Citizens band radio (also known as CB radio), used in many countries, is a land mobile radio system, a system allowing short-distance person-to-person bidirectional voice communication between individuals, using two way radios operating on 40 channels near 27 MHz (11 m). CB operation does not require a license, and it may be used for business or personal communications. Like many other land mobile radio services, multiple radios in a local area share a single frequency channel, but only one can transmit at a time. The radio is normally in receive mode to receive transmissions of other radios on the channel; when users want to talk they press a “push to talk” button on their radio. Transmitter power is limited to 4 watts. CB radios have a range of about 1 to 10 miles, depending on terrain, for line of sight communication; however, various radio propagation conditions may intermittently allow communication over much greater distances.