The application can attempt to manually control the flow of data by using the following IOCtls:

• Transmit Immediate (ASYNC_TRANSMITIMM)
  
• Stop Transmit Behave as if XOFF Received (ASYNC_STOPTRANSMIT)
  
• Start Transmit Behave as if XON Received (ASYNC_STARTTRANSMIT)

The physical device driver automatically controls the flow of transmitted data based upon the reception of XON/XOFF characters. This is referred to as Automatic Transmit Flow Control (XON/XOFF). The physical device driver also attempts to control the flow of data that is received by automatically transmitting XON/XOFF characters to the system it is communicating with, based on the amount of space left in the receive queue. This is referred to as Automatic Receive Flow Control (XON/XOFF).