Ahmed, Here is the link the serial comunication tutorial available on www.8052.com

http://www.8052.com/tutser.phtml

RTS - Request to Send.
CTS - Clear to send.
are not used to determine when the data is coming but are used to inform the connected device weather the other device is ready or not accept further data.

for example If you are connecting a PC with a 8051.
RTS - would mean that PC is ready to accept data from 8051.
CTS - would mean that 8051 is ready to accept data from PC.

CTS and RTS probably you would use when your read buffer is full and before processing the accepted data you have no rool left to accept further data. for example your 8051 can pull CTS when it can'nt accept any further data. PC will stop sending data untill CTS is again to its normal sate.