Expand Configuration and Management Manual (H06.21+, J06.10+)

Subsystem Description
Expand Configuration and Management Manual — 529522-013
17 - 9
Expand Subsystem and the OSI Reference Model
Expand Subsystem and the OSI Reference
Model
The Expand line-handler process and $NCP components of the Expand subsystem
contain some of the functions defined in the lower five layers of the OSI Reference
Model. The Expand subsystem does not provide any Application Layer or Presentation
Layer functions; these functions in addition to some Session Layer functions, are
provided by the message and file systems.
This subsection describes these topics:
•
Expand Line-Handler Process Layer Functions on page 17-10
•
$NCP Layer Functions on page 17-12
Figure 17-2 compares the Expand subsystem’s protocol layers to the OSI Reference
Model.
Note. The Expand subsystem was not designed to match the OSI framework. The OSI
Reference Model is used in this discussion as a common point of reference to help explain the
functions of the various layers of the Expand subsystem.
Figure 17-2. Expand Subsystem Protocol Layers
VST011.vsd
Application Layer (7)
Presentation Layer (6)
Session Layer
Transport Layer (4)
Network Layer (3)
Data Link Layer (2)
Physical Layer (1)
User Applications
Message and File
Systems
$NCP
Communications
Hardware
Expand Line-
Handler
Process
Expand OSI Layers