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Appendix B. OSI Model and Frame Relay Technology Overview
The Router and Frame Relay software in ATLAS involves layer 3 and layer 2 data processing. The OSI model is not limited to digital data networks, but can be extended to such networks as the U.S. Postal Service. The examples below should clarify the roles of the first three layers and how they interface with each other. Example 1 relates the OSI model to the process of mailing a letter.
Example 1: OSI Model Related to Process of Mailing a Letter
Upper Layers |
| Letters and Advertisements | |
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Layer 3 | Network | Envelopes and Boxes | |
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Layer 2 | Data Link | Mailbags | |
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Layer 1 | Physical | Planes and Trucks | |
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Send Process |
| Receive Process | |
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Person A writes a letter. | Upper Layers | Person B reads the letter. | |
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Person A places the letter in |
| Person B opens envelope | |
an envelope, addresses it to | Layer 3 | and removes the letter. | |
person B, and puts envelope |
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in mailbox. |
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The envelope is collected |
| The mailbag is opened and | |
from mailbox and placed | Layer 2 | the envelope is placed in | |
into a mailbag destined for | person B’s post office box. | ||
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post office B. |
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A truck takes the mailbag | Layer 1 | The truck delivers the | |
and drives to post office B. |
| mailbag to post office B. | |
| Roads and |
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| Interstates |
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Since the postal service specifies how mail is transferred between layers, the person addressing the letter only needs to know the address of the person receiving the letter to pass the letter down to the next layer. The letter writer has no knowledge of the details of mailbags and moving letters between post offices, but knows to place the letter in the mailbox so that the post of- fice delivers the letter to the reader. The lower layers have no knowledge of the letter, but take responsibility for getting it to the appropriate location.
ATLAS 550 User Manual |