over a wide distance as people move around on foot or in cars and public transportation. FINITE SPECTRUM FOR WIRELESS NETWORKS On wireless networks, spectrum consists of the invisible electric energy used to trans- mit signals. The spectrum over which signals are carried is a critical asset that carri- ers require to operate their mobile networks. Governments regulate the allocation spectrum because the availability of mobile networks in national defense and public emergencies is vital. They also designate uses for each portion of spectrum and set the terms of use by carriers and other entities that use spectrum. The Division of Airwaves into Frequencies Spectrum is divided into and allocated by frequencies. A frequency is the number of times per second that a radio wave completes a cycle. A cycle can be imagined as a letter S lying on its back, such that it appears similar to looking at a cross-section of an ocean swell. A cycle is complete when energy passes through an entire radio wave from the highest to the lowest portions of the wave. For a visual depiction of a com- plete wavelength cycle, see Figure 7-1. Frequency is measured in hertz, which refers to the number of complete cycles per second. Thus, for a frequency of 30 million hertz-or megahertz (MHz), as it is called-energy passes through 30 million resting S's in one second. Mobile and Wi-Fi Networks Radio Wavelengths Long wavelength, low frequency Short wavelength, high frequency Figure 7-1 A comparison of short, high-frequency wavelengths and long, low-frequency wavelengths. The Characteristics of Short and Long Wavelengths Spectrum is divided into frequency ranges from low ranges of frequencies of about 30MHz to 300MHz allocated to government agencies, local police, highway, and state police to high frequencies such as 38 to 40 gigahertz (GHz) for fixed, point-to-point wireless service. Each range of frequencies is measured by the wavelength in the range. Wavelengths in low frequencies, such as AM radio at 100 kilohertz, measure 9,000 feet (3,000 me