There are two channels between the transmitter and receiver: an outward and a return channel. Each participant in a bidirectional connection can thus be either a transmitter or a receiver. In bidirectional operation, a distinction is made between those in which only one station can transmit and the other receive, i.e. half- duplex operation, and those in which both stations can transmit and receive simultaneously, i.e. duplex or full-duplex operation.