OSI/AS Programming Manual

What OSI/AS Connections Are Possible?
Introduction
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Layer, and the Network Layer. For local area networks, TLAM performs the functions
of the Physical Layer and the Data Link Layer. (However, note that the functions of
the OSI internet protocol (IP) in the Network Layer are performed not by NSP
processes, but by TSP processes.)
The Transport Layer functions are provided by TSP (transport service provider)
processes. TSP processes are supplied by the OSI/TS (Open Systems
Interconnection/Transport Services) subsystem. The OSI/TS subsystem runs over
X25AM for wide area networks or TLAM for local area networks.
For both wide area networks and local area networks, OSI/TS performs the functions
of the Transport Layer. For some protocol combinations, it also performs the functions
of the IP in the Network Layer.
The Session, Presentation, and Application Layers
Layers 5, 6, and 7 of the OSI Reference Model are the Session Layer, Presentation
Layer, and Application Layer, respectively. The processes that provide these services
are called TAPS (Tandem application, presentation, and session) processes. TAPS
processes are part of the OSI/AS subsystem.
The APS procedures, another important part of OSI/AS, allow your application
programs to access the services provided by the TAPS processes.
What OSI/AS
Connections Are
Possible?
The following rules apply to connections made using OSI/AS:
Your application can have a number of connections active at the same time, using
the same or different OSI/AS subsystems.
Your application can have many connections with the same or different remote
applications.
Your application can have connections with other applications using the same
OSI/AS subsystem as your own application. Your application can even have
connections with itself—that is, with the same application process. However,
because this type of connection is not common, this manual always refers to the
other end of the connection as the “remote application.”