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What makes up a wireless LAN (WLAN)?  Is it just the Access Point(s) and any associated WLAN controller(s)?  Does it include anything else?  Personally, I think of wireless LANs as a system.   In my mind, there are three main parts to this system:

1. The infrastructure
2. The clients
3. The environment

The Infrastructure

This includes not only the access point(s) and the WLAN controller(s), but any of the routing & switching gear used for uplinks.  Also included in the infrastructure would be any of the network infrastructure / services leveraged by the wireless LAN (examples include RADIUS, Network Time Protocol, Syslog, etc.) 

The Clients

The IEEE 802.11 specification refers to these as “stations”.   Clients/stations could be computers such as laptops or tablet PCs.  Other examples of wireless stations include dual-mode phones, handheld scanners, etc.  When thinking about the clients, realize this includes not only the hardware, but the software (such as a client driver or management supplicant). 

The Environment

This most commonly refers to the Radio Frequency environment.  The RF environment can be evaluated with a spectrum analyzer, which can show you the signal level of your APs, the noise floor, and any interference in the area.  Interference can come from other Wi-Fi devices, or non Wi-Fi devices (such as microwave ovens, cordless phones, and Bluetooth).  I also think of the Physical environment, which is the type of construction materials used, the amount of vegitation in the area, the terrain/line-of-sight, etc. 

I often get called out in troubleshooting situations.  In many of these cases, the infrastructure has been evaluated as a potential source for issues (for example, are the authentication/encryption settings correct), but the clients and the environment have been neglected. 

Conclusion

In these troubleshooting scenarios, it is imperative to take a thorough look at all three components.  Not only should you take a look at the configuration file of the infrastructure, but ensure that wireless clients have the most up-to-date drivers.  Look to see that clients power-save and roaming agressiveness settings are appropriate for the application.  Double-check to see there is no interference from Wi-Fi or non Wi-Fi sources, etc.  In short, evaluate your WLAN as a system.