ng a variety of devices, including note- books, PDAs, and emerging mobile devices. Without reliable and secure mobile access to enterprise networks, mobile workers will not be able to be productive. Another area in which we want to use our freedom to be mobile is with the Internet. The mobile Internet is already a reality in many places, and it will become more prevalent everywhere in the near future, as more and more Internet- ready mobile phones and other wireless devices make their way into people's pockets, accessories, and clothing. According to Ray Kurzweil, in his brilliant book The Age of Spiritual Machines, wireless is predicted to become the dominant form of access to the Internet by 2009. Mobile entertainment is yet another appli- cation area, with interest and services growing daily. These trends, as well as the trend toward machine-to-machine communications, sensor-based networks, and Chapter 13 Wireless Communications Basics portable computing in the form of wearables, contribute to our avid interest in wireless communications and systems. We used to think of wireline facilities as the default and of wireless as some- thing extra that needed to be justified. Wireless was traditionally justified only in situations such as when wire could not be physically applied (e.g., in a jungle, in a desert, on a mountaintop); when wire could not be economically justified (e.g., when the expense of burying cable was too great); when time was an issue (the time to deploy a cable system can range from 9 to 18 months, while wireless sys- items can be deployed in 3 to 6 months); and of course, when mobility was a requirement. We are now beginning to see mixed-media approaches (including both wireline and wireless solutions), depending on the situation. For example, we may want to bring fiber as close as possible to an end node, but to engage in a more mobile lifestyle, we might use a wireless link for the last few meters. This chapter discusses the history of wireless communication, wireless com- munications regulati