In synchronous time division multiplexing( STDM), transmission frames are defined, each consisting of a certain number of time slots of a fixed size. The situation is different for asynchronous time division multiplexing (ATDM), which does not use fixed reserved time slots.
In STDM, a transmission channel is uniquely identified by the position of the time slot within the transmission frame, and the time slot of a channel is always synchronous with respect to the transmission frame. The transmission frame of the synchronous time division multiplex consists of the synchronization character and the individual channels. If the same number of bits is transmitted in each channel, then, for example, channel 1 starts by inserting its xx bits, it is followed by channel 2, also with xx bits, then channel 3, and so on. The first transmission frame is then finished when all channels have inserted their data. The synchronization channel follows with which the next transmission frame begins.