Flow control provides a mechanism for suspending transmission while one device is busy or for some reason cannot further communicate. The DTE and DCE must agree on the flow control mechanism used for a communication session. There are two types of flow control: hardware and software.
Hardware Flow Control
Software Flow Control
Uses dedicated ASCII control characters (XON / XOFF) to control data transmission. Software flow control requires text-based communication data or other data that does not contain any XON or XOFF characters.