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6mode with two clients: A at 130 Mbps and B at 39 Mbps. t wtook the values of 50, 80, and 100 ms in the evaluation.Figure 7 shows the impact on throughput. When t w wasdoubled from 50 to 100 ms, the throughput of A improved byabout 40% while that of B reduced by about only 15% and theaggregate network throughput improved by about 20%. Thisis because, as the opportunism is increased along with t w , Agot more chance and B got less chance of using the channel.Figure 8 shows the impact on jitter. The impact is oppositeto that on throughput. When t w was doubled, the jitter of Awas reduced from 5 ms to about 3.5 ms while the jitter ofB increased from 7.7 ms to 10 ms just because A got morechance and B got less chance of using the channel. However,both of their jitters are much lower than what is required forreal-time applications.B. Evaluation with SimulationWe developed all three proposed algorithms into the simulatorNS3 [25] and comprehensively evaluated the performancewith extensive simulations. Our algorithms were comparedwith the default CSMA/CA without opportunism for each case.The evaluation began with a simple infrastructure topologyhaving one access point and two clients, then studied theimpact of hidden terminal on opportunistic access, comparedwith opportunistic transmission in the case of mobility, andfinally evaluated the performance on a multiple-hop meshnetwork. All experiments were performed with constant bitrate (CBR) UDP traffic at a rate of 10 Mbps for 5 minuteswith packet size of 1K bytes. The results are averaged over50 runs for each case. In simulations, α in Formula 1 wasset to 1.7 and the weighted window t w was set to 50 ms.In the following performance figures, “Original” representsthe default algorithm in the CSMA/CA, “Overlapped” forthe Overlapped Contention, “Segmented” for the SegmentedContention and “Normal” for the Normal Distribution BasedBackoff Selection.1) Triple-node Infrastructure Topology: We first evaluatedthe throughput and jitter performance of opportunistic accessover the original algorithm in a simple topology: one accesspoint and two client nodes. All nodes were in radio rangeof each other. These client nodes transmitted packets to theaccess point at different bit rates. One client node was set toa 54 Mbps, but the other node changed its bit rate from theIEEE 802.11 rate set consisting of 6, 12, 24, 36, and 48 Mbps.Throughput Performance: Figure 9 plots the throughputperformance when one client changes its bit rate each timeto imitate the changes of channel conditions. The X-axisrepresents the different bit rates and the Y -axis represents thethroughputs of opportunistic access algorithms and the originalone in the CSMA/CA. From the figure, opportunistic accessimproves the throughput by approximately 30 - 100% basedon channel conditions as compared to the original algorithm.Delay Performance: Figure 10 plots the measured averagedelay at different bit rates The opportunistic access shows asignificant improvement in delay as compared to the originalaccess. This is because the high bit rate nodes in opportunisticaccess has very less contention time and transmits packetsrapidly. The lower bit rate nodes experiences a much largerdelay comparitively beacause of collision due to back-offterminate at the same time and the contetion window sizegets doubled everytime when it encounters a collision. Yet theoverall network performance in delay is improved significantlyover the default CSMA/CA method. To show more clearly, wefurther studied the delay performance with increase in trafficrate Figure 11. As expected, the delay gets increased as CBRtraffic rate increases because more packets are transmittedfrequently. Although the variation in individual packet delaybetween high bit rate nodes and low bit rate nodes is high,the overall network performance of opportunistic access isimproved.Jitter Performance: Figure 12 shows the measured jittersat different bit rates. Jitter is the delay variation between twoconsecutive successful packet receptions and the result plottedis the average of delay variation of total received packets.Surprisingly, the opportunistic access outperforms the originalone in jitter as well. This is because, with opportunistic access,the high bit rate nodes obtain much smaller initial contentionwindows than those in the default CSMA/CA and the selectedbackoff in the contention is thereby significantly reduced.As a result, although lower bit rate nodes experience largerjitters than higher bit rate nodes, the overall jitter across thenetwork is improved because of the shortened time spent onthe contention with opportunistic access.Throughput (Mbps)876543210Throughput of Proposed AlgorithmThroughput of Original AlgorithmJitter of Proposed AlgorithmJitter of Originial AlgorihtmAggregate ThroughputC->DB->EB->EC->DJitter (ms)Throughput (Mbps)43.532.52A1.51AggregateAggregateAggregateAABBB0.50tw=50ms tw=80ms tw=100mstw value (ms)Jitter (ms)109B8 B76 AA543BA210tw=50ms tw=80ms tw=100mstw value(ms)Fig. 6: Ad-hocFig. 7: Impact of t w on ThroughputFig. 8: Impact of t w on Jitter
