An almost synchronous stream consists of a sequence of proper messages and pauses transmitted over a directed channel. The paper presents the transformational design of a communicating component which coordinates two almost synchronous streams. A proper message on one input channel is forwarded to the corresponding output channel as soon as there arrives a proper message on the other input channel as well. We refine the stream-based input/output behaviour to a state-based implementation exploiting two major transformations. The first transformation refines the component's infinite behaviour to a finite behaviour. The second transformation implements the component's finite behaviour by a state transition machine. The component's internal state is extracted from the input histories using a history abstraction function. Altogether, we explicate a formal method how to refine a specified infinite input/output behaviour of a communicating component by a state transition machine in the setting of almost synchronous streams.