PollEv

Question 1

Implications of the use of ARQ for QoS? (2)

Answer 1

• Reduces PER, increases delay jitter
• Reduces PLR, reduces throughput (slightly)

Question 2

Implications of the use of buffering for QoS?

Answer 2

Reduces throughput jitter, increases delay

Question 3

Successful transmission of data in 802.11 DCF protocol?

Answer 3

Question 4

Scenario: Node gnerates a CW of 6 slots. Sense medium to be free for 2 lots then sense another node accessing the channel. What should the node do when it next sense the channel to be free?

Answer 4

Transmit Data

Question 5

Scenario: Node generates a CW of 6 slots. Sense medium to be free for 6 slots then transmits its data and recieves an ACK. What should the node do next?

Answer 5

Contend again w/ new random CW

Question 6

What duration should DIFS be compared to SIFS?

Answer 6

DIFS > SIFS

(DCF Inter Frame Space) > (Short Inter Frame Space)

Question 7

What duration should PIFS have?

Answer 7

SIFS < PIFS < DIFS < EIFS

(Point co-ordinated Inter Frame Space)

Question 8

Scenario: A node waiting to transmit data sense the channel to be free. The channel remains free until it completes counting down its CW. The data waiting should be sent is larger than the RTS / CTS threshold. What should the node do?

Answer 8

Transmit RTS

Question 9

Scenario: A node waiting to transmit data sense the channel to be free. The node completes contention and transmits an RTS. No CTS is recieved. What should the node do?

Answer 9

Infer a collision has occurred, increase CW range and generate a new CW

Question 10

Scenario: A node waiting to transmit data sense the channel to be free. Before the node counts its CW to 0, it detects another CTS frame on the medium. What should the node do?

Answer 10

Set a NAV (Network Allocation Vector), wait and then contend again