7Throughput (Mbps)43.532.521.510.50Average Jitter (ms)2.221.81.61.41.210.80.60.4AAggregateBAAggregateBtw=50ms tw=80ms tw=100mstw value (ms)Fig. 9: ThroughputAOriginalOverlappedSegmentedNormalAggregate0.26 12 24 36 48Bit Rate (Mbps)Fig. 12: Average JitterBAverage Packet Delay (ms)Average Back-off (Ts)1110987651.110.90.80.70.60.50.40.30.2OriginalOverlappedSegmentedNormal0.16 12 24 36 48Bit Rate (Mbps)Fig. 10: Average Delay4OriginalOverlapped3 SegmentedNormal26 12 24 36Bit Rate (Mbps)48Fig. 13: Average BackoffThroughput (Mbps)Average Packet Delay (ms)131211109140012001000800600400200OriginalOverlappedSegmentedNormal01 2 3 5 10 15 20CBR Traffic Rate (Mbps)Fig. 11: Traffic Rate Vs Delay8Original7OverlappedSegmentedNormal66 12 24 36Bit Rate (Mbps)48Fig. 14: Hidden Terminal ProblemAverage Backoff Time: To further investigate how theopportunistic access improves the throughput and jitter, wecollected the backoff time in terms of time slots of eachtransmission. Figure ?? shows the average backoff time persuccessful transmission for each access algorithm. Averagebackoff slot time per packet is calculated as the sum ofindividual packet backoff time slots over the total number ofpackets transmitted successfully. From the figure, the opportunisticaccess algorithms consume much less time in backoffthan the original access, up to 400% less at 48 Mbps. Theoverall network performance improves because 1) the equalprobability access in the original algorithm results in almostconstant average backoff, and 2) the opportunistic accessalgorithms spend less time on backoff, namely contention.Discussion on Opportunistic Access Algorithms: Amongthe proposed three opportunistic access schemes, the SegmentedContention yields in general the best performance fromFigure 9 to ??. This is because the channel access of nodesat different bit rates is deterministically prioritized when theircontention windows are segmented. Nodes with lower ratesnever get a smaller backoff value than the ones with higher bitrates. The Overlapped Contention and the Normal DistributionBased Backoff result in almost identical performance. This isbecause both of them have a similar expectation in selectingbackoff values for the nodes at the same bit rate. Because theyboth have contention windows starting at “0”, the higher bitrate nodes may still be occasionally beaten by nodes at lowerbit rates.2) Impact of Hidden Terminal Problem: In this case, thenetwork was configured to still have one access point and twoclient nodes with one at 54 Mbps and the other varying itsbit rate, but the clients do not hear each other. RTS and CTSare disabled to fully stimulate the hidden terminal problem.Figure 14 plots the measured throughputs at different rates.The hidden terminal problem does hurt the performance of allalgorithms. The figure tells that the overall network throughputis still improved by the opportunistic access, although theimprovement is reduced to some degree by the hidden terminalproblem. To show more clearly the effect of hidden terminal,we further studied the percentage of packet loss ratio ofthe network. Packet loss ratio percentage is the differencebetween total transmitted packets and total received packetsover the total number of transmitted packets. Figure 16 showsthat packet loss ratio is considerably reduced in opportunisticaccess which explains the performance improvement of opportunisticaccess as compared to the original access.3) With RTSCTS: Although opportunistic access performsbetter than original one, the measured throughput performanceis mainly due to high bit rate nodes in the presence of hiddenterminal. This is because the high bit rate nodes wins thechannel more frequently due to its initial smaller contentionwindow size, suppressing the channel acces to lower bit ratenodes. Hence the channel should be reserved by nodes beforeits access to provide the fairness among the nodes. We haveenabled the RTS/CTS reservation technique for this purpose.As expected, the opportunistic access outperforms the originalone in throughput and gives the throughput proportional to its
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Page 9 and 10: 9Where r i and n are the allocated
Page 11: 11Opportunistic transmission based

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