Citizens' Radio Service, commonly called Citizens Band radio (also known as CB radio), is in many countries a system of short-distance radio communications between individuals on a selection of 40 channels within the 27-MHz (11 m) band. Citizens' Band is distinct from the FRS, GMRS, MURS and the Amateur Radio Service (ham radio). In many countries, CB operation does not require a license, and (unlike Amateur Radio) it may be used for business or personal communications. Like many other two-way radio services, Citizens' Band channels are shared by many users. Only one station may transmit at a time; other stations must listen and wait for the shared channel to be available.