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XOFF/XON is a protocol for controlling the flow of data between two computers or between two devices. During data communication, a connection is usually established via ports. The "X" in the name means "transmitter," so XON and XOFF are commands for switching a transmitter on or off, respectively. When sending binary data, the XON/XOFF command may not be recognized because it is character encoded.
The actual code used by XON is equivalent to the ASCII code for the Ctrl-Q keyboard combination (DC1, which is 17 in decimal), whereas that for XOFF it is the equivalent ASCII code for Ctrl-S (DC3, which is 19 in decimal). For example, when a USB drive is attached via a USB port of a computer and a data file is to be sent from the computer to the USB drive, when the receiver's buffer reaches a point where it cannot accept any more data, it sends an XOFF command to the transmitter. As soon as the transmitter reads the XOFF signal, it stops the transmission process and only continues after getting a corresponding XON command.