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Network Based on Self-Routing Principle #



IEICE TRANS. ELECTRON., VOL.E82–C, NO.2 FEBRUARY 1999

187

PAPER

Joint Special Issue on Photonics in Switching: Systems and Devices

All-Optical Code

Division Multiplexing Switching Network Based on Self-Routing Principle?
Isamu SAEKI?a) , Member, Shouhei NISHI?? , Student Member, and Koso MURAKAMI?? , Member

SUMMARY The tera-bit order capacity of ultrahigh-speed and wide-band networks will become necessary to provide highly advanced multimedia services. In conventional networks, electronic circuits limit the speed capability of the networks. Consequently, all-optical networks are essential to realize ultrahighspeed and wide-band communications. In this paper, we propose the con?guration of an all-optical code division multiplexing (CDM) switching network based on self-routing principles and the structure of a nonlinear all-optical switching device as one of the key components for the network. We show that the required performances of the optical devices used in the CDM switching fabric are lower than those used in the TDM and illustrate the basic transmission characteristics of the switching device utilizing FD-BPM. To evaluate the multiplexing performance, we demonstrate the maximum number of channels under an error-free condition and the BER characteristics when the Gold sequence is applied as one of the CDM code sets, and show that the network of the sub-tera-bit order capacity is realizable by adopting TDM, WDM and CDM technologies. We also illustrate the packet assembly method suitable for self-routing transmissions and one of network architectures where the proposed switching fabric can be exploited. key words: ?????? ? ×? ? ? ? ? × ?? ???? ?? ? ? ? × ? ?
???? ? ? ???? ? ? ??

1.

Introduction

In the highly advanced information processing society of the 21st century, the tera-bit order capacity of ultrahigh-speed and wide-band networks will become necessary to realize advanced multimedia communications, for example, video on demand, telemedicine and virtual reality. In conventional networks, optical ?bers are used as transmission lines, but switching nodes are composed of complex electronic circuits. Therefore, O/E and E/O conversions are inevitable at each node and electronic circuits limit the speed capability of the networks. Consequently, all-optical networks, where optical signals are switched by optical control signals, are essential for realizing near-future networks of the tera-bit order capacity [1]. Ultrahigh-speed all-optical
Manuscript received June 18, 1998. Manuscript revised September 21, 1998. ? The author is with the Computation Center, Osaka University, Ibaraki-shi, 567-0047 Japan. ?? The authors are with the Department of Information Systems Engineering, Graduate School of Engineering, Osaka University, Suita-shi, 565-0871 Japan. a) E-mail: saeki@center.osaka-u.ac.jp ? This paper is also published in IEICE Trans. Commun., Vol.E82-B, No.2, pp.239–245, February 1999.

networks require autonomous routing principles, because call setup functions and signal synchronization in each node make switching node con?gurations complicated and expensive. As for optical multiplexing methods, the time division multiplexing (TDM) and the wavelength division multiplexing (WDM) methods have mainly been studied. The former utilizes high-speed and large-capacity optical memory devices, and also needs strict time synchronization between communicating nodes [2]–[4]. The latter does not require internode time synchronization, but necessitates complicated optical devices such as wavelength converters and ?lters. Moreover, even if both TDM and WDM technologies are adopted, the realizable multiplexing capacity is insu?cient. On the other hand, the optical code division multiplexing access (CDMA) method has drawn researchers’ attention [5]–[8]. Although the CDMA is characterized by the on-demand access method, this study will deal with the connection oriented method, called CDM hereafter. The outstanding features of the optical CDM method are listed below. Firstly, optical CDM encoders and decoders can be realized with much simpler optical devices compared to the optical TDM and WDM methods. Secondly, it does not require time synchronization control systems, unlike the TDM method. Lastly, it will be possible to connect wireless and wired networks seamlessly [2], [3], for the CDMA method has been practically deployed especially in mobile wireless network systems. Thus far, optical CDM encoding and decoding systems have mainly been studied, such as the experiment on the coherent CDM code converter [9]. On the other hand, the photonic switch based on CDM was reported [10], but there are very few reports that consider its application to large-sized public networks [17]. In this paper, we propose the con?guration of an all-optical CDM switching network based on selfrouting principles and the structure of a nonlinear alloptical switching device as one of the key components for the network. In Sect. 2, we illustrate the packet assembly method suitable for self-routing transmissions, the con?guration of the all-optical CDM switching fabric and the network architecture where the proposed switching fabric can be exploited. We also show that the required performances of the optical devices used

IEICE TRANS. ELECTRON., VOL.E82–C, NO.2 FEBRUARY 1999

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Fig. 2 Fig. 1 Optical packet format.

