11-2 CHAPTER 11: FLOW CONTROLHardware andSoftware FlowControlThere are two kinds of flow control: hardware and software. BusinessModems support both, but your computer and communications softwaremust also support the kind of flow control you choose.Hardware FlowControlBusiness Modems implement hardware flow control by detecting that abuffer is 90% full and then interrupting the Clear to Send (CTS) signal tostop the flow of data. When the buffer drops back to 50% full, theBusiness Modem sends CTS to restart the flow of data.Software FlowControlBusiness Modems implement software flow control by detecting that abuffer is 90% full and then sending special characters in the data streamto stop the flow of data. When the buffer drops back to 50% full, theBusiness Modem sends special characters in the data stream to restart theflow of data.The problem with software flow control is that the characters used tostop (Q) and start (S) the flow of data can occur naturally inthe data flow. Enabling software flow control instructs the BusinessModem to recognize and act on these characters, even if they are notintended to control the data flow.Using software flow control may prove satisfactory if you're transferringtext files only.The start command is called XON (for transmit on) and the stopcommand is called XOFF (transmit off). You can change the charactersused. See Registers S22 and S23 in Appendix B, Alphabetic CommandSummary.
