In order to understand how and why 802.11 (WiFi) devices behave as they do, it is invaluable to perform a wireless packet capture ("sniffer".) This can be especially important when working with Cisco TAC to resolve a technical problem. The following articles will help you to choose and use a wireless sniffer.
To analysis these captures, refer to the links below, they are designed to be read in order since each document will build upon the preceding document. Bear in mind that when reading any wireless trace, its a good idea to understand the 802.11 Wireless specifications. While these documents will do a great job at helping you understand the packet flow and what to look for in a wireless trace, they are not meant to teach the 802.11 Wireless specifications.