Appendix B. OSI Model and Frame Relay Technology Overview
B-2 ATLAS 550 User Manual 61200305L1-1
TheRouterandFrameRelaysoftwareinATLASinvolveslayer3andlayer2
data processing. The OSI model is not limited to digital data networks, but
canbeextendedtosuchnetworksastheU.S.PostalService.Theexamples
belowshouldclarifytherolesofthefirstthreelayersandhowtheyinterface
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
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.
Upper Layers Letters and Advertisements
Layer 3 Network Envelopes and Boxes
Layer 2 Data Link Mailbags
Layer 1 Physical Planes and Trucks
Send Process Receive Process
Person A writes a letter. Upper Layers Person B reads the letter.
Person A places the letter in
an envelope, addresses it to
person B, and puts envelope
in mailbox.
Layer 3
Person B opens envelope
and removes the letter.
The envelope is collected
from mailbox and placed
into a mailbag destined for
post office B.
Layer 2
The mailbag is opened and
the envelope is placed in
person B’s post office box.
A truck takes the mailbag
and drives to post office B.
Layer 1
Roads and
Interstates
The truck delivers the
mailbag to post office B.
