Why bother?

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Layer Overlap!

Exam Summary




Chapter Analogy!



Why have the Open System Interconnection Model at all?

The answer to this question is in the two key words "model" and "open".


"It provides a way to understand how an internetwork operates"

The OSI Model helps us understand how networks work.  How does it do this? By breaking down the concept of internetworking into step-by-step tasks that can be easier understood.

Internetworking is complex. Several tasks are undertaken to transform the keystrokes that you send to an e-mail program on your PC to electrical signals that leave the network interface card of your PC to a mail server, across copper cabling. You also need to understand how the electrical signals can find their way across the globe to a recipient on the other side of the world.

A layered model breaks down this complex internetworking problem into several tasks, and assigns each task to a specific layer, making it possible to understand.


"It serves as a guideline or framework for creating and implementing network standards, devices and internetworking schemes"

If you’ve ever wept with frustration because the travelling iron you bought at the airport in London cannot be plugged into the power socket of your hotel room in Paris, you might appreciate this point.

The key here is standards. When the networking industry took off in the early eighties, many vendors started to manufacture networking equipment – cables, network interface cards, interfaces, etc. Each had their own idea about how to go about it. The result was that it became impossible to create a network by using different vendor’s products – they just did not fit together.

Having an open standard, “open” in this case meaning not proprietary (not belonging to anyone), means all manufacturers stick to one well-known standard. This results in what is called "inter-vendor interoperability” (which means networking equipment made by different manufacturers will be compatible).

What then are the advantages of having the OSI model?


"It allows for dividing the interrelated aspects of network operation into less complex elements"

Tasks, tasks, tasks. By chopping up the concept of internetworking into specific, definable tasks, you make it less complex, easier to understand, more manageable, etc.  In the OSI model, each task is represented by a layer.


"It enables engineers to specialise design and development efforts on modular functions."

Because the different tasks of internetworking are now so clearly defined, engineers and developers can each concentrate on the task they do best. For example, software developers can concentrate on writing networking software and not worry about cabling standards. Manufacturers of network interface cards need not worry about programming issues, etc.


"It provides the ability to define standard interfaces for "plug and play" compatibility and multi-vendor integration."

This follows from having an open standard. Because standards are based on an open (not proprietary) model, all vendors, manufacturers and developers are consulting the same "blueprint" so to speak. For this reason, all their equipment, software, etc will be compatible with each other, regardless of who the manufacturer is.

With these ideas in mind, the people at the ISO proceeded to create a layered model which is what we know today as the OSI Model.

It has seven layers in all.  The topmost layer is layer number 7 (not 1), and the layer at the bottom of the model is layer number 1.  The OSI Model therefore looks like this:

Now you understand why the OSI model was created, and the advantages of having such a model you may now…

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User-friendly Language!

Throughout this e-book, I will always start by quoting a phrase in techno-speak, (which I will underline and place in italics). “Techno-speak” is the jargon-filled language that is often used in text books and exams.  Do not let it intimidate you.  I will always follow this with an explanation in clear English, as I’ve done on this page.