Con?guration of all-optical CDM switching fabric.

addresses or higher layer tags. in the CDM switching fabric are lower than those used in the TDM. In proposing new systems, the feasibility of constructing devices should be taken into consideration. Therefore, in Sect. 3, we propose the model of the all-optical switching device and show the basic transmission characteristics utilizing FD-BPM. In Sect. 4, to evaluate the multiplexing performance, we demonstrate the maximum number of channels under an error-free condition and the bit error rate (BER) characteristics when the Gold sequence is applied as one of the CDM code sets. We also show that the network of the subtera-bit order capacity is realizable by adopting TDM, WDM and CDM technologies. 2. Switch Con?guration and Network Architecture 2.2 All-Optical CDM Switching Fabric Figure 2 shows the proposed con?guration of the alloptical CDM switching fabric, where packets described in the previous subsection are switched based on selfrouting principles. The switch consists of the following modules; the junctions, the decoders, the signal regenerators, the routing processors and the encoders [9]. Basically, CDM switching is realized by converting the CDM codes of input signals to those of output signals according to routing tags. In Fig. 2, input signals multiplexed by the CDM codes are divided by the junction to n lines, where n is the number of channels. The CDM decoders demodulate the divided signals by tapped-delay lines. The decoded signals are sent to the signal regenerators to remove interfering noises by optical hard-limiters. Signals of the packet detection bit are extracted from regenerated signals and sent to the routing processors. The regenerated signals, except those of the packet detection bit, are divided into the head parts of routing tags and other parts, which are sent to the routing processors and the encoders, respectively. To distribute the bit sequences, we can utilize either optical switches or dividers with controls for receiving messages. At the inputs of the encoders, signals of the packet detection bit are added to the packet signals. Referring to the head parts of routing tags, the routing processors generate the CDM codes and drive the encoders. Encoded signals are merged by the junction and sent to the output of switches. Figure 3 shows the con?guration of the all-optical CDM encoder. The encoder has a con?guration similar to that of a bipolar optical encoder [9], except that switches or phase shifters are controlled by optical control signals from the routing processors. Note that the decoder can be realized by inverting the input and the output of the encoder and setting the switches or phase shifters statically according to the CDM code. Figure 4 shows the con?guration of the all-optical CDM routing processor. Signals of the head part of

2.1 Packet Assembly Method Figure 1 shows the proposed optical packet format suitable for all-optical self-routing transmissions. The variable-length routing tag contains complete routing information, that is, a set of output port numbers of all passing nodes to the destination. To simplify the routing processing, each bit referred by switches corresponds to each output channel of switches, namely, only one bit representing the desired output channel is set to ‘1,’ and the others are set to ‘0.’ When packets are sent out from switches, referred bits at the head of routing tags are discarded, which increases the occupation rate of payloads in packets and ensures that the routing information for the next node is always located in the forefront of routing tags. In the upper part of Fig. 1, we show the packet as a sequence of labeled ?elds. Note that the packet is transmitted beginning with the leftmost ?eld. The packet detection bit indicates that the packet is transmitted beginning with this bit. In this format, the packet detection bit is assumed to be one bit ‘1,’ but several bits can also be adopted considering the accuracy. The extra header contains information tags such as source

SAEKI et al: ALL-OPTICAL CDM SELF-ROUTING SWITCHING NETWORK

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Fig. 3

Con?guration of all-optical CDM encoder.

Fig. 5

Network architecture.

