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PRP-WLAN OPNET Model 

PRP-WLAN OPNET Model 

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Conference Paper
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Redundancy techniques based on the combination of multiple diverse communication channels are an established countermeasure to improve performance characteristics of wireless communication systems. Besides parallel redundancy in the space and frequency domain, serial redundancy in the time domain can be utilized. It is known that the parallel appro...

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... D reflects the queuing IV. delay OPNET time M ODEL of the duplicated packets at the This respective section presents ETH interface the simulated with delay OPNET queue. model. Firstly, it addresses the general PRP implementation on OPNET without having any sort of interference, to verify that the results from OPNET regarding PRP match those seen in [6] and [7]. The second part presents a novel perspective on implementing Time Diversity on the PRP model. This part is analyzed theoretically first, then OPNET simulations results are presented to verify the expected results. IV. OPNET M ODEL This section presents the simulated OPNET model. Firstly, it addresses the general PRP implementation on OPNET without having any sort of interference, to verify that the results from OPNET regarding PRP match those seen in [6] and [7]. The second part presents a novel perspective on implementing Time Diversity on the PRP model. This part is analyzed theoretically first, then OPNET simulations results are presented to verify the expected results. The model in [6] and [7] is implemented on the sending end as a node connected to a switch, which in turn is connected to two Access Points (APs) as seen in Fig. 4. The two APs operate on the non-interfering channels 1 and 9 using the IEEE 802.11g protocol. The same data (a payload of 100Bytes) is sent on both channels as described by the PRP specifications. In the described model, the switch does all the functions of the PRP RedBox; it performs packet duplication, one for each channel, with the addition of a byte trailer and a sequence ID to each packet. On the receiving side, a similar infrastructure is used to receive both copies of the data. It analyses the sequence ID, strips the trailer from the packet and discards any already received duplicate. The sending node is termed sensor while the receiving node is termed actuator. OPNET simulations show that the results from this model match with the expected results from PRP by having the sent data identical to the received data from the sensor to the actuator. In this section, the PRP system is presented with added immunity gained through Time Diversity as described in Section III.C. This is done by adding a time shift between the data being sent on both of the PRP channels. In order to determine the ideal time shift, and also to present a method that can be used to determine the ideal time shift suitable for a given application, time varying interferers are employed and their effect on the system is studied. A similar approach was used in [9-11] but utilizing different non time-varying interferers. The interferers used in this research are periodic, with a duty cycle of 50%, i.e., the period of time during which these interferers are on is equal to the period during which they are off. Fig. 5 shows a sample of the output of the Pulsed Jammer, a type of jammer model in OPNET, which induces the required effect. Two PRP systems are analyzed, one with a sampling period of 5ms and the other with a sampling period of 10ms. All possible time shift values are addressed. For the 5ms PRP, the experimented time shift ranges from 1ms to 4ms with increments of 1ms. As for the 10ms PRP, a range from 1ms to 9ms is tested with increments of 1ms. Each of these time shifts for both systems is tested under 5 different cases of 50% duty cycle interferers. The values of On-Time/Off-Time (On/Off) for the 5 interferers used are (1ms/1ms), (2ms/2ms), (3ms/3ms), (4ms/4ms) and (5ms/5ms). In other words, the time periods for the jammers are 2ms, 4ms, 6ms, 8ms and 10ms. A theoretical analysis of the results expected from these experiments is presented next, followed by a verification based on OPNET simulations using the Pulsed Jammer OPNET node model to represent the interference. Results from this part are based on the assumption that when a jammer is active during the sending of the data, any packets being transmitted at this time instance will be automatically dropped. This occurs whether the PRP system is transmitting on the first channel or the second one due to the fact that the jammer targets the entire Wi-Fi spectrum when active at this time instance. As such, the only possibility for the data to be successfully transmitted through PRP is when at least one or both of the channels are sending the same copy of the data when the jammer is off. To conclude which time shift is ideal for each case, the total number of received packets for each scenario is analyzed. For each one of the sampling periods, 24 packets are transmitted (the lowest common