OSI/AS Programming Manual

What Is the API?
Introduction
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What Is the API? The application programmatic interface (API) provided by OSI/AS is the Tandem
interface between applications and the OSI upper-layer environment. You access the
OSI/AS subsystem through the API by calling the APS procedures.
When you call an APS procedure to initiate a session, presentation, or ACSE operation,
the API passes on the request for services, if necessary, to one or more OSI/AS
subsystem processes. These processes, in turn, may pass the request and associated
data to one or more OSI/TS subsystem processes. Your application is not aware of the
processing that takes place, but the interface returns status information indicating the
success or failure of the call. You can call additional procedures to obtain more
information about your request and to retrieve any data sent to you.
Your application programs need not be aware of the underlying architecture of the
OSI/AS environment. Nevertheless, whoever performs system management at your
installation must be aware of the architecture and of your local configuration, and
must make certain preparations before you run your application programs.
What Is an OSI/AS
Subsystem?
Figure 1-1 shows the relationships among the various components of the OSI/AS
environment and a communications application that uses OSI/AS. As shown in the
figure, an OSI/AS subsystem consists of three main parts:
An OSI manager process
A Management Information Base (MIB)
The Tandem application, presentation, and session (TAPS) processes that provide
services and protocols for the upper three OSI layers
You access the OSI/AS subsystem from your application program by using the APS
procedures described in this manual. OSI/AS subsystem management is described in
the Tandem OSI/AS Configuration and Management Manual.
When examining the figure, note the following facts:
The transport service provider (TSP) and network service provider (NSP)
processes are provided not by OSI/AS, but by the underlying Tandem
subsystems: OSI/TS for TSP processes, and X25AM or TLAM for NSP processes.
Note, however, that the OSI manager process does provide some management
functions for the TSP and NSP processes.
Several OSI/AS subsystems can run on a single Tandem system.
The processes in the OSI/AS environment can be distributed among several
Tandem systems (nodes) in an Expand network.