Mechanism
When one end of a data link is unable to accept any more data (or approaching that point), it sends XOFF to the other end. The other end receives the XOFF code, and suspends transmission. Once the first end is ready to accept data again, it sends XON, and the other end resumes transmission.
For example, imagine a computer connected to a slow printer. The computer starts sending data to the printer. Since the computer is faster than the printer, the printer falls behind and approaches a situation where it would be overwhelmed by the data. The printer reacts to this by sending XOFF to the computer. The software on the computer "sees" the XOFF, and temporarily stops sending data. Once the printer has "caught up", it sends XON to the computer. The computer sees XON, and starts sending data again.
XOFF/XON can be employed in both directions—for example, two teleprinters connected to each other.
Read more about this topic: Software Flow Control
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