Wireless communication uses electromagnetic waves to carry signals. These waves require a line of sight, and therefore are obstructed by the Earth's curvature. The purpose of communications satellites is to retransmit the signal around the Earth's curve allowing communication between widely separated points. Communications satellites use a wide range of radio and microwave frequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" that certain organizations can use. This band allocation minimizes the risk of signal interference.