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# Assign2 - transmission diameter and assuming a packet of 10...

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WLAN Assignment 2 Question 1: Consider a CSMA/CA 802.11 WLAN with 4 stations A, B, C and D. The 4 nodes are using the RTS/CTS mechanism to transmit data. The following are values used: DIFS = 34µsec SIFS = 16µsec Slot Time = 9µsec ACK = 20µsec RTS = 24µsec CTS = 20µsec At a certain point in time, the nodes are such that: A is hidden from C and D D is hidden from A and B B is hidden from D C is hidden from A The following diagram will give you a better idea Assume that A and D have data to transmit. A will transmit to B while D will transmit to C. At time t = 0, A and D start to sense the medium for DIFS. Assume that A and D had chosen 0 as their random number before a transmission. Draw separate time lines for A,B,C and D to show what is happening in the medium. Show any possible collisions. Are A and D able to transmit their data packets to B and C respectively. In brief explain why? A B C D A’s range B’s range C’s range D’s range

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Answer: Assuming that all nodes have the same transmission speed/strength, have the same
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Unformatted text preview: transmission diameter and assuming a packet of 10 slot times on nodes A and D we see that the data gets transmitted successfully by A and D. A and D transmit an RTS packet to B and C respectively. These RTS packets are received by B and C. B and C reply by sending out CTS back to A and D. These two CTS packets collide in the region where B and C intersect. However this collision is not felt in the region of A intersection B. Similarly the collisions between the CTS packets are not felt in the region of C intersection D. Thus, A and D receive the CTS packets and transmit the data. The data goes through successfully the B and C without interference since A is hidden from C and D and vice versa for D. An ACK is sent out which collides between B and C however this ACK too like the CTS reaches A and D ending transmission. RTS A B C D CTS DATA Time DIFS SIFS SIFS SIFS 74 34 94 110 200 58 216 236 CTS DATA RTS ACK ACK ACK DIFS...
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Assign2 - transmission diameter and assuming a packet of 10...

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