ServersThe OSI Model - Part One

The OSI Model – Part One




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.

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