Suppose the propagation delay is ?, SIFS is ?, DIFS is 3?, and RTS and CTS are 5?, 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 100? 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?
(a) If a terminal has a packet ready to transmit, it will sense the medium first. If the
medium is idle for DIFS, it will transmit RTS, then waits for the CTS from the
receiver. Therefore, the earliest time for the receiver to send CTS massage is:
3? + 5? + ? + ? = 10?.
(b) When the sender receives the CTS from the receiver, it waits for SIFS, and then
sends its data packet. Therefore, the shortest time for the receiver to send the
ACK signal is:
3? + 5? + ? + ? + 5? + ? + ? + 100? + ? + ? = 119?.
Note that we should include the propagation delay also.
(c) This is to give priority to some packets. For example after transmitting the DATA
packets the receiver should be allowed to send ACK packets without any
collisions. To accomplish this, we transmit the ACK packets immediately after
SIFS time interval before any other node can transmit and interrupt the ACK
packet.
(d) No. SIFS is the turnaround time required to change from transmit (receive) mode
to receive (transmit) mode. Thus it is compulsory to have finite SIFS time.
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