Communications Engineering. Data transmission over a communications channel between two machines is characterized by the direction of the exchanges, the transmission mode and the synchronization between transmitter and receiver. As regards the direction of the exchanges, a distinction between simplex, half-duplex and full-duplex connections is feasible. The transmission mode describes the number of bits that are simultaneously transmitted on a transmission line. In this context, a distinction between serial and parallel transmission is feasible. Serial connections are more often used than parallel connections. As a consequence of the usage of serial connections that are performed on single links, synchronization between transmitter and receiver is required. This synchronization can either be asynchronous, in which each information bit is sent at irregular time intervals, or synchronous, in which transmitter and receiver are paced by the same clock.