P6.17. Suppose the propagation delay is a, SIFS is a, DIFS is 30, and RTS and CTS are 50, respectively, for CSMA/CA with RTS/CTS. (a) What is the earliest time for the receiver to send the CTS message? (b) If the data packet is 100a long, what is the shortest time for the receiver to send the ACK signal? (c) Explain why SIFS is kept smaller than DIFS. (d) Can you make SIFS = 0?