The objective of this exercise is to realize how high distance and high bandwidth affect the performance of stop and wait. The distance from Earth to a distant planet is approximately 9 x 10° km. Assume that the frame size is 8 Kbits and the speed of light is 3 108 m/s. Assume that the bit rate is 10 Mbps. a) What is the efficiency (channel utilization) if a stop-and-wait protocol is used?
b) Suppose we use a window protocol. What should be the window size in frames to achieve the maximal efficiency (channel utilization)?
c) Plot the efficiency versus the distance when the bit rate is set to 10 Mbps. Discuss this plot.
d) Plot the efficiency versus the bit rate when distance is set to 9 x 109 km. Discuss this plot.

Consider a slotted aloha system, where the time slot equals the fixed duration of each packet. assume that there are 4 stations a, b,c, d sharing the medium. (a) stations a, b,c, d receive one packet each from higher layers at times 1.3, 1.5, 2.6,5.7 respectively. show which transmissions take place when, according to the slottedaloha protocol; describe all transmissions until all four packets have been successful. when needed, each station has access to the following sequence of random number, provided by a random number generator and drawn uniformly between 0 and 1: (1) station a draws numbers: 0.31, 0.27, 0.78, 0.9, 0.9, 0.11, 0. (2) station b draws numbers: 0.45, 0.28, 0.11, 0.83, 0.37, 0.22, 0. (3)station c draws numbers: 0.1, 0.2, 0.3, 0.4, 0. (4) station d draws numbers: 0.36, 0.77, 0.9, 0.1, 0.1, 0.1, 0.1, 0. (b) in slotted aloha, a station transmits in each time slot with a given probability. what probabilities would you assign to each of the four stations so as to: (i) maximize the efficiency of the protocol? (ii) maximize fairness among the four stations? (c) will the efficiency increase or decrease if we modify slotted aloha as follows: (i) get rid of slots and allow stations to transmit immediately? (ii) implement carrier sensing? (iii) implement collision detection? (iv) implement collision avoidance?

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