An analog signal has a base carrier's radio frequency signal, which is modified in some way to amplify the strength of the signal or vary the frequency to add information to the signal. An analog signal can be represented as a series signal to a signal carrier known as sine waves because carrier waves are analogous to the fluctuations of the human voice or other sound that is being transmitted. Advanced Mobile Phone System (AMPS), the first common cell phone system in the U.S., uses a range of frequencies between 824MHz and 894 MHz for analog cell phones. A Hertz (Hz) is equal to cycles per second, while a MegeHertz (MHz) is equal to one million cycles per second. The frequencies chosen to be used in analog voice channels are 30 kHz wide, because it gives voice quality that is comparable to a wired telephone. The transmitter and receiver frequencies of each voice channel are separated by 45 MHZ, to keep them from interfering with each other. Each carrier has 395 voice channels and 21 control channels for activities of registration and paging. Each cell only uses about one-seventh of its frequencies. This helps a hexagonal cell and the six cells in the grid to all use the frequencies. Digital cell phones use the same radio technology in a different way. For example, digital phones change voice into binary information (1 and 0) and then compress it. This compression allows ten digital phones to occupy the same frequency space as one analog cell phone.