 Hardware handshaking is done with extra signals, often the RS-232 RTS/CTS or DTR/DSR signal circuits.Generally, the RTS and CTS are turned off and on from alternate ends to control data flow, for instance when a buffer is almost full.DTR and DSR are usually on all the time and, per the RS-232 standard and its successors, are used to signal from each end that the other equipment is actually present and powered-up.