Analysis of Parallel Redundant WLAN with Timing Diversity

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1 Analysis of Parallel Redundant WLAN with Timing Diversity Marco T. Kassis, Omar A. Mady, Hassan H. Halawa Electronics Engineering Department American University in Cairo Cairo, Egypt {marco_kassis; omar_mady; Ramez M. Daoud, Hassanein H. Amer Electronics Engineering Department American University in Cairo Cairo, Egypt Markus Rentschler Hirschmann Automation &Control GmbH Neckartenzlingen Baden-Wuerttemberg, Germany Hany M. ElSayed Electronics and Communication Department Cairo University Giza, Egypt Abstract Abstract 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 approaches can significantly improve performance characteristics like jitter and reliability, when applied for wireless packet-based data transmission. In this work, an investigation on the performance impact of additional time diversity on top of the parallel redundancy approaches is performed. An OPNET simulation model is created and analysed for its performance characteristics. Keywords- industrial automation; time diversity; interference; redundancy; wireless communication systems. I. INTRODUCTION Wireless data communication technologies like WLAN according to IEEE (Wi-Fi) [1] are promising for the use in industrial applications, compared to the IEEE (Ethernet) [2] standard currently used in many industrial applications. However, Wi-Fi shows time-variable and nondeterministic error characteristics. For industrial applications with tight reliability requirements, such as guaranteed maximum latency times for packet transmission, standard WLAN is often not suitable. In previous work it was shown that redundancy techniques based on diverse communication channels (see Fig. 1) are an effective countermeasure to improve performance characteristics of WLAN on a stochastic basis [3, 4]. In Brennan s classical 1959 paper [3], the elementary forms of diversity for wireless communication have been described. But unlike in Brennan's days, the focus today is on the transmission of longer signal sequences, such as for example Ethernet packets. Based on this, specific performance characteristics regarding diversity gain can be observed. Figure 1. PRP-WLAN System This previous work was dealing with parallel redundancy based on the IEC [5] Parallel Redundancy Protocol (PRP) that was utilized as a post-detection Selection Combiner according to Brennan s classification scheme. With this approach, all packet data is simply sent simultaneously over two parallel paths and on the receiving side, out of the two branches the first arriving Ethernet packet is selected and further processed towards the end application. The second packet -if arriving- is discarded. We have named this type of combiner a Timing Combiner, since a significant performance improvement is gained through this timing behavior based on the immediate processing of the first arriving packet. This has been demonstrated in [6, 7]. This approach however fully relies on parallel redundancy in the space and frequency domain. Adding serial redundancy in the time domain on top of it could have the potential to introduce further improvements, especially for interference behavior that affects both frequency bands simultaneously for short periods of time. This assumption is studied in this work. The paper is structured as follows: In section II, some background of wireless diversity systems is outlined; section III shortly describes how the parallel redundancy protocol implicitly supports frequency diversity and how it can be enhanced for time diversity. Section IV develops an analytical model as well as a channel model on OPNET Network Modeler [8] and presents both results. Finally, section V concludes on the findings and the possible implementation of the previously described time diversity principle depending on the application and noise characteristics in the factory floor.

