1. Request To Send/Clear To Send :
RTS/CTS, as name suggests, is a flow mechanism in which Data Terminal Equipment (DTE) i.e. host assets or send RTS to Data Communication Equipment (DCE). This is simply done to indicate that host is ready to send data and modem can initiate or form a communication channel. Then DCE further asserts or send CTS to grant permission i.e. yes you can send data. RTS simply indicates that host wants to send some data where as CTS simply indicates that yes you can start sending data. Here, host can be computer or any other device and DCE can be modem.
2. Data Terminal Ready/Data Set Ready :
DTR/DSR, as name suggests, is a flow mechanism in which DTE i.e. host asserts or send DTR to DCE i.e. modem simply to indicate that host is ready for communication and modem can start or create a communication channel. DCE then further assets DSR to simply indicate that receiver is ready for communication.
Difference between RTS/CTS and DTR/DSR :
