ADTRAN 550 user manual Example 1 OSI Model Related to Process of Mailing a Letter

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
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		Person B opens envelope
an envelope, addresses it to	Layer 3	and removes the letter.
person B, and puts envelope
in mailbox.
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.
post office B.
A truck takes the mailbag	Layer 1	The truck delivers the
and drives to post office B.		mailbag to post office B.
	Roads and
	Interstates

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.

B-2 ATLAS 550 User Manual 61200305L1-1