The mechanism by which a modem controls the rate at which it receives data from another modem. You can also use flow control to describe data rate control mechanisms between other devices, such as computers and attached printers, or between CSU/DSUs (Channel Service Unit/Data Service Units) and routers. Flow control is sometimes equated with handshaking, but the term “handshaking” specifically refers to flow control negotiations that take place at the beginning of a communication session, while the term “flow control” also can apply to data transmission management during an active communication session. In general modem technologies, two basic types of flow control exist:
Software flow control is slower and less reliable than hardware flow control because a user, program, or line noise might inadvertently generate a stop signal for the remote modem. In addition, software flow control is used only for transmitting ASCII text information, not for binary data files, because the binary data might contain the Ctrl+S stop character and cause the remote modem to stop transmitting data.
Graphic F-17. Flow control.