Using the OSI model to troubleshoot video

Last Updated on November 28, 2016 by Dave Farquhar

I was at church on Sunday and the video projection wasn’t working. After a few minutes of watching everyone struggle, I volunteered to take a look, and working together, we were able to get the video working again using a simple, repeatable methodology: Using the OSI model to troubleshoot video.

I’m going to share that methodology now.

Normally we think of the OSI model in terms of networking, but it makes sense to apply the same type of thinking elsewhere too. Everywhere in the computer, it’s encoding, transmitting, and decoding electrical impulses, so if we step through the layers, we can quickly make sure we’re not missing something obvious.

Here are the layers of the OSI model:

Data Link

It helps to start at one end and work toward the other. I tend to start wherever is closest–if I’m at the keyboard, I start with the application that’s running. If the cabling is accessible, I’ll start at the physical layer.

Sometimes the lines between layers are very blurry, and in this case they are, but even in the case of video, the progression was logical.

Physical. I started by looking at the cables. Were they plugged in and running where they were supposed to go? Since I’d never seen this computer system before, this gave me a quick assessment of how it worked too. I knew at that point it had a PNY video card with two outputs.

Data Link. Was the monitor getting video? It indicated it was, in spite of the black screen. Had I been getting “no signal,” I would have known to step back to the physical layer and start reseating or replacing cables.

Network, Transport, Session, Presentation. Here’s where things get fuzzy. Knowing it had a PNY video card, I knew there was an Nvidia or AMD driver involved. I checked the Nvidia driver when I found it, and saw it was functioning, saw the second monitor, and the second monitor was enabled. Next, I opened a window and dragged it off the edge of the screen. It showed up on the bigger malfunctioning display. Where in the stack these steps lie concern me less than making sure I covered them.

Application. “It’s this program,” I said, pointing at the app that was running. It wasn’t using the secondary display the way it normally did. So they shut down the app and restarted it and video started working.

I ended up taking the long way home, but still, all those checks took less than 90 seconds to step through, and it ensured we didn’t miss anything in the middle.

One large company in St. Louis looks for people who use this methodology in their job interviews. I won’t call the company out by name, but it provides them a competitive advantage. I know of other companies that administer expensive and cumbersome tests to learn what this company is learning with an assessment that takes less than five minutes. It’s worth doing, because when you find people who can do this, you know you have a lot to work with even if their other skills might be a bit raw. It can help you find the talent other companies are overlooking.


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