2.3 Network Architecture In this subsection, we show one of the all-optical selfrouting network con?gurations, where the proposed switching fabric can be exploited. The holonic optical network [11] was proposed as one of the highly advanced multimedia network systems. The network is characterized by distributed cooperative networking based on autonomous management and all-optical tera-bit order networks. Figure 5 shows the con?guration of the network. The network consists of an optical transport network (OTN) and a personal navigation network (PNN). The OTN is an all-optical self-routing network with simple structures, that facilitate complex controls with optical logical gates. In the OTN, information is transferred and switched between the multiplexers of sending nodes and the demultiplexers of receiving nodes, integrating TDM, WDM and CDM technologies. The PNN is an intelligent electronic network that compensates for simple functions of the OTN. Sending stations obtain complete routing information from the PNN, which refers to the network map and sets connections while avoiding congestion and contention at the beginning of communications. Thus, user terminals can assemble packets including entire routing information. Each optical switching node autonomously performs the routing function while referring only to the routing tags in packets, that is, without exchanging routing information between switching nodes. See [11] for the detailed concept of the Holonic network. 3. All-Optical Switching Device

Fig. 4

Con?guration of all-optical CDM routing processor.

routing tags are separated into individual bit signals by optical delaylines and memories. To memorize these signals for the time length of incoming packets, optical memories are controlled by signals of the packet detection bit, which are delayed by optical delaylines for an appropriate time length. Output signals from optical memories are sent to the optical matrix and converted to those of the CDM code bit for the desired output channels. Note that vertical and horizontal lines are statically connected, and correspond to output channels and the CDM code bits, respectively. The CDM code bit signals are discarded after driving the CDM encoders. The proposed all-optical self-routing CDM switching fabric is composed of optical junctions, hardlimiters, delaylines, memories and switches. The important feature of the CDM switching fabric is that the required performances of the optical devices are lower than those used in the TDM; for example, the necessary capacity of the memories is much smaller and phase synchronization is simpler. This feature is especially fascinating for realizing ultrahigh-speed and wide-band networks in the nearfuture.

Among the optical component devices listed in Sect. 2.2, optical junctions, hard-limiters and delaylines are relatively easier to realize than optical memories and switches. Because optical memories in the proposed switch can be constructed by optical switches

IEICE TRANS. ELECTRON., VOL.E82–C, NO.2 FEBRUARY 1999

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Fig. 7 Transmission characteristics of signal and control light for Pcin . Fig. 6 Structure of nonlinear Mach-Zehnder all-optical switching device.

utilizing bistable optical materials, we focus on optical switches as the key components. Optical switching devices for the switching fabric require the following properties. First, the switching devices must be controlled by optical signals. To control optical signals optically, we must make use of nonlinear optical e?ects, which are caused by the nonlinear response of the polarization. Second, the switching devices must show ultrafast response characteristics. Basically, since nonlinear optical e?ects show a faster response speed and optical pulse widths can be shortened much more than electronic ones, we can expect that alloptical switching devices exhibit much faster switching speed than those controlled by electronic signals. In a recent study of an all-optical switching device utilizing phase modulation, the operation speed was reported to be as high as 160 Gbps [12]. In this section, we propose the model of the alloptical switching device using phase modulation in the nonlinear Mach-Zehnder interferometer. To demonstrate the primary switching characteristics of the device, we numerically analyze the proposed structure utilizing the iterative ?nite di?erence beam propagation method (iterative FD-BPM) [13] with the transparent boundary condition [14], [15]. 3.1 Nonlinear Mach-Zehnder All-Optical Switching Device Figure 6 shows the structure of the nonlinear MachZehnder all-optical switching device. We simulate the propagation characteristics of the device with continuous lightwave inputs. The wavelengths of signal and control lights are λs = 0.9 ?m and λc = 0.515 ?m, respectively. Although the wavelengths are di?erent, the refractive

