er devices in the immediate area (up to 30 feet [10 m]). UWB is like a twenty-first-century version of Marconi's spark-gap transmitter, which was based on short electromagnetic pulses, transmitting a whopping total of 10bps. However, UWB can send more than 100Mbps, with the potential of up to 1Gbps. The basic concept is to develop, transmit, and receive an extremely short- duration burst of radio frequency energy, typically a few tens of picoseconds (tril- lionths of a second) to a few nanoseconds (billionths of a second) in duration. UWB can not only carry huge amounts of data over a short distance at very low power but also has the ability to carry signals through doors and other obstacles that tend to reflect signals at more limited bandwidths and higher power. Familiar forms of radio communications use what is called a carrier wave. Data messages are impressed on the underlying carrier signal through modulation of the amplitude, frequency, or phase of the wave in some way and then are extracted upon reception. UWB does not employ a carrier wave; instead, emissions are com- posed of a series of intermittent pulses. By varying the pulses' amplitude, polarity, timing, or other characteristics, information is coded into the data stream. This is similar to the technique used in radar applications. UWB operates at a very low power level, 0.2 milliwatts, thus restricting its range to distances of 300 feet (100 m) or, more typically, as little as 30 feet (10 m). Chapter 15 WMANs, WLANs, and WPANs Because the energy levels of the pulses are simply too low to cause problems, inter- ference from UWB transmitters is generally not an issue. A UWB transmitter radi- ates only 1/3,000 of the average energy emitted by a conventional 600-milliwatt mobile phone, which means it reduces many of the health concerns being expressed and studied in relationship to cellular and PCS networks. Advantages and Disadvantages of UWB UWB offers a number of advantages, including the fact that there is growing demand for greater wireless data 