DCF timing problem. Note that you should draw out the message exchange diagram before presenting the computation.

Node A is sending frames to Node B using the IEEE 802.11 DCF. Node B has no data to send. Refer to the DCF related Figures in the last page.

Assume that SIFS equals 20 ?sec. and DIFS is 50 ?sec.

Node A has many MAC frames to send. Assume that receipt of the acknowledgement from sending a previous frame has just been completed, and this is time t = 0. The next three frames to be sent are frames #1, #2, and #3.

Transmission speed = 2 Mbps

Slot time = 5 ?sec.

Propagation delay is negligible.

The length of the data portion of each MAC frame is 2000 bits.

The header and trailer of the MAC frame total 34 octets.

RTS, CTS, and ACK messages are 34 octets each.

Assume that if Node A needs to backoff, it will pick successive backoff time slots as follows: 16, 5, 3, 28, 14.

a. Assume that when Node A tries to send its frames it always finds the medium to be idle. Node A is not using RTS/CTS. Give the times when the acknowledgements will be received for Frames #1, #2, and #3.

Time when acknowledgement is received for Frame #1: ________________

Time when acknowledgement is received for Frame #2: ________________

Time when acknowledgement is received for Frame #3: ________________

b. Assume that when Node A tries to send its frames that it always finds the medium to be idle. Node A is using RTS/CTS this time. Give the times when the acknowledgements will be received for Frames #1, #2, and #3.

Time when acknowledgement is received for Frame #1: ________________

Time when acknowledgement is received for Frame #2: ________________

Time when acknowledgement is received for Frame #3: ________________

c. Assume that when Node A tries to send its frames that it always finds the medium to be busy when first trying to send each frame and for 50 microseconds after that. It does not find the medium busy after that 50 microseconds (during which it is trying to send that frame). At the beginning of trying send Frame #2, it also finds the medium busy for 50 microseconds, then not busy. Same for Frame #3. Node A is using RTS/CTS this time.

Give the times when the acknowledgements will be received for Frames #1, #2, and #3.

Time when acknowledgement is received for Frame #1: ________________

Time when acknowledgement is received for Frame #2: ________________

Time when acknowledgement is received for Frame #3: ________________