Specifications

Models
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layers. Also, the layers above Session Control interface directly
with it. For this reason the upper three layers are sometimes
referred to as the "end user."
Modules of the same type in the same layer communicate with each other
to provide their services. The rules governing this communication and
the messages required constitute the protocol for those modules.
Messages are typically excnanged between equivalent modules in
different nodes. However, equivalent modules within a single node can
also exchange messages.
A brief description of each layer follows in order from the highest to
the lowest layer:
User Layer. The highest layer, the User Layer supports user
services and programs. Programs such as the Network Control
Program, which interfaces with the Network Management Layer,
and file transfer programs, which interface with the Network
Application Layer, reside in the User Layer.
Network Management Layer. The Network Management Layer is the
only one that has direct access to each lower layer for
control purposes. Modules in this layer provide user control
over and access to network parameters and counters. These
modules also perform
UD-line dumping, down-line loading, and
testing functions.
Network Application Layer. Modules in the Network Application
Layer support network functions, such as remote file access
and file transfer, used
by the User and Network Management
Layers.
Session Control Layer. The Session Control defines the
system-dependent aspects of logical link communication, which
allows messages to be sent from one node to another in a
network. Session Control functions include name-to-address
translation, process addressing, and, in some systems, process
activation and access control.
End Communication Layer. The End
Communication Layer defines
the system-independent aspects of logical link communication.
Routing Layer. Modules
in the Routing Layer route messages,
called packets,
between source
and destination nodes.
Data Link Layer. The Data Link Layer defines the protocol
concerning data integrity and physical channel management.
Physical Link Layer. The Physical Link Layer encompasses a
part of the device driver for each comrnunications device plus
the communications hardware itself. The hardware includes
interface devices, modems, and the communication lines.