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Table Of Contents  The TCP/IP Guide
 9  The Open System Interconnection (OSI) Reference Model

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Indirect Device Connection and Message Routing
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OSI Reference Model Layers
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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 logically—the 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 don’t use it as a way of trying to learn about how TCP/IP works—or courier services, for that matter.

Anyway, I hope the example helps make some sense of all of this OSI stuff!

 


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OSI Reference Model Layers
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