Operation in which transmission occurs in one and only one preassigned direction. Synonym one-way operation. Note: Duplex operation may be achieved by simplex operation of two or more simplex circuits or channels. 2. Operating method in which transmission is made possible alternately in each direction of a telecommunication channel, for example by means of manual control. Note: In general, duplex operation and semiduplex operation require two frequencies in radiocommunication; simplex operation may use either one or two. [NTIA] [RR] Note 2: These two definitions are contradictory, however, both are in common use. The first one is used in telephony and the second one is used in radio. The user is cautioned to verify the nature of the service specified by this term.