Specifications

The Open Systems Interconnect (OSI) Model
3-2 Ethernet LAN Standards
Figure 3-1. Open Systems Interconnect (OSI) Model
Application of the OSI Model
The perception of network operation appears as a direct peer-to-peer
communication to the user. The user message appears to go from the
sending application layer directly to the receiving application layer as if
the devices were directly attached. In actuality, the user message is routed
from the sending application layer down through the other layers of the
system. Each layer adds to or modifies the message according to the
network operating system’s protocol for each layer. The message passes
through all the layers of the system before appearing on the data channel
(cable or communications media) at the physical layer.
From the data channel the message passes upward through the same
layers at the destination device. As the message progresses from layer to
layer, each layer strips off information that was added by its counterpart
in the transmitting station. The result is the same message as was
originally sent, arriving at the top of the destination application layer.
Each layer performs a specific function with respect to the complete
communications process. The functions of each layer are as follows:
• LAYER SEVEN: Application Layer–The application layer is the user’s
interface with the network. This layer directly interacts with user
application programs to provide access to the network. All other layers
exist to support the requirements of this layer. The application layer is
usually involved with tasks such as electronic mail and file transfer.
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Application
Presentation
Session
Transport
Network
Data Link
Physical
1913-02