There are two types of flow control: hardware (RTS/CTS) and software (XON/XOFF). In XON/XOFF flow control, either end can send a stop (XOFF) or start (XON) character to the other end to control the rate of incoming data. In RTS/CTS flow control, the computer and the modem use the RTS and CTS lines respectively to control the flow of data (for example, when the modem's buffer is full). For modems, RTS/CTS flow control is the preferred method as it is more efficient and reliable.
Modems can usually be configured to use hardware flow control. For example, the atdialHAY dialer supports RTS/CTS flow control using the &K3 command.
Hardware flow control for outgoing calls is supported by the atdialer dialer. All of these dialers recognize strings of the form STTY=settings in their /etc/uucp/default files. For example, you can enable bidirectional hardware flow control for the dialTBIT dialer by creating a file /etc/uucp/default/dialTBIT that contains the following line:
STTY= RTSXON CTSXON
Hardware flow control for incoming calls is configured by adding the RTSXON CTSXON flags to the appropriate /etc/ttydefs entry referenced by the /etc/inittab entry for the modem tty port.
The CTSXON flag is often used by itself to prevent the modem buffer from overflowing when speed conversion is used for incoming calls. For example, assume a MultiModem V.32 is set for a fixed speed of 9600bps to the computer. When a 2400bps connection calls in, the computer sends data to the modem at 9600bps, which the modem can only send out at 2400bps. The CTSXON flag in /etc/ttydefs tells the serial driver to stop sending data when the modem drops CTS. This allows the modem to signal when it is busy and prevent data overruns.