2 II. DIVERSITY SYSTEM TAXONOMY In this section a summary of some of the basic concepts involved in the domain of wireless diversity systems is presented, according to Brennan s widely accepted nomenclature [3]: Space diversity: The same signal is transmitted in parallel over several different propagation paths. Time diversity: The same signal is transmitted more than once, but at different time instants. Frequency diversity: The same signal is transmitted in parallel over several frequency channels or spread over a wide frequency spectrum. All these diversity techniques have the goal to bring at least one of the signal copies in the best possible quality over one of the redundant channels to the receiving side. Brennan [3] has also classified the basic diversity combining methods and grouped them as follows: Gain Combining: The instantaneously received signals of all branches are added to increase the signal-to-noise-ratio (SNR) of the overall signal at the receiver. This method requires the received signals in the same phase. It is commonly distinguished between Maximal-Ratio Combiner and Equal-Gain Combiner. Scanning (Switching) Combining: The receiver switches to another branch when the signal of the currently selected branch drops below a predefined threshold. Selection Combining: Only the strongest signal of all branches is selected, the other signals are ignored. This is more efficient than scanning combining. In [7], the following term for the new combining technique was introduced: Timing Combining: The fastest arriving signal of all branches is selected, the other signals are ignored. III. PARALLEL REDUNDANT WLAN The Parallel Redundancy Protocol (PRP) according to IEC [5] was developed to achieve seamless redundancy for highest availability of Ethernet networks. In [6] and [7] it was on an experimental basis utilized as diversity combination method on the wired Ethernet interfaces of two independent and diverse IEEE WLAN channels to operate two point-to-point links in parallel (Fig. 2). It could be demonstrated that the PRP-WLAN system achieves a significant improvement not only in packet loss but especially in timing behavior compared to a single WLAN channel. The PRP-WLAN system gains a significant improvement of synchronicity. A. Redundancy Box as Timing Combiner The PRP RedBox can be at the receiving side modeled as a selection combiner, where out of the two branches the better signal is selected, in this case the first -or faster- arriving Ethernet packet, which is then immediately further processed. The second packet -if arriving within the PRP timing windowis discarded. This can be described as follows: { A if (t A< t B ) X = (1) B otherwise t A and t B reflect the arrival times of a duplicated packet at the respective interface. Thus this is called a Timing Combiner. A major influencing factor in the PRP-WLAN system is that Ethernet packets are a longer sequence of signal instants, which are sequenced into smaller pieces to be transmitted over and reassembled on the receiving side. This is depicted in the sequence chart in Fig. 3, where the impact on the timing behavior is highlighted. Figure 2. PRP-WLAN Timing Behavior B. Redundancy Box Diversity Capabilities The PRP-WLAN system can by nature utilize space and frequency diversity. Since the same data is sent redundantly over two different non-interfering wireless channels, the system is immune to interference targeting any one of the two frequency channels individually at different points in time. In that case, the RedBox combiner is still able to extract the information being sent from at least one of the two copies of the data sent over the channels. C. Redundancy Box Enhancement for Timing Diversity Adding time diversity to the PRP-WLAN system can possibly make it more immune to certain types of noise. To introduce time diversity in the parallel redundancy protocol, the transmitting side of the PRP RedBox needs to be enhanced by a delay line in one of the two sending paths (Fig. 3). This can be done by creating a certain time shift in the time between the transmission of the first channel s data and the second channel s data. By adding this time shift, the system is likely to be more capable of avoiding the negative impacts of time-varying interference. Figure 3. Time Diverse PRP-WLAN Architecture

3 This Timing Splitter can be described as follows: t = t t A B = t t D reflects the queuing delay time of the duplicated packets at the respective interface with delay queue. X X + t IV. OPNET MODEL 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. A. PRP Model Implementation on OPNET 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 g 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. D Figure 4. PRP-WLAN OPNET Model 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. B. PRP System with Time Diversity Capabilities 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 (2) 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. Figure 5. Pulsed Jammer OPNET Output 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. 1) Theoretical Expectations 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

4 Figure 6. Theoretical analysis of 5ms sampling period PRP system with 3ms time shift (5ms best) 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. TABLE I. NUMBER OF SUCCESSFUL TRANSMISSIONS PER SYSTEM ON 10ms SAMPLING PERIOD (OUT OF 120 PACKETS) Time Shift (ms) Single Channel System PRP System Channel 1 Channel 9 PRP TABLE II. NUMBER OF SUCCESSFUL TRANSMISSIONS PER SYSTEM ON 5ms SAMPLING PERIOD (OUT OF 120 PACKETS) Time Single Channel System PRP System Shift (ms) Channel 1 Channel 9 PRP 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 timevarying interference. 2) OPNET Verification of PRP Time Diversity 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. Figure 7. Theoretical analysis of 5ms sampling period PRP system with 4ms time shift (5ms worst)