indices and the nonlinear coe?cient for the control light are assumed to be equal to those for the signal light for simplicity. The structure is uniform in the ydirection and the nonlinear dielectric region (shaded) has only Kerr-like nonlinearity. Two 3-dB couplers (length L1 = 3000λs ) are jointed by parallel straight cores (length L2 = 2200λs ). The width of cores, the smallest distance between two cores and the bent angle of cores are d = 4λs , g = 4λs and θ = 0.25? , respectively. The refractive indices of cores and claddings are nf = 1.55 and ns = 1.545, respectively. The refractive index and the nonlinear coe?cient of the nonlinear dielectric region are nf and n2 = 10?9 m2 /W, respectively. Here, we assume the nonlinear material to be the liquid crystal MBBA, which has a relatively large nonlinear dielectric coe?cient. The signal and the control lights are launched into port 1 and port 2 with input powers Psin = 2.1 mW/mm and Pcin , respectively. The signal light will be equally divided by the input 3-dB coupler (L1 ) and then they will propagate along the straight cores (L2 ) without coupling. Two guided waves will be coupled again by the output 3-dB coupler (L1 ). On the other hand, the control light does not couple in the input 3-dB coupler because of its short wavelength, and propagates along the right core. Since the L2 region of the right core is nonlinear, two guided signal lights will be coupled di?erently and the transmission coe?cient in each output port will vary with the input power of the control light. 3.2 Numerical Results Figure 7 shows the transmission characteristics of the signal and the control lights in each output port for Pcin . In Fig. 7, the transmission coe?cients of the signal light vary periodically with Pcin , whereas those of the control light keep almost constant values. The control power needed to change the paths of the signal light

SAEKI et al: ALL-OPTICAL CDM SELF-ROUTING SWITCHING NETWORK

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(a)Pcin = 0

Fig. 9

Field amplitude evolution of control light.

shifted by the same ratio, we can expect basic transfer characteristics similar to those shown in Figs. 7–9. To analyze the accurate transfer characteristics for the shifted wavelengths, we must measure the optical constants. 4. Performance Analysis

(b)Pcin = 0.44 (mW/mm)

Fig. 8

Field amplitude evolution of signal light.

is estimated to be 0.44 mW/mm, which is considerably small compared to the input power of the signal light of 2.1 mW/mm. Figures 8(a) and 8(b) show the ?eld amplitude evolutions of the signal light for Pcin = 0 and Pcin = 0.44 mW/mm, respectively. These ?gures illustrate well that the paths of the signal light are all-optically controlled. Figure 9 shows the ?eld amplitude evolutions of the control light for Pcin = 0.44 mW/mm. Since the wavelengths of the signal and the control lights are different, the control light launched into port 2 does not couple in the 3-dB coupler and propagates along the right nonlinear core. Therefore, the paths of the signal light are switched by the relatively lowpower of the control light. In this analysis, we selected the wavelengths of 0.9 ?m and 0.515 ?m, but even if the wavelengths are

In general, the selection of a set of CDM codes in?uences the BER performance of network systems. To increase the number of channels in CDM network systems, we must choose a class of CDM codes that have less cross- and auto-correlation properties as well as a large family size. The Gold sequence [16] and the M sequence are two major CDM codes. The Gold sequence is easily generated and has a much larger family size than the M sequence. In this section, to evaluate the capacity of the proposed network system, we calculate the maximum number of channels under an error-free condition and the BER characteristics using the Gold sequence. 4.1 Maximum Number of Channels under Error-Free Condition First of all, we assume that there is no quantum and thermal noise. According to the following algorithm, we calculate the maximum numbers of channels for the Gold sequence generated by the n-bit shift register. 1. Take two of the generated M sequences. Generate the (2n ? 1) Gold sequences by calculating XOR for the pair with all phase di?erences. 2. Take two of the Gold sequences generated in 1. Calculate the maximum and minimum crosscorrelations of the pair. 3. Perform 2, for all of the Gold sequences generated in 1, and select the maximum and mini-

IEICE TRANS. ELECTRON., VOL.E82–C, NO.2 FEBRUARY 1999

192

Fig. 10

Maximum number of channels with Gold sequence.

mum cross-correlations. Calculate the di?erence between them. 4. Perform 3, for all pairs of the M sequences and de?ne C as the smallest value of the di?erence. 5. To achieve an error-free condition, the received signal level should be greater than the maximum summation of all correlation noises from other channels, which can be expressed as C(M ? 1) < 1, (1) and

Fig. 11

BER performances with Gold sequence.


