In an 802.11 WLAN, suppose the duration of the propagation delay is k, transmiss

Question

In an 802.11 WLAN, suppose the duration of the propagation delay is k, transmission time for RTS, CTS or ACK frames is 5k, SIFS is k, and DIFS is 3k. Assume the sender and the receiver use CSMA/CA with RTS/CTS enabled.

Once the sender finishes sending an RTS (say at time 0), the earliest time before it can completely receive the corresponding CTS is _____ k.

suppose the sender sends a frame that is 100k long (transmission time). The minimum total time that elapses in the exchange of RTS/CTS/data/ACK is ______ k.

Measure time at the sender, from the beginning of the transmission of the RTS until the complete receipt of the ACK.

Explanation / Answer

The steps in CSMA/CA with RTS and CTS are as follows:

Propagation delay =k

Transmission time for RTS=5k

Transmission time for CTS=5k

Transmission time for ACK=5k

SIFS=k

DIFS= 3k

Now since sender when complete transmission of RTS is assumed at 0 so transmission time of RTS is not included here.

(a) If the sender has a packet ready for send then it will first wait for DIFS , send RTS and wait for CTS to receive.

The earliest time before it can completely receive the corresponding CTS is

DIFS+CTS+Propagation delay=3k+5k+k+k=10k

(b) Now if the sender receives the CTS it waits for SIFS and then send data packet

Sender sends a frame that is 100k long (transmission time). The minimum total time that elapses in the exchange of RTS/CTS/data/ACK is

DIFS+RTS+delay+CTS+delay+SIFS+100k+k+ACK

3k+5k+k+5k+k+k+100k+k+5k=122k

Measure the time at sender and cross check the same with the calculation.

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