XON/XOFF software flow
Transmitter on/transmitter off (XON/XOFF) flow controls involves the sending of data transmission control characters along the data stream (TxD and RxD). For this reason it is referred to as software flow control.
When data is sent to a modem, it is placed in a buffer. Just before that buffer reaches its maximum capacity, the modem will send an XOFF character to the system and the system will stop transmitting the data. When the modem's buffer is almost empty and ready for more data, it will send an XON character back to the system causing more data to be sent.