274 C h a p t e r 13 · S e r v e r Tr o u b l e s h o o t i n g upper Layers Finally, if you've tested the first three layers and can't find a problem there, the network infrastructure is probably working fine. It's time to move to the upper layers. Look at the configuration settings in your server applications and ser- vices. Verify that the authentication mechanisms are installed and configured correctly. Try using different tools and software that provide the same basic functionality. Do they work? If so, there may be a compatibility problem with the specific application you're using and the hardware on which it is operating. As you can see, the OSI model method of troubleshooting can help you both focus and move through a sequence of testing procedures until you find the actual source of the networking problem. I've only touched on the concept here, but you can take it further by learning more about what happens at each layer and the tools that can be used to test at that layer. For example, you can use a spectrum analyzer to test and troubleshoot the physical layer of a wireless net- work and a protocol analyzer to inspect the data-link layer of wired and wireless networks. For more informa- tion on the OSI Model, see Microsoft Windows Networking Essentials book by Darril Gibson (Sybex, 2011).