two services provided by the Session Layer. Each such service has a
number of primitives associated with it. The OSI primitives provided by OSI/AS are
listed in Tables 2-1 through 2-4.