XON/XOFF flow control

Flow control

There are six different links involved in a connection: transmitting computer to transmitting modem, transmitting modem to receiving modem, receiving modem to computer, and the reverse of all three links. They can all have different data transmission rates. When the receiving modem is temporarily unable to

accept the data, it needs a way to tell the transmitting modem to slow down or wait for it. Flow control is the method by which a modem controls the rate at which other modems send data to it.

If flow control is improperly configured, it may be impossible to connect to a remote system, the transfer rate may be considerably slowed, or the connection may be disrupted. If you see many errors and retransmittal of data when downloading files, check both the modem and the communication program settings for flow control. Flow control settings for both the communication program and the modem must be the same. Many communication programs set these automatically based on the modem, but some must be configured separately.

Hardware flow control

Hardware flow control(RTS/CTS) depends on the modem to control the flow of data. This should be used with all high-speed modems or modems that compress data.

Software flow control

Software flow control(also called XON/XOFF or CTRL+S/CTRL+Q) uses data characters to indicate that the data flow should start or stop. This enables a modem to send a control character to signal another modem to stop transmitting while it catches up.

Software flow control is slower and usually less desirable than hardware flow control. Software flow control is used only for transmitting text. It cannot be used for binary file transfer because binary data may contain the special flow control characters.