PCm (u) =

?∞

PCm?1 (u ? x) P0 · δ(u) (3)

+ P1 · PC (x) dx

where M is the number of channels. Thus, the maximum number of channels under the error-free condition Mmax is Mmax = 1 +1 , C (2)

PC1 (u) = δ(u),

(4)

where δ(x) is Dirac’s delta function and P0 and P1 are the probabilities with which transmitters send signals of ‘0’ and ‘1,’ respectively. Supposing that P0 = P1 = 1/2, the BER PEm is given by


where x is the integer part of x. Figure 10 shows the calculated maximum number of channels versus register length for 5 ≤ n ≤ 10. The ?gure illustrates that the maximum number of channels increases with the register length. If the register length is 10, the achievable number of channels under the error-free condition is 9, where the period of sequences is 1023 chips. Assuming that one physical link has a 100GHz band width, the link can carry 9 CDM channels of 100 GHz/1023 bits?1 100 Mbps signals, which can also be multiplexed by the TDM method. Since the band width of optical ?bers is about 10THz, the entire data transfer rate with one optical ?ber is estimated at 100 Mbps × 9 × (10 THz/100 GHz) = 90 Gbps, adopting the TDM, WDM and CDM methods in the proposed network system. 4.2 BER Characteristics To evaluate BER characteristics, we ?rst calculate the probability density function PC (u) of cross-correlation for the Gold sequences generated in the previous subsection. Assuming that m transmitter and receiver pairs communicate continuously with the on-o? keying, the probability density function of the interference from (m ? 1) transmitters PCm (u) is given by PEm =
?∞≤T h≤∞ T h?1

min

P0
Th

PCm (u)du (5)

+P1

?∞

PCm (u)du ,

where T h is the threshold of decoders. Figure 11 shows the calculated BER characteristics for 5 ≤ n ≤ 10. Taking the same consideration as that described in the previous subsection, the entire data transfer rate is estimated to be 100 Mbps × 12 × (10 THz/100 GHz) = 120 Gbps, under the condition that n = 10 and PEm < 10?10 . As a result of the analysis in this section, we showed that the network of sub-tera-bit order capacity is realizable by adopting TDM, WDM and CDM technologies. Although the data transfer rate estimated in the analysis is su?ciently large to meet the demand of networks for the time being, developments in CDM sequences will further enlarge network capacities, the detailed examination of which will be the subject for future studies. 5. Conclusion

We proposed the con?guration of the all-optical CDM

SAEKI et al: ALL-OPTICAL CDM SELF-ROUTING SWITCHING NETWORK

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switching network based on self-routing principles and the structure of the nonlinear all-optical switching device as one of the key components for the network. The main feature of the proposed network is the fact that the required performances of the optical devices are lower than those used in the TDM and that it does not require time synchronization control systems. We then demonstrated the basic switching characteristics of the device by simulating the propagation characteristics utilizing FD-BPM. We also evaluated the multiplexing performance of the CDM switching system by demonstrating the maximum number of channels under the error-free condition and the BER characteristics when the Gold sequence is applied as one of the CDM code sets. We showed that the network of the sub-tera-bit order capacity is realizable by adopting TDM, WDM and CDM technologies. Moreover, we illustrated the packet assembly method suitable for self-routing transmissions and one of the network con?gurations where the proposed switching fabric can be exploited. In the future, we will con?rm the behavior of the proposed switching fabric by experiments.
References [1] S.B. Alexander, R.S. Bondurant, D. Byrne, V.W.S. Chan, S.G. Finn, R. Gallger, B.S. Glace, H.A. Haus, P. Humblet, R. Jain, I.P. Kaminow, M. Karol, R.S. Kennedy, A. Kirby, H.Q. Le, A.A.M. Saleh, B.A. Scho?eld, J.H. Shapiro, N.K. Shankaranarayanan, R.E. Thomas, R.C. Williamson, and R.W. Wilson, “A precompetitive consortium on wide-band all-optical networks,” IEEE J. Lightwave Technol., vol.11, no.5/6, pp.714–735, 1993. [2] S. Park, K. Tsukamoto, and S. Komaki, “Proposal of direct optical switching CDMA for cable-to-the-air system and its performance analysis,” IEICE Trans. Commun., vol.E81-B, no.6, pp.1188–1196, June 1998. [3] S. Park, K. Tsukamoto, and S. Komaki, “Radio highway networks using optical CDMA with optical polarity reversed correlator,” Proc. MWP’97, vol.1, no.1, pp.227–230, Sept. 1997. [4] H. Harada, S. Kajiya, K. Tukamoto, S. Komaki, and N. Morinaga, “TDM intercell connection ?ber-optic bus link for personal radio communication systems,” IEICE Trans. Commun., vol.E78-B, no.9, pp.1287–1294, Sept. 1995. [5] J.A. Salehi, “Code division multiple-access techniques in optical ?ber networks—Part I: Fundamental principles,” IEEE Trans. Commun., vol.37, pp.824–833, Aug. 1989. [6] J.A. Salehi and C.A. Brackett, “Code division multipleaccess techniques in optical ?ber networks—Part II: Systems performance analysis,” IEEE Trans. Commun., vol.37, pp.834–842, Aug. 1989. [7] T. Ohtsuki, “Performance analysis of direct-detection optical asynchronous CDMA system with double optical hardlimiters,” IEICE Technical Report, CS96-46, July 1996. [8] H.M.H. Shalaby, “Performance analysis of optical synchronous CDMA communication systems with PPM signaling,” IEEE Trans. Commun., vol.43, no.2/3/4, pp.624–634, Feb./March/April 1995. [9] K. Kitayama, “Error-free 1.24Gb/s 4-chip coherent CDM code converter,” ECOC Conference Publication, no.448, pp.117–120, Sept. 1997. [10] Y. Takushima and K. Kikuchi, “Photonic switching using

