|
Understanding The OSI Reference Model: An Analogy
I have attempted in this discussion
of the OSI Reference Model to provide as much plain English
explanation of how it works as I could. However, there are situations
where a good analogy can accomplish what lots of descriptions cannot.
So, I am going to attempt to illustrate the key OSI model concepts (layers,
vertical
communication, horizontal
communication, data
encapsulation and message
routing) by way of a real-life analogy.
You can be the judge of whether it is a good analogy or
not. Just remember that no analogy is perfect.
Our scenario seems relatively simple
and common: the CEO of a Fortune 500 company needs to send a letter
to the CEO of another large company. Simple, right? Just like firing
up your browser and connecting to your favorite Web site is simple.
However, in both cases, a lot goes on behind the scenes
to make the communication happen. In the analogy shown in Table 17
below, I compare these real-world and "cyber-world communications.
Note: (Yes, the first CEO could fly to the other one's town in his Lear Jet, take him out for a lobster-and-martini dinner and hand him the letter there. Please play along, will you? Oh, and sorry to any CEOs for the blatant stereotyping. J |
Table 17: OSI Reference Model Real-World Analogy
Phase
|
OSI Layer
|
CEO Letter
|
Web Site
Connection (Simplified)
|
Transmission
|
7
|
The CEO of a
company in Phoenix decides he needs to send a letter to a peer of his
in Albany. He dictates the letter to his administrative assistant.
|
You decide you
want to connect to the web server at IP address 10.0.12.34, which is
within your organization but not on your local network. You type the
address into your browser.
|
6
|
The
administrative assistant transcribes the dictation into writing.
|
(Generally,
with a web site connection, nothing happens at this layer, but format
translation may be done in some cases.)
|
5
|
The administrative
assistant puts the letter in an envelope and gives it to the mail room.
The assistant doesn't actually know how the letter will be sent, but
he knows it is urgent so he says, get this to its destination
quickly.
|
The request
is sent via a call to an application program interface (API), to issue
the command necessary to contact the server at that address.
|
4
|
The
mail room must decide how to get the letter where it needs to go. Since
it is a rush, the people in the mail room decide they must use a courier.
The envelope is given to the courier company to send.
|
The
Transmission Control Protocol (TCP) is used to create a segment to be
sent to IP address 10.0.12.34.
|
Routing
|
3
|
The courier
company receives the envelope, but it needs to add its own handling
information, so it places the smaller envelope in a courier envelope
(encapsulation). The courier then consults its airplane route information
and determines that to get this envelope to Albany, it must be flown
through its hub in Chicago. It hands this envelope to the workers who
load packages on airplanes.
|
Your computer
creates an IP datagram encapsulating the TCP datagram created above.
It then addresses the packet to 10.0.12.34. but discovers that it is
not on its local network. So instead, it realizes it needs to send the
message to its designated routing device at IP address 10.0.43.21. It
hands the packet to the driver for your Ethernet card (the software
that interfaces to the Ethernet hardware).
|
2
|
The
workers take the courier envelope and put on it a tag with the code
for Chicago. They then put it in a handling box and then load it on
the plane to Chicago.
|
The
Ethernet card driver forms a frame containing the IP datagram and prepares
it to be sent over the network. It packages the message and puts the
address 10.0.43.21 (for the router) in the frame.
|
1
|
The plane flies
to Chicago.
|
The frame is
sent over the twisted pair cable that connects your local area network.
(I'm ignoring overhead, collisions, etc. here, but then I also ignored
the possibility of collisions with the plane. J)
|
2
|
In
Chicago, the box is unloaded, and the courier envelope is removed from
it and given to the people who handle routing in Chicago.
|
The
Ethernet card at the machine with IP address 10.0.43.21 receives the
frame, strips off the frame headers and hands it up to the network layer.
|
3
|
The tag marked
Chicago is removed from the outside of the courier envelope.
The envelope is then given back to the airplane workers to be sent to
Albany.
|
The IP datagram
is processed by the router, which realizes the destination (10.0.12.34)
can be reached directly. It passes the datagram back down to the Ethernet
driver.
|
2
|
The
envelope is given a new tag with the code for Albany, placed in another
box and loaded on the plane to Albany.
|
The
Ethernet driver creates a new frame and prepares to send it to the device
that uses IP address 10.0.12.34.
|
1
|
The plane flies
to Albany.
|
The frame is
sent over the network.
|
2
|
The
box is unloaded and the courier envelope is removed from the box. It
is given to the Albany routing office.
|
The
Ethernet card at the device with IP address 10.0.12.34 receives the
frame, strips off the headers and passes it up the stack.
|
Reception
|
3
|
The courier
company in Albany sees that the destination is in Albany, and delivers
the envelope to the destination CEO's company.
|
The IP headers
are removed from the datagram and the TCP segment handed up to TCP.
|
4
|
The
mail room removes the inner envelope from the courier envelope and delivers
it to the destination CEO's assistant.
|
TCP
removes its headers and hands the data up to the drivers on the destination
machine.
|
5
|
The assistant
takes the letter out of the envelope.
|
The request
is sent to the Web server software for processing.
|
6
|
The
assistant reads the letter and decides whether to give the letter to
the CEO, transcribe it to email, call the CEO on her cell phone, or
whatever.
|
(Again,
in this example nothing probably happens at the Presentation layer.)
|
7
|
The second CEO
receives the message that was sent by the first one.
|
The Web server
receives and processes the request.
|
As you can see, the processes
have a fair bit in common. The vertical communication and encapsulation
are pretty obvious, as is the routing. Also implied is the horizontal
communication that occurs logicallythe two CEOs seem to be connected
despite all that happens to enable this to occur. Similarly, the two
assistants are logically connected as well, in a way, even though they
never actually converse. Of course, this example is highly simplified
in just about every way imaginable, so please dont use it as a
way of trying to learn about how TCP/IP worksor courier services,
for that matter.
Anyway, I hope the example helps
make some sense of all of this OSI stuff!
If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support! |
|
|
Home -
Table Of Contents - Contact Us
The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.
|