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8bit rates because of channel reservation technique and longterm temporal fairness provided by our scheme. The packetloss ratio of our scheme is also considerably reduced in thepresence of RTS/CTS channel reservation technique as shownin Figure 17 which increases the overall network performanceof opportunistic access.4) Comparison with Opportunistic Transmission under Mobilityand Auto Rate: We also evaluated the performance inan infrastructure topology of multiple flows with mobility. Wetested topologies with a different number of nodes varyingfrom 2 to 6 nodes with one flow between each node andthe access point. The maximum transmission range of a nodewas set to 30m and mobility was enabled such that eachnode would be moving randomly within the bounded area of50m×50m at a speed of 5 m/s. The nodes sometimes move outof range and packet losses occur more frequently. Because ofmobility, the supported bit rate has to be dynamically adapted.RRAA [20] rate adaptation algorithm was enabled for thispurpose. In addition, we also implemented Opportunistic AutoRate (OAR) protocol as a representative of opportunistictransmission algorithms to compare with the opportunisticaccess in this experiment setting.Figure ?? shows the throughput performance of the opportunisticaccess algorithms, the original access and OARwith mobility and RRAA in the case of multiple flows.The X-axis represents the number of flows and the Y -axisrepresents the throughput for the entire network. From thefigure, the opportunistic access (our algorithms) and the opportunistictransmission (OAR) have close performance whenthe network only has a couple of nodes, but the opportunisticaccess gradually outperforms the opportunistic transmissionas the number of mobile nodes increases. This is becauseof the difference in the opportunism between access andtransmission. With the growing number of mobile users, theuser diversity increases as well. OAR does not exploit userdiversity and it uniformly selects a user to transmit. As a result,it does not favor the user with the best channel conditionto use the channel each time. On the contrary, opportunisticaccess exploits user diversity. With more users, it is morelikely that someone is at a very high bit rate. Because theopportunistic access always favors the user with the highestbit rate to use the channel, the overall network performance isimproved. In addition, in this environment, the mobility andrate adaptation introduce fast channel variations. This mayresult in transmission failure when OAR opportunisticallytransmits a burst of frames if the channel degrades beforethe opportunistic transmission finishes. However, because theopportunistic access only transmits one frame per contention,its short transmission time is resilient to the fast variations.Moreover, the variations generate new user diversity in thenetwork that facilities the opportunistic access to exploit forhigh network performance. The performance figure also showsthat the overall throughput increases slowly when the numberof flows grows because of the increasing contention amongthe flows.5) High Performance of Opportunistic transmission overOpportunistic access: In this case, the network is still configuredwith infra-structure topology of multiple flows butwithout mobility and auto rate algorithms. The number offlows are increased from 2 to 6 by gradually adding onenode each time. All nodes transmit packets to the accesspoint. Because of stable channel conditions and less networkoverhead, opportunistic transmission OAR protocol transmitsmultiple frames when it gains access to the channel whereasopportunistic access relies on one frame transmission per contentionand network overhead is more comparitively.AlthoughOAR protocol shows a high performance over opportunisticaccess scheme as shown in Figure 19, opportuinistic accessshows a better performance than default CSMA/CA method.6) Fairness Index Measurement: Jain Fairness Index is themost commonly used metric to measure the fairness variationusing the individual flow throughput over wireless networks.Basically Jains Fairness rates the individual throughput variationwith respect to number of flows. If resources are allocatedequally, then Jain fairness index will have the maximum value1 and vice-versa.Jain Fairness Index is given byI J = (∑ ni=1 r i) 2n ∑ ni=1 r2 i(5)Throughput (Mbps)15141312111098Original7Overlapped6SegmentedNormal56 12 24 36 48Bit Rate (Mbps)Fig. 15: Hidden Terminal Problem withRTSCTSPacket Loss Ratio (%)757065605550454035OriginalOverlappedSegmentedNormal306 12 24 36 48Bit Rate (Mbps)Fig. 16: Packet Loss RatioPacket Loss Ratio (%)757065605550454035OriginalOverlappedSegmentedNormal306 12 24 36 48Bit Rate (Mbps)Fig. 17: Packet Loss Ratio with RTSCTS
