Despite their popularity and wider deployment, IEEE 802.11 WLANs have been found to be vulnerable to security threats soon after their emergence requiring adoption of security measures. However, the introduced security measures didn't provide solutions for Denial of Service (DoS) attacks. This book characterizes the DoS attacks based on their ease of applicability and the degree of severity they introduce, and evaluates countermeasures for efficiency and effectiveness in defending against the attacks. Among various DoS attacks, Authentication Request Flooding (AuthRF) and Association Request Flooding (AssRF), which are practical and needing lesser effort to cause damage, were selected for study using OMNET++ simulation environment embedding INET Framework. Designs and models have been developed for the selected attacks and for two versions of defenses: countermeasure and enhanced countermeasures. The studied attacks, AuthRF and AssRF, required less effort and caused severe damage. Both versions of defenses, Countermeasure and Enhanced countermeasure, are effective although they incur delay.