The OSI Model – Houston We Have a Problem

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Houston: “What is the problem?”

Communication between systems what it’s all about. Things sometimes go wrong with servers and computers that provide services to clients\customers, whether it is a website, e-mail, database, etc… When something goes wrong with the connectivity between systems these are some of the thoughts that will run through your mind:

“What in the world just happened?
Does the software or apps have a wrong configuration?
Did we lose power somewhere? Are the servers down?
Who tripped over the cables?
*Sigh* Will this turn into a 24 hour shift to fix the problem?”

You have to acquire a logical thinking process to troubleshoot problems experienced in the Information Technology field. The more skilled you are at troubleshooting the quicker you can come to a conclusion on how to fix the problem. You do not want to spend your wheels, so to speak, longer than necessary. Right?

Then the worker who is worthy of his or her wages can relax with a cold beer. *Scratches head* I mean a tall cold glass of lemonade!

The OSI Model and Troubleshooting

In 1984, the public learned from the International Organization for Standardization that OSI stands for Open Systems Interconnection. Explaining in everyday language the OSI model’s function is to help you visualize your servers’ service connections to everything in a 7 layer logical order the communication\connectivity occurs.

  • Application
  • Presentation
  • Session
  • Transport
  • Network
  • Data Link
  • Physical

Special Exam Mnemonic Learn it and pass the test!

“All People Seem To Need Data Processing”

The Lower OSI levels comprise of the Physical, Data Link, Network and Transport. This deals with what you would most likely troubleshoot first. Why? Because it starts with a physical inspection.

Are all cables (Ethernet, Fiber etc…) and networked cards connected? If they have power, are they transmitting the correct protocol such as TCP/IP? Back in the 80’s to the early 2000’s companies were using token ring, IPX, and other protocols we rarely use today, if at all. For the most part, you can quickly verify your connections by visual inspection and by using the Ping, in addition to command line troubleshooting tools. Often the lights on the network card and switch ports will provide feedback according to the colors and their meaning in the device’s manual.

The Upper OSI levels comprise of the Session, Presentation, and Application layers. The upper levels are involved when you have 2 devices like a computer and even smart phones communicating with one another through a session. When there is a problem on this level most likely there is a glitch in the operating system, program, or app code that can in some cases turn out to be a flaw or bug. The upper levels involve DNS and encryption. Sometimes it may require a simple restart of one or more of the systems to flush out obsolete settings and software.

This is why you will hear from technicians tell you to restart the system(s) first before you contact them.

We could go into extensive details, but for the most part, the systems administrator will work on the first 4 layers in this way:

  1. Connections
  2. Power
  3. Configurations

If you recently added new hardware or changed configurations, you should check your firewall, routers and switch settings for proper port forwarding and routing protocols.

Over time, you will learn your systems like the back of your hand and will know that Port 18 is about to go out or the e-mail server just needs to be restarted for software patch updates.

In time, the main parts of the OSI models that apply to you as a systems administrator or developer will become second nature to you even if you forget at times the OSI Model Proper layer names.

So when someone tells you “Houston, we have a problem.”

You can reply, “All cables, servers, and switch configurations are a go, restart your client’s system first and then I will look at the program or app you are using.

The OSI Model Demystified

This YouTube teaching video by Eli the Computer Guy will expound further on the terms used in the article.

In the video Eli states the DNS is on the network level. You will also hear IT consultants say, as in the article above, that it is on the Application layer.

It is certainly understandable why you will find knowledgeable system administrators state DNS is on the network level. It is due to the reason routers, firewalls and smart switches can and are configured with DNS settings.

All the same, according to the ISO Standards for the OSI model, programs (like e-mail) on the application layer services use the DNS.


James Lynch

POCBOOKS.COM “If something you read changes your life, it becomes your reality.” The author of the Product of Culture Trilogy James is also an Information Technology Professional and Consultant "Whether a person is aware of it or not, like technology, everything has an answer. You can't be afraid to ask questions, You just may receive what you have been looking for!"

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