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EEE521

EEE521. Computer and Data Communications Semester 2 2011-2012 Mohd Nazri Mahmud Session 2b-29 Feb 2012. OBSERVATIONS. OBSERVATIONS

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EEE521

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  1. EEE521 Computer and Data Communications Semester 2 2011-2012 MohdNazri Mahmud Session 2b-29 Feb 2012

  2. OBSERVATIONS OBSERVATIONS 1. Throughputs for both schemes increase with load until their corresponding maximum achievable throughput area reached, then their throughput decrease as the load increases.

  3. OBSERVATIONS 2. In between G=0.1 to the optimum loads, the throughputs for both schemes increase at greater rates for the lower values of a

  4. OBSERVATIONS 3. max achievable throughputs are also higher for lower values of a (S = 0.2, 0.5, 0.8, 0.9, 1) 4. max throughput occur at more intense loading for lower values of a.(G = 0.105, 1-5, 10, 50,

  5. OBSERVATIONS 5. At lower load (G<0.1) their throughputs are both low and comparable.

  6. OBSERVATIONS 6. At higher load beyond the optima, the throughputs for the 1-persistent CSMA drop more rapidly than the throughputs for the non-persistent CSMA.

  7. OBSERVATIONS 7. Interestingly, if the parameter, a = 0 , the throughput for the non-persistent CSMA does not decrease with increasing load whereas the throughput for the 1-persistent CSMA descrease to zero as G approaches 10. What does it mean ? a=the ratio of the propagation time/frame time (ieTp is much smaller compared to the frame time (meaning stations are closer to one another is smaller area.

  8. ANALYSIS OBSERVATIONS 1. Throughputs for both schemes increase with load until their corresponding maximum achievable throughput area reached, then their throughput decrease as the load increases. WHY? At lower load there are few packets per frame time. This means that there are sufficient gaps between the start times of succesive transmissions. These gaps are longer enough to prevent the frames from overlapping with one another in the medium. Without collision, the frames that are delivered successfully equals the number of frames transmitted by source stations. At higher load closer to the optimum point, the rate of increase of throughput is decreasing because the interarrival times between successive frames becomes shorter and shorter. Some of the frames collide with one another and cannot be successfully delivered. When the maximum point is reached, the rate of increase in throughput is equal to 0. An increase in loading will no longer increase the throughput because all additional frames cause collision. Beyond this point, the throughput drops drastically because the probability of collision increase with the loading. Eventually, all frames transmitted collide and S=0.

  9. OBSERVATIONS 2. In between G=0.1 to the optimum loads, the throughputs for both schemes increase at greater rates for the lower values of a WHY? a=1 means that the Tp=Tfr. a<1 means that Tp<TFr.Tp <<<< Tfr, a approaches 0. Lower Tp implies short distance between source and destinations. Faster delivery. Also, frames gets delivered fast before there is a chance of being hit by another frame.

  10. OBSERVATIONS 3. max achievable throughputs are also higher for lower values of a (S = 0.2, 0.5, 0.8, 0.9, 1) 4. max throughput occur at more intense loading for lower values of a.(G = 0.105, 1-5, 10, 50) Why? Same reasons as 2.

  11. OBSERVATIONS 5. At lower load (G<0.1) their throughputs are both low and comparable. Why? No collision. Throughput = input

  12. OBSERVATIONS 6. At higher load beyond the optima, the throughputs for the 1-persistent CSMA drop more rapidly than the throughputs for the non-persistent CSMA. Why? There are higher chance of collision for 1-persistent because it immediately send again when the medium is idle whereas non-persistent use random backoff in which the likelihood of concurrent retransmissions are reduced.

  13. OBSERVATIONS 7. Interestingly, if the parameter, a = 0 , the throughput for the non-persistent CSMA does not decrease with increasing load whereas the throughput for the 1-persistent CSMA drops to zero as G approaches 1000. Why? Same as 6

  14. Computer and data communications session 3A • Monday, 5/3/2012 - 9-11am at Data Communication Lab Room 2.04. • Group Experiential Learning on the Local Area Network. • Download and print the Lab Sheet from my teaching webpage

  15. REMINDERS • Group survey paper presentation • Select paper by today(by 5pm) and email me the title • Search, download and start preparing for presentation in Week 4 or maybe week 5. • If title is not decided – selection by the lecturer. • Individual review paper assignment • Select by Friday and email me your chosen paper.

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