Serial Communications

I'm currently working on serial communications among CNC Controllers and PCs. This communication is based on Xon/Xoff Flow
control. In this communication PC send a Packet of 16k to the controller.But problem is that when controller sends to PC an Xoff
character during transmission of any packet, PC receives this character after sending almost 16 to 20 charcters, this means
Controller lost so many data during transmission because PC does not quickly stop the transmission.
How to stop the transmission as soon as possible?
Environment: Windows 95/NT , Language: Visual C.