Wireless computer networks are playing a major role in modern communications. Laptop computers, palm PCs, cell phones, and embedded systems in common appliances and automobiles may be connected to form an ad hoc network or to a fixed network infrastructure such as the Internet through wireless access points. Wireless access points have been installed in office buildings, homes, airports, hotels, highway service stations, and other facilities, providing people with unprecedented conveniences and flexibilities to obtain and exchange information. People today, for example, can connect their laptop computers to the Internet while sitting in their own backyards or while waiting for flights in the airports. Wireless computer networks have started a new revolution in the information industry.