Q3 Project 2: Carrier Sense Multiple Access: CSMA (30pt) In WiFi systems, nodes
ID: 2266730 • Letter: Q
Question
Q3 Project 2: Carrier Sense Multiple Access: CSMA (30pt) In WiFi systems, nodes adopt CSMA MAC protoool to access to the shared media through coutention. At the beginning CWmin 31 meaning that there are 32 slots available fr users. Answer the following questions. (All answers limit to 1-2 sentences, please.) 1) At the beginning, if sode 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 2and n = 3 active users, respectively node A do at this moment and Why? is the range of throughput at this moment? a fair protocol 7 How does the protocol realize it 2) Finally, sode A won the channel content and sent package but did not get ACK after SIFS. What did 5) Assume that there is one active user, n = 1. The packet size is ioaytes and date rate 24Mbits/s. What ) II 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 DCFExplanation / Answer
Given that in wifi systems nodes adopt the CSMA( carrier sense multiple access) MAC(media access control) protocol to access shared media through contention.
At the begining CWMin = 31 i.e there are 31 slots available for users.
In this protocol node itself verifies the traffic before transmitting.
(a)
At the begining, if node A pick up the back off counter BO=10 , the probability to win the channel contention means the channel should not be activated with a formal disagreement. Given that after 3slots node A detected that some other node began to sent package. Hence the probability to win the channel actication is 10 /(31-3) = 0.357
Node A Has better chance to win the channel contention in the next round, as the probablity is going to increase if we increase the number of rounds.
(B)
Node A won the channel content and sent package but did not get the ACK after SIFS (Short interframe spacing) Then the Node A should make a request for the ACK soon as the packet has already been sent. Without ACK of the sent package, the Node A has no access to send the other packet in the next round.
(c)
Assuming that there is one active user n=1, The packet size = 1024 bytes and the data rate is 24Mbits/sec
generally speaking 8bits = byte, hence the data rate is 24/8 = 3Mbytes/sec
Hence the range of throughput is packet size/ data rate
Pcket size = 1024bytes = 1Kbyte
hence range = 1/3*1024= 3.25*10-4 s = 0.325 ms
(d)Given that there are 12 active nodes .
Hence there are 12 packets delivered by the users.
totally there are 31 nodes .
hence from the given definition, fairness coefficient = 12/31 = 0.387
In IEEE 802.11 is a WLAN network in which medium access control layer employs the distribution coordination function(DCF). hence the IEEE 802.11 has been effected by the fairness problem. The enhanced CS algorithm is used in this network to improve the fairness of the network.
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