ADTRAN Atlas 550 User Manual page 214

Appendix B. OSI Model and Frame Relay Technology Overview
Example 1:
B-2
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.
OSI Model Related to Process of Mailing a Letter
Upper Layers
Layer 3
Layer 2
Layer 1
Send Process
Person A writes a letter.
Person A places the letter in
an envelope, addresses it to
person B, and puts envelope
in mailbox.
The envelope is collected
from mailbox and placed
into a mailbag destined for
post office B.
A truck takes the mailbag
and drives to post office B.
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
Letters and Advertisements
Network Envelopes and Boxes
Data Link Mailbags
Physical Planes and Trucks
Receive Process
Upper Layers Person B reads the letter.
Person B opens envelope
and removes the letter.
Layer 3
The mailbag is opened and
the envelope is placed in
Layer 2
person B's post office box.
Layer 1 The truck delivers the
mailbag to post office B.
Roads and
Interstates
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