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spread spectrum technique,” Electron. Lett., vol.30, no.5, pp.436–438, March 1994. K. Murakami, S. Uno, K. Kinoshita, M. Higashida, and T. Takine, “Distributed cooperative networking architecture for multimedia communications based on autonomous routing,” Proc. ITC-OSCC’96, vol.1, pp.83–86. K. Suzuki, K. Iwasaki, S. Nishi, and M. Saruwatari, “Errorfree demultiplexing of 160 Gbit/s pulse signal using optical loop mirror including semiconductor laser ampli?er,” Electron. Lett., vol.30, no.18, pp.1501–1503, Sept. 1994. H. Yokota, K. Kimura, and S. Kurazono, “Numerical analysis of an optical X coupler with a nonlinear dielectric region,” IEICE Trans. Electron., vol.E78-C, no.1, pp.61–66, Jan. 1995. G.R. Hardley, “Transparent boundary condition for beam propagation,” Opt. Lett., vol.16, no.9, pp.624–626, May 1991. G.R. Hardley, “Transparent boundary condition for the beam propagation method,” IEEE J. Quantum Electron., vol.28, no.1, pp.363–370, Jan. 1992. W. Huang and K. Kitayama, ”Optical pulse code division multiple access utilizing coherent correlation detection,” Proc. ICT’97, pp.1221–1226, April 1997. S. Nishi, I. Saeki, and K. Murakami, “All-optical CDMA self-routing switch for holonic network,” Proc. ITCCSCC’98, vol.1, pp.347–350, July 1998.

Isamu Saeki received his B.E. and M.E. degrees from Osaka University, Osaka, Japan, in 1991 and 1993, respectively. Since 1996, he has been a research assistant at the Computation Center, Osaka University. His research interests include photonic switching network and all-optical switching devices. He is a member of IEEE.

Shouhei Nishi received a B.E. degree in Information Systems Engineering from Osaka University, Osaka, Japan, in 1997. Since 1996, he has been engaged in the study of all-optical network systems. In 1997, he started to work for his Master’s degree at Osaka University, Osaka, Japan. His current research interest is in network systems.

Koso Murakami received his B.E., M.E. and D.E. degrees from Osaka University, Osaka, Japan, in 1971, 1973 and 1991, respectively. From 1973 to 1995, he was with Fujitsu Laboratories Ltd., engaged in research and development of digital switching systems, ATM switching systems and photonic switching technologies. From 1995 to 1998, he was a professor and a director of R&D at the Computation Center, Osaka University. Since 1998, he has been a professor in the Department of Information Systems Engineering of Osaka University. His research interests include high-speed multimedia networks and intelligent information networking architecture. He is a member of IEEE and the Information Processing Society of Japan.



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