5 Figure 8. Theoretical analysis of 10ms sampling period PRP system with 2ms time shift (10ms best case 1) 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. TABLE III. PERCENTAGE OF SUCCESSFUL TRANSMISSIONS PER SYSTEM ON 5ms SAMPLING PERIOD Figure 9. OPNET analysis for 5ms sampling period PRP system with 3ms time shift (5ms best) Time Shift (ms) of channel 1 of channel 9 of PRP % 50.0% 70.8% % 50.0% 71.7% % 50.0% 82.5% % 50.0% 65.0% TABLE IV. PERCENTAGE OF SUCCESSFUL TRANSMISSIONS PER SYSTEM ON 10ms SAMPLING PERIOD Figure 10. OPNET analysis of 5ms sampling period PRP system with 4ms time shift (5ms worst) Time Shift (ms) of channel 1 of channel 9 of PRP % 50.0% 73.3% % 73.3% 95.0% % 46.7% 95.0% % 73.3% 90.0% % 26.7% 83.3% % 53.3% 88.3% % 26.7% 88.3% % 53.3% 80.0% % 26.7% 80.0% Figure 11. OPNET analysis of 10ms sampling period PRP system with 2ms time shift (10ms best case 1) Figure 12. Theoretical analysis of 10ms sampling period PRP system with 3ms time shift (10ms best case 2)

6 Figure 13. Theoretical analysis of 10ms sampling period PRP system with 1ms time shift (10ms worst) Figure 14. OPNET analysis of 10ms sampling period PRP system with 3ms time shift (10ms best case 2) Figure 15. OPNET analysis of 10ms sampling period PRP system with 1ms time shift (10ms worst) V. CONCLUSION 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 researched. REFERENCES [1] IEEE , Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications; available at [2] IEEE Ethernet Standard; available at [3] D.G. Brennan, "Linear diversity combining techniques," Proc. IRE, vol.47, no.1, pp , June [4] H. Beikirch, M. Voss, A. Fink, Redundancy Approach to Increase the Availability and Reliability of Radio Communication in Industrial Automation ; Proceeding of: 14th IEEE International Conference on Emerging Technologies and Factory Automation ETFA, Mallorca- Spain, September [5] H. Kirrmann, M. Hansson, P. Muri, IEC PRP: Bumpless recovery for highly available, hard real-time industrial networks ; Proceeding of the IEEE International Conference on Emerging Technologies and Factory Automation ETFA, Patras-Greece, September [6] M. Rentschler, P. Laukemann, Towards a Reliable Parallel Redundant WLAN Black Channel ; Proceeding of the IEEE International Workshop on Factory Communication Systems WFCS, Lemgo/Detmold-Germany, May [7] M. Rentschler, P. Laukemann, Performance Analysis of Parallel Redundant WLAN ; Proceeding of the IEEE International Conference on Emerging Technologies and Factory Automation ETFA, Krakow- Poland, September [8] Official Site for OPNET: [9] E.E. AbdelReheem, Y.I. El Faramawy, H.H. Halawa, M.A. Ibrahim, A. Elhamy, T.K. Refaat, R.M. Daoud and H.H. Amer, "On the effect of interference on Wi-Fi-based Wireless Networked Control Systems," Proceedings of the IEEE IET 8th International Symposium on Communication Systems, Networks and Digital Signal Processing CSNDSP, Poznan-Poland, July [10] H.H. Halawa, A. Elhamy, M.A. Ibrahim, E.E. AbdelReheem, Y.I. Faramawy, T.K. Refaat, R.M. Daoud and H.H. Amer, Sensor/Actuator Mobility in Noisy Wi-Fi based Networked Control System, Proceedings of the IEEE International Conference on Mechatronics ICM, Vicenza-Italy, February [11] Y.I. Faramawy, M.A. Ibrahim, H.H. Halawa, A. Elhamy, E.E. Abdel Reheem, T.K. Refaat, R.M. Daoud and H.H. Amer, Multicasting for Cascaded Fault-Tolerant Wireless Networked Control Systems in Noisy Industrial Environments, Proceeding of the IEEE International Conference on Emerging Technologies and Factory Automation ETFA, Krakow-Poland, September 2012.