Question

In WiFi systems, nodes adopt CSMA MAC protocol to access to the shared media through contention. At the beginning, CWmin # 31 meaning that there are 32 slots available for users. Answer the following questions. (All answers limit to 1-2 sentences please.) 1) At the beginning, if node A pick up the back off counter BO-10, what is probability to win the channel contention ? However, after 3 slots node A detected that some other node began to sent package. Does node A has a better chance to win the contention in next round? Consider the two cases that there are n#2and n", 3 active users, respectively node A do at this moment and Why? is the range of throughput at this moment? Finally, node A won the channel content and sent package but did not get ACK after SIFS. What did )Assume that there is one active user, n -1. The packet size is 1024bytes and date rate 24Mbits/s. What ) If we define the fairness as the ratio of the number of packets delivered by a user and the total ones by all users, for the case having 12 active nodes, what is the fairness coefficient? Is the IEEE 802.11 DCF a fair protocol 7 How does the protocol realize it.

Answer 1

Answer 1)

Back off time =10,

n= number of users=2.

Waiting time =K*T

Where k= [0,2ⁿ – 1]

When both A and B choose K =1 0
Data is transmitted at the same time, So collision.

When A chooses K = 0 and B chooses K = 10
A sends the data and B is waiting for its chance, A wins.

When A chooses K = 10 and B chooses K = 0
B sends the data and A is waiting for its chance, hence B wins.

When both A and B choose K = 10
Collision occurs.

Winning Probability for A = 1/4

Winning Probability for B = 1/4

Case-2 : When there are 3 users

Back off time =10,

n= number of users=3.

Waiting time =K*T

Where k= [0,2ⁿ – 1]

performing the same syntax used above. We get,

Winning Probability for A = 5/8

Winning Probability for B = 1/8

Answer 2)

The node should re-send the data, because the server will get up-link and will not entertain/acknowledge the down-link.

Answer 3)

throughput = packet size / time [s]

Total number of packets delivered=1024 bytes.

data rate=24 Mbits/s

So, throughput=1024/24=.42.66

Answer 4)

Jain's Fairness Index (Lizr Fairness Index- where x is the normalized throughput (in Kbps) of the i-th TCP flow and n is the number of connections.

Using the above formula for fairness index. The result lies between 1/n to 1. 1/n will be the worst case and 1 will be the best case. When all the users get the same allocation, it will at maximum point.

Yes, IEEE 802.11 DCF is a fair protocol. There is used a controller for central supervision which is take care by an operator. A DCF holds many local controllers which will be helpful for realizing.