The OSI Model - Part One

By ServerWatch Staff (Send Email)
Posted Feb 21, 2001


by Ryan Smith

For many MCSE's out there, the OSI Model is something that consisted of a few questions on the soon to be retired Networking Essentials 70-058 Microsoft exam.  Amazingly, with the new Windows 2000 track, MCSE's won't be required to have any knowledge of the OSI Model.

BACKGROUND:

For many MCSE's out there, the OSI Model is something that consisted of a few questions on the retired Networking Essentials 70-058 Microsoft exam. Amazingly, with the new Windows 2000 track, MCSE's won't be required to have any knowledge of the OSI Model.

The International Standards Organization (ISO) began developing the Open Systems Interconnection (OSI) reference model in 1977.  Since then, the OSI model has become the de facto standard for network communications. 

MCSE's who were required to take Microsoft's Networking Essentials 70-058 exam were required to have knowledge of the OSI model.  However, that knowledge of the OSI model consisted mainly of needing to memorize the order of the model and possess a basic knowledge of what task each of the seven layers is responsible for.

Many MCSE's simply memorized the OSI model and passed Networking Essentials 70-058.  However, it's a different story today.  New MCSE's that are embarking on the Windows 2000 MCSE track are NOT required to have knowledge of the OSI model.  None of the "core four" exams has anything relating to the OSI model and since Networking Essentials is being retired, it looks like new MCSE's are going to be without any knowledge of the OSI model. 

If you have any intention or desire to ever pursue any type of Cisco certification, be warned.  The OSI model is part of the bread and butter of the Cisco Certified Network Associate (CCNA) track.  Without a very solid understanding and knowledge of the OSI model, you're going to have a difficult time with any Cisco related certification.

As suprising as it may seem to some MCSE's out there, it's not just for certification purposes that the OSI model comes into play.  I'm sure at least half of the MCSE's out there are also responsible for their companies WAN and Internet links.  Any time you are dealing with troubleshooting these type of WAN and Internet connections, you are going to be dealing with tech support at your ISP who knows the OSI model backwards and forward (any has no problem quoting it to you at that!) A solid understanding of the OSI model will certainly help you deal with your ISP support group on any WAN or Internet related issues as well.

The OSI Model:

So by know you're probably dying to get more information about the OSI model, right? In a nutshell, there's really nothing to the OSI model. It's not even tangible. It's a conceptual framework that is used so that we can better understand the complex interactions that are happening.  The OSI model doesn't do any functions in the networking process.  It's not a piece of code or software that can be broken or upgraded.  The OSI model takes the task of internetworking - that is, host to host networking - and divides that up into what is referred to as a vertical stack.  This vertical stack consists of seven different layers:

  1. Physical
  2. Data Link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

Many mnemonics have been designed to assist with the memoritization of the OSI model. One such mnemonic is "Please Do Not Throw Sausage Pizza Away"

Please Physical
Do Data Link
Not Network
Throw Transport
Sausage Session
Pizza Presentation
Away Application

A second mnemonic for memorizing the OSI model in top down order is "All People Seem To Need Data Processing"

All Application
People Presentation
Seem Session
To Transport
Need Network
Data Data Link
Processing Physical

Because the OSI model is layered, it allows vendors to implement specific functionality into their networking devices.  For example, a network card is a Layer 1 (Physical) device, a switch is typically a Layer 2 (Data Link) device while a router is a Layer 3 (Network) device.  This layering approach allows vendors to more easily interoperate with other vendors devices because all of their devices are following a predetermined road map, the OSI model.

In Part Two of this article, I'll cover each of the seven layers of the OSI model individually and present more detail to how the OSI model is utilized by networking.

Ryan Smith Home

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