multiple for the time periods under study). Each of these scenarios is studied under the 5 types of jammers discussed leading to a maximum of 120 packets transmitted per analysis. Fig. 6 and Fig. 7 show the best and worst time shifts for the 5ms sampling period of the PRP system, which are 3ms and 4ms respectively, having a total number of PRP received packets of 99 and 78 out of 120 respectively. As for the 10ms sampling period, time shifts of 2ms and 3ms demonstrate the best case with 114 packets received out of 120, as shown in Fig. 8 and Fig. 12. The 1ms time shift, on the other hand demonstrates the worst time shift for the 10ms sampling period, having 88 packets out of 120 received, demonstrated in Fig. 13. Tables I and II show a compilation of the total number of packets received over PRP for each of the values of the time shifts addressed for the 10ms and 5ms systems respectively. The tables verify that the PRP system demonstrates a significant improvement over the single channel system even at the least effective time shift. It also demonstrates how time diversity features, when carefully chosen, result in a significant improvement of system performance under time- varying interference. Simulations of the PRP model with the Pulsed Jammer on OPNET have proved that the theoretical expectations match the correct behavior of the PRP under time diversity features. Results for the 5ms sampling period are shown in Fig. 9 which shows the performance of the system under the 3ms time shift. This is expected to be the ideal time shift based on the aforementioned theoretical analysis. The first subplot shows the performance of channel 1, 3ms time shifted channel 9 and the resulting overall PRP system behavior in the presence of (1ms/1ms) pulsed jammer. Similarly, subplots 2, 3, 4 and 5 show the performance of the three systems in the presence of the (2ms/2ms), (3ms/3ms), (4ms/4ms) and (5ms/5ms) pulsed jammers. Fig. 10 shows the worst time shift for the 5ms sampling period of the PRP which is 4ms. Similarly, Figs. 11, 14 and 15 show the two best case scenarios, which are 2ms and 3ms time shifts and worst case scenario, which is 1ms time shift respectively when using the 10ms sampling period PRP system. The results reaffirm that the ideal time shift for the 5ms case is 3ms, while for the 10ms sampling period the 2ms and 3ms time shifts both demonstrate superior behavior over the other time shifts. The results also demonstrate how the PRP generally shows superior behavior over the single channel system. It is also shown that the time diversity generally improves the performance of the system through increasing the number of received packets over PRP significantly especially under the impact of time-varying interference. It is also verified that choosing an ideal value for the time shift is a critical decision that impacts the extent to which the PRP system improves its performance in the presence of time-varying interference. Tables III and IV show the packet reception of the PRP system based on both the expected theoretical results as well as the OPNET simulations of the time shifted PRP under the effect of time varying interferers. The results show that there is always an optimum value for the time diverse PRP system to perform at, which is determined based on the sampling period being used by the original PRP system as well as the time-varying nature of the interferers. V. C ONCLUSION This work demonstrates how PRP can be utilized not only to realize a wireless diversity system for the space and frequency domain, but also for the time domain by enhancing the RedBox with added time diversity features. This system has superior behavior over the single channel system as well as the regular PRP system under time-varying interference. The time varying interferers, Pulsed Jammers in OPNET, used in the analysis and OPNET simulations were chosen to be periodic with duty cycles of 50%. These caused the packets transmitted over any of the wireless channels operating in the Wi-Fi spectrum to be dropped when the jammer is active, while on the other hand the packets would transmit successfully when the jammer is off. Theoretical analysis was performed and it shows that the best and worst time shifts for the 5ms sampling period of the PRP system are 3ms and 4ms respectively. While for the 10ms sampling period, the 2ms and 3ms time shift achieves the best performance. However, the 1ms time shift is determined as the worst time shift, but is still shows better performance than a single channel system. The theoretical analysis matches the OPNET simulation results. The analytical methodology and its verification with the OPNET modeler as applied in this paper could be used to evaluate the ideal time shift for different applications given the specific sampling period as well as the nature of noise available. The optimum time shift will be the one with the least number of packet losses. However, the artificial delay introduced by the “Timing Splitter” in one of the parallel redundant channels might potentially affect the performance behavior on pure stochastic fading. This should also be further ...

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