The OSI model is a great tool that we can use to isolate any problem a computer is having.
It is actually a model used to show how data travels up, through the layers if you will, from one computer to another.
- Physical
- Data Link
- Network
- Transport
- Session
- Presentation
- Application
So there it is. We can work our way from the bottom to the top, or from the top to the bottom. Typically you would actually look at the layers with the Application layer at the top.
Anyway, how does it relate to troubleshooting?
It basically takes you through every thing a computer needs to do in order to take input from the user, set up a means and method for talking with another computer, encode it properly, attach an address to the data, and physically send it over the cable. When things stop working, it’s a great resource you can use to just use the process of elimination to rule things out.
The Application Layer
Ask yourself if other applications are having the same problem.
For example, if you are getting an error when you try to print, try to print from a different application.
If you get the same message, try another, and another. If you continually an error message, the problem is very likely not isolated to the application, and you need to look deeper. If you only get the error message within that application, depending on the error, I would either look it up on the internet, look through your system’s event logs, or look for another place the application might be logging. Where to proceed next from this is a bit of a personal call- it depends on how much experience you have with this type of problem. If you’re unfamiliar with it, you probably need as much information as you can get. Look for logs, and start with the eventlog.
The Presentation Layer
On this layer, such things as encryption, formatting, and compression take place. The name of the layer actually describes it best, if something doesn’t look right, say you’re using an instant messenger client and the text is a garbled mess, things may be going wrong here. A printer driver might be an example of something that operates in some capacity on this layer. If you’ve ever used the wrong printer driver for a printer and tried to print something out, it doesn’t work very well. You’ll notice that it happens from every application, so you can rule that out, and you’ll also notice that the layers below this one also seem fine. Usually you just need to reinstall an application or specific file (DLL?) to resolve errors here.
The Session Layer
A “session” can be thought of as similar to a phone call. When you call someone, a certain process takes place where your phone connects with your phone company and gives you a dial tone. Once you dial the number and someone picks up the phone on the other end, a session has been established.
This layer is responsible for making that call. The establishment of a connection with the right parameters, protocols, etc happens here. If an end-to-end connection is not being made properly, look to this layer.
I would argue that Windows Firewall operates on this layer. Reason being you can tell it to allow or disallow certain applications through, rather than just ports or protocols.
The Transport Layer
Aah, this layer is a bit fuzzy. It provides error recovery, flow control, and sort of acts as a mediator between the top layers and the bottom layers. Up to now the layers have been somewhat software-ish in nature. Most of what happens up to now can be done differently depending on the operating system. Below this layer is more hardware centered. This is the bridge. It can be difficult to troubleshoot here, however if things don’t seem to be getting to the other end and the lower layers have been ruled out, this could be a possibility. ”Dumb” load balancing runs here.
The Network Layer
Establishes, maintains, and terminates a connection. You may notice a similarity here to the session layer; this is more or less the hardware equivalent. There’s actually a big difference. A session can use multiple connections, or multiple sessions can use a single connection, depending on how the application works. A connection on this layer is cut and dry- it either exists or doesn’t.
Routing and relaying are done at this layer as well, so most routers run here. Some run on level 4 but that’s for another day. The various routing protocols sit here.
The Data Link Layer
I always have trouble with defining this layer. It’s the first/last level of “sorting” that gets done on any “frame” of information. It knows how to recognize what is the start and end of a frame for passing to the next layer, and is responsible for making sure of a reliable connection.
The Physical Layer
This would be the actual network card and cable. Technically this layer defines everything from how the NIC is made to the RJ-45 jack to the cable and which standards it follows. But in troubleshooting this layer is much more broad and from it are derived two of my favourite questions;
“Is it plugged in?” and “Is it turned on?”
Useful? Leave me a comment. :-)
Originally posted 2009-04-06 22:01:19.
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