OSI/AS and OSI/TS Supplement (Includes RFC-1006 Support)

Overview of the Tandem OSI Architecture
Introduction to RFC-1006
107751 Tandem Computers Incorporated 1–5
OSI/AS and OSI/TS on Top
of TCP/IP
Figure 1-4 shows the combination of the OSI upper layers and the TCP/IP lower
layers. The three OSI upper layers remain the same and provide the application
services. The OSI Transport Protocol Data Units (TPDUs) interface with the TCP/IP
Transport Layer through the sockets interface. Figure 1-5 illustrates the subsystem
interfaces.
Figure 1-4. Tandem OSI Architecture: OSI/AS and OSI/TS Over TCP/IP
Services and Protocols
Applications/Application Services
Application Services
Application Services
OSI Transport Protocols
Transmission Control Protocol
Internet Protocol
Network Protocol
Hardware
Layers
Application
Presentation
Session
Transport
Transport
Internet
Network
Physical
Objects Passed
Between Layers
APDUs
PPDUs
SPDUs
TPDUs
TCP Segments
IP Datagrams
Network-Specific Frames
Bits
OSI/AS
OSI/TS
TCP/IP
020
Subsystem Interfaces
As shown in Figure 1-5, OSI/TS interfaces with TCP/IP through socket library
routines and with OSI/AS through file-system procedures. OSI/AS applications
interface with the OSI/AS subsystem through the application programmatic interface
(API) by calling Application, Presentation, and Session (APS) procedures. (The APS
procedure calls provide a procedural interface to OSI/AS, just as the file-system
procedures provide a procedural interface to the file system.) When the APS
procedure is called to initiate a session, a presentation, or an ACSE operation, the API
passes 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 the
OSI/TS subsystem process through file-system procedures.