Modern mobile phones are relatively simple in design but include a great deal of extremely complex technology that allow them to function. A basic mobile phone consists of a small speaker, a small microphone, an antenna, a radio transmitter, a tiny hard drive, a portable microchip called a SIM card and a lithium-ion battery. All of this is generally encased in a plastic frame or shell of some variety. Together, these elements provide a remarkable device that is commonly referred to as a mobile phone Mobile phones communicate via radio waves. In order to work, a mobile phone must be in communication with a cell site. This is why mobile phones are sometimes referred to as cell phones or cellular phones. Cell sites are large radio antennas, not unlike those that transmit FM radio, and they are often placed on top of tall buildings or at the highest geographical point in an area. One cell site gives about an 8-mile radius of cellular coverage for mobile phones. The mobile phones communicate with cell sites using radio waves, which are then transmitted from the cell site to a wired phone network. As mentioned above, mobile phones only have an effective range of roughly 8 miles. This means that they must be within 8 miles of a cell site in order to function properly. Mobile phones also have relatively limited battery capacities. Although lithium-ion battery technology has improved dramatically over time, battery life is still fairly short. After a few hours of talking time, a cell phone will need to be recharged before being used further. If left idle and not used, however, most cell phones can go up to several days before requiring recharging.