Expand Configuration and Management Manual (G06.24+)

Subsystem Description
Expand Configuration and Management Manual—523347-008
18-9
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 the following topics:
•
Expand Line-Handler Process Layer Functions on page 18-10
•
$NCP Layer Functions on page 18-12
Figure 18-2 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 the following discussion as a common point of reference to help
explain the functions of the various layers of the Expand subsystem.
Figure 18-2. Expand Subsystem Protocol Layers
011CDT .CDD
Application Layer (7)
Presentation Layer (6)
Session Layer (5)
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