<> Unscramble the multi site application shown in the figure below CSMA/CA Principle of wireless transmission process .

<> answer

For easy viewing , I give the assumption that the figure above is from t0 It's time t7 time 5 Data to send from sites .

From the legend, you can see the descriptions of four kinds of legends . You can see t0 Site only at all times 3 Data to send ( The yellow arrow indicates ), site 3 Is the frame listening channel free . If you are free, you need to wait for one DIFS time .

Enough waiting for one DIFS time , site 3 Busy ( Taking up the medium , Other sites even with data , It can't be sent either ).

Supplement the status of other sites in the above process : site 3 stay t2 Sending data at any time , that t2 The status of other stations before the time is as follows :

site 3 Waiting for one DIFS time , site 1 There's data to send, too .

site 1 stay t0 reach t1 At this time, we find that the channel is free , Start counting .

That's the count t1 time , Find that the channel is not idle ( site 3 It's done counting , Start sending data , Media occupied ).

site 2 And sites 5 At the same time at the site 3 When sending data , You need to send data, too , But we can't send data yet , Only waiting sites 3 After sending data , Wait for another one DIFS Time before sending data .

Now site 1, site 2, site 5 stay t2 Data needs to be sent before any time , But the channel is occupied . here , Need to generate a random waiting time . In principle, the waiting time generated by these three sites is different .

Waiting for site 3 After sending data , Need to wait for one DIFS.
Wait for one DIFS after , Random waiting time generated before each need to wait .

You can see the site 2 Minimum waiting time for , Then the site 2 Start channel occupation , send data .

Process description :

site 1, site 2, site 5 Within the time of waiting at the same time .

site 2 Enough waiting , It takes up the channel , At this time, the site 1 And sites 5 It's listening. The channel is busy . Then temporarily terminate the wait .

The remaining waiting time is still valid , Need to be kept for later use .

site 1 And sites 5 Discovery site 2 I'm done , You need to wait for one DIFS time , After waiting , Enter a waiting time .

At this time, the site 4 Need to send data , But the frame hearing channel is still busy , It randomly generates a waiting time . Then I still need to follow the site 1, site 5 Waiting for one DIFS time .

site 1 Remaining waiting time :

site 4 The generated random waiting time is exactly the same as that of the site 5 The remaining waiting time is the same :

Now site 4 And sites 5 At the same time, it enters the status of sending data .

Due to site 4 And sites 5 There was a conflict , Now expand the size of the interval of the generated random waiting time , Double and enlarge .

Then wait for one at the same time DIFS time .

site 1 Keep waiting according to the time left before , site 4 And sites 5 Another random sampling of a random waiting time .

Now site 1 The time is relatively short , So send it first .

and so on , Until the end .

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