In data communications systems, control of the rate at which data are transmitted from a terminal so that the data can be received by another terminal. Note 1: Transmit flow control may occur between data terminal equipment (DTE) and a switching center, via data circuit-terminating equipment (DCE,) or between two DTEs. The transmission rate may be controlled because of network or DTE requirements. Note 2: Transmit flow control can occur independently in the two directions of data transfer, thus permitting the transfer rates in one direction to be different from the transfer rates in the other direction.