9Where r i and n are the allocated resources to user i and thetotal number of users respectively.Figure 20 shows that the the fairness of opportunistic accessscheme gets increased as the number of flows increases.Thisis because the opportunistic access actually reduces the contentiontime of high bit rate nodes, thereby giving opportunismto send more packets but the lower bit rate node remain inits regular channel access time. The fairness of opportunistictransmission is also compared with opportunistic access whichshows that the resources are allocated equally over the longterm.7) Multi-hop Mesh Topology: In this scenario, 30 nodeswere randomly and statically distributed in a 150m×150m areaand bit rates (12, 24, 36, 48, 54 Mbps) were assigned to thehops in a uniform distribution. Since the radio range was 30m, these nodes form a multi-hop mesh network. Source anddestination pairs of flows were preselected among the nodesand the number of flows was increased from 1 to 9. OLSR [27]protocol was used to route the packets over the network.Throughput (Mbps)54.543.532.52OriginalOverlapped1.5SegmentedNormal11 3 5 7 9Number Of FlowsFig. 21: Multi-hop Mesh TopologyThe network throughput performance is plotted in Figure ??.The opportunistic access shows an average throughput improvementof up to 40% over the original access. This isbecause the original access takes the same initial contentionwindow upper bound regardless of bit rates, but the opportunisticaccess obtains a smaller initial contention window upperbound than that in the original access if the bit rate is largerthan the basic rate. As a result, the opportunistic access spendsless overhead time on contention than the original access if thechannel condition for the single flow is good. As the number offlows grows, the throughputs decrease because of the increasedcollisions among flows. The Overlapped Contention and theoriginal access boost their throughputs in the case of 9 flowsbecause their uniform selection of backoff from overlappingcontention windows helps dilute the collision.VI. DISCUSSIONThis section presents some issues with opportunistic accessas observed during the performance evaluation. We also brieflydiscuss the tradeoff between the opportunistic transmission andaccess.A. Performance Outage in Chain TopologyDuring the performance evaluation with simulation, wetested a 5-hop chain network where the sender and receiverwere placed at the extremities of the chain with 4 nodesbetween. Each node had a buffer of 130 frames. The bit ratesof these 5 hops were set in four different patterns as shown inFigure 22. Pattern A has ascending bit rates (12, 24, 36, 48,54 Mbps) along the transmission path, B has descending bitrates (54, 48, 36, 24, 12 Mbps), C has hybrid ordering withlower rates first (12, 24, 54, 36, 48 Mbps), and D has hybridordering with high bit rates first (36, 48, 54, 12, 24 Mbps).Fig. 22: Four Chain TopologiesFigure 23 plots the throughput performance of all algorithmsin the four bit rate distribution patterns. From the figure,the opportunistic access is significantly outperformed by theoriginal access in the decreasing order case (B) and slightlyoutperformed in the hybrid order that has high bit rates first(D). This performance outage problem occurs because theopportunistic access transmits more packets in the early hopsThroughput (Mbps)12.51211.51110.5109.598.5OriginalOverlapped8Segmented7.5NormalOAR72 3 4 5Number Of Flows6Fig. 18: MobilityThroughput (Mbps)1816141210864OriginalOverlappedSegmentedNormal2OAR02 3 4 5 6Number of FlowsFig. 19: No MobilityFairness Index1.210.80.60.4OriginalOverlappedSegmentedNormalOAR0.22 3 4 5 6Number of FlowsFig. 20: Fairness
Page 1 and 2: Opportunistic Random Access with Te
Page 3 and 4: 31) Overlapped Contention: In the f
Page 5 and 6: 5Contention Window17515012510075502
Page 7: 7Throughput (Mbps)43.532.521.510.50
Page 11: 11Opportunistic transmission based

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