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hwk3-4-Su10-sol - CNT5106C Computer Networks Summer 2010...

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CNT5106C Computer Networks, Summer 2010 Instructor: Prof. Ahmed Helmy Homework #3 & 4 (this is worth the grade of 2 homeworks) On the Data link layer, MAC protocols and Wireless Networking Q1. (9 points: 3 x 3) MAC layer utilization For an Ethernet LAN (shared bus) the data rate was increased from 10Mbps to 100Mbps. I. How will the utilization (U) of this network change? [Calculate U for each case] II. Suggest two ways in which we can return the utilization to what it was before (Increasing or decreasing another parameter and by how much? Show your reasoning.) III. One person argued that increasing the number of stations attached to the LAN would reduce the idle time on the LAN and hence increase the utilization. Do you agree? A1: I. For Ethernet U=1/(1+5a), where a = Tprop/Ttrans. But Ttrans=#bits/data rate. Since the data rate was increased, so Ttrans is decreased (by a factor of 10), and hence a is increased by a factor of 10, and subsequently U is decreased. II. We can either increase the # of bits per frame by a factor of 10 (so that Ttrans would decrease by 10), or decrease the length of the Ethernet network/cable by a factor of 10 (so that Tprop would increase by 10), or a combination thereof (i.e., increase the # of bits by 5 and the length by 2, so on). III. No. Increasing the number of stations would increase the probability of collisions and so will bring the utilization down. Q2. (13 point: 3 + 10) I. Why do we need a medium access control (MAC) protocol? Shouldn’t routing, queuing and flow control at the data link layer be sufficient? II. Mention ten MAC protocols and classify them according to their type (e.g., random access, etc.). Ans I. No. Because of collision on the shared medium due to multiple accesses from connected stations. Hence a coordination (or collision resolution, avoidance of detection) mechanism is needed. These functions are not performed by routing, queuing or flow control mechanisms. II. 1. Frequency division multiplexing (FDM), 2. time division multiplexing (TDM), 3.Random access which includes: a. Aloha, b. slotted Aloha, c. CSMA, d. CSMA/CD, e.CSMA/CA, and 4. Token based: a. token ring w/ release after transmission, b. token ring with release after reception, and c. polling based MAC. Q3. (6 points) Reason mathematically about the relationship between the utilization in Aloha and utilization in the slotted Aloha MAC protocols.
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A3. Students need to write the formulas used in each of the schemes (depending on the collision probability). For Aloha, probability of collision exists anytime during the slot in which the frame is sent, and also if a frame is already in flight (i.e., that was sent during the previous time slot). P(success by given node) = P(node transmits) . P(no other node transmits in [t 0 -1,t 0 ] . P(no other node transmits in [t 0 ,t 0 +1] = p . (1-p) N-1 . (1-p) N-1 = p . (1-p) 2(N-1) … choosing optimum p and then letting n -> infty ...
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