The OSI Model - Reworked

July 25, 2007

The Open Systems Interconnection Basic Reference Model (OSI Model) or 7-layer model has been around for decades. It was actually developed in 1977 by the ISO as an abstract model of networking protocols which is divided into seven layers. Each layer interacts with the layer beneath it or above it depending in the directionality of the conversion. The 7-layer model was written from a broader point of view and in today's world is not really used as it was intended. The most common protocol and the one most of us have heard of is the Internet protocol know as TCP/IP. The TCP/IP model only uses four of the layers to more simplify the architecture making it streamlined and easier for most to understand. Here are both models and a brief description and example of each.

OSI Model (7 layer)

  1. Physical – The electrical and physical connections for devices (example: wires, electrical signals, hubs, network cards)
  2. Data Link – Functional means of transferring data between network devices via switches and protocols (example: Ethernet, Token-Ring and switches)
  3. Network – This layer is responsible for transferring data between multiple networks via routing protocols (example: Internet Protocol (or IP), ARP, and RIP)
  4. Transport – this layer provides a reliable transparent transfer control of data between hosts (example: TCP and UDP)
  5. Session – This layer controls the connections between hosts. Establishes, maintains, and terminates connections between hosts. (example: NetBIOS and DNS)
  6. Presentation – This is the layer the data is transformed and formatted to provide a standard interface for the Application layer (example: ASCII to XML conversion)
  7. Application – Provides services and data to user defined applications (example: RPC, FTP, HTTP)

 
TCP/IP Model (4 layer)

  1. Network Access – This is the physical layer like cables, hubs, switches, and routers necessary for communications
  2. Internetworking – This is the IP address and layer that allows hosts to be able to find one another on the Internet
  3. Transport – Connection protocols like TCP and UDP operate here. This layer deal with the opening, maintaining, and closing connections between hosts
  4. Process/Application – High level protocols like HTTP, FTP, SMTP, POP3 operate

Back in the day when I got started in the networking field for a small ISP in Dallas, I had to study the OSI model for Cisco exams. I had to use anagrams to remember the different layers such as (P-D-N-T-S-P-A) "Please Do Not Throw Sausage Pizza Away" or in reverse (A-P-S-T-N-D-P) "All People Seem To Need Domino's Pizza" so I could keep them all straight. Once I was actually starting to get my hands wet in the field, I found the OSI model to come in handy when trying to troubleshoot networking issues. I had to morph the definitions of the layers to fit my needs, and used them as a daily reference on how to isolate issues and come to a resolution by stepping up and down the layers. Here is what I used to simplify my life when troubleshooting a dial-up connection (analog, ISDN and sometimes a T1).

Network Troubleshooting (7 layer/step)

  1. Physical layer – Is there good working cable between point A and point B? (the router and switch let's say). Have you tested the cable to make sure it works?
  2. Data link layer – Is there a link light on the router or switch? Is it plugged into the correct ports on both ends? Are the port speeds and duplex settings on either end matched up? (10/full, 100/full or 1000/full)
  3. Network layer – Can I ping across the link from the router to the switch? Am I using the correct IP address information?
  4. Transport – Am I able to get out of the local network? Is there a firewall that might be blocking something? Is the default gateway setup correctly?
  5. Session – Am I able to reach (ping) the end host I are trying to reach? (the web server in this case)
  6. Presentation – Is the service I am trying to reach installed and running? (like IIS or Apache) Is there a firewall blocking inbound requests? (hardware or software)
  7. Application – Is there actually content on the server to present? (HTML pages) Does the web server config have the appropriate permissions applied and pointed to the correct directory for content?

I know this might seem a little simplistic, but sometimes getting back to the basics is the best way to solve problems. It is also an effective way to teach people interested in networking how to troubleshoot issues that come up in our industry on a daily basis. I hope you find this approach useful and apply it in your environment.

A funny little known factoid is that when we started this company a couple years ago, the OSI model actually came up when designing our logo. When brainstorming and jotting down ideas one of our founders (guess who?) used the 7 Layer theme to design our current logo. Shows you how influential the OSI model has been in today's Internet driven world.

Let's see what anagrams you can come up with for "P-D-N-T-S-P-A" or "A-P-S-T-N-D-P" Give me your best shot. (keep it clean though!)

-Ric

Comments

August 26th, 2007 at 3:05pm

A-P-S-T-N-D-P:

All People Seem To Need Dirt P...

Fill in for yourself, you said to keep it clean. :P

 

      

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Comments

August 26th, 2007 at 3:05pm

A-P-S-T-N-D-P:

All People Seem To Need Dirt P...

Fill in for yourself, you said to keep it clean. :P

 

      

Leave a Reply

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