software flow control is a method of flow control used in computer data links, especially rs-232 serial. it uses special codes, transmitted in-band, over the primary communications channel. these codes are generally called xoff and xon (from "transmit off" and "transmit on", respectively). thus, "software flow control" is sometimes called "xon/xoff flow control". this is in contrast to flow control via dedicated out-of-band signals — "hardware flow control" — such as rs-232 rts/cts.