We know from the previous pages that a signal can have many frequency components. The frequency components are called frequency spectrum. The frequencies are spread out over 1,000 to 5,000 Hz. The bandwidth of frequencies is 4,000 Hz. Frequency spectrum of a signal is all frequency components compose a signal in the frequency domain. Bandwidth of a signal is representing the width of its frequency spectrum. The frequency band is very important in signal transmission. The frequency bands of signals should not be overlapped with each other. The frequency overlapping of the signals means that there are interferences. Therefore, there is agreement on the use of radio waves. Agreement on use of radio waves is avoid radio wave disturbance particularly when radio waves are used without permission, each country has its own radio regulation laws and controls the use of radio waves. There are also international agreements on the use of radio waves to promote the efficient use of radio waves.