i need an explination on how these answers were obtained? The diagram at right s
ID: 3821346 • Letter: I
Question
i need an explination on how these answers were obtained?
The diagram at right shows a WIFI network with an access point, X and three hosts, A, B and C. The large circles indicate the coverage areas of the three hosts. The coverage area for X is not shown, but you may assume that it includes all three hosts. Assume RTS/CTS are not used. Suppose X is transmitting a packet at time 0 and finishes sending it at time 100 mu s. Also, A gets a packet to send at time 50 that takes 100 mu s to send and is assigned a backoff timer of 100. B gets a packet at time 70 that takes 200 mu s and is assigned a backoff timer of 50. C gets a packet at time 90 that takes 150 mu s and is assigned a backoff timer of 150. For each of the three hosts, what time do they start sending their packets? You may ignore the inter-frame spacing and the time required for asks. A starts sending at time 200 and finishes at 300 B starts sending at 150 and finishes at 350 C starts sending at 350 and finishes at 500 Of the three packets sent, which are successfully delivered on the first attempt? Only the one from C is successfully delivered. For each packet that is not successfully delivered on the first attempt, approximately when does the sending host learn that the packet was lost and must be sent again? Hosts leant of lost packets from the absence of ACKs. Here, A would expect an ACK at 300 and so would learn of the lost packet when the ACK fails to arrive at time 300. Similarly, B would learn of its lost packet at time 350. Now, suppose RTS/CTS is enabled. In this case, approximately when does each host send its data packet? You may assume that the time needed to send RTS, CIS and ACK packets is negligible. A sends at 400. B sends at 150. C. sends at 550.Explanation / Answer
1.
Lets understand the meaning of Backof Timer first.
Backof Timer:
When a station wants to transmit, it picks up a random number called as the backoff timer, from which it counts down before deciding to send.
- A gets the packet at time 50, but since X is still transmitting till time 100. A will wait.
At time 100, A senses the channel to be free and waits for the Backof Timer period which happens to be 100.
So at time 200, A starts sending and finishes at 300.
- B gets the packet at time 70, and senses the channel to be busy till 100 as X is still transmitting.
At time 100, B senses the channel to be free, and waits for the backoff timer period which happens to be 50.
So, B starts sending at 150 and finishes at 350
Please note that A and B are in different zones whereas A and C are in the same zone.
- C gets the packet at time 90, and senses the channel to be busy till 100 as X is still transmitting.
At time 100, C senses the channel to be free, and starts to decrement its Backof timer which happens to be 150.
When the backoff timer value is 50, that is when the actual time is 200, A begins to transmit and hence C will defer access to the medium
till A completes at 300. At 300, the medium is available again, C starts decrementing the backoff timer from where it previously let off.
So at time 350, C starts sending and finishes at 500.
2.
Since C starts to transmit after A and B has completed, C is successfully delivered.
3.
ACKs are only obtained after the packet reaches the destination.
Hosts learn of lost packets from the absence of ACKs. Here, A would expect an ACK at approximately
300µs at which it completes transmission and so would learn of the lost packet when the ACK fails to arrive at that time.
Similarly, B would learn of its lost packet at time 350µs.
4.
RTS/CTS (Request to Send / Clear to Send) is the optional mechanism used by the 802.11 wireless networking protocol
to reduce frame collisions introduced by the hidden node problem.
All the 3 nodes A, B and C will start decrementing there Backof timer at 100.
Since B's backoff timer value is 50, it will begin transmissiting at 150. Before that it will send the RTS/CTS to X and gets the
permission to send till 350 by X.
Now A and C at time 150 senses the channel to be busy and defers decrementing the backoff timer till B completes at 350.
A's backoff timer left to be decremented = 50.
C's backoff timer left to be decremented = 100.
At 350, both A and C senses the channel to be free and starts decrementing the backoff timer from where they left it earlier.
A will complete decrementing the backoff timer first at 400. while C still has 100 more left to decrement.
At this stage, A gets permitted by X to transmit till 500 and A compeltes the transmission.
Now C will start to decrement again the remainning 50, and at 550 will get permitted by X.
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