JP2018046428A - Repeater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the occurrence of a continued communication disabled state with any other repeaters.SOLUTION: In the reception processing 60 of a frame transmitted from other one repeater which constitutes an ad hoc network, a processing unit in a repeater executes, synchronous establishment processing 61 based on a synchronous signal part, error correction processing (62-64) to a data part, and count processing 65 which counts the number of times of error correction executed on the data part in the error correction processing, as the number of times of corrections for each data part, and also executes discrimination processing 66 which discriminates whether or not the number of times of corrections in the data part matches with a prescribed condition. On discriminating in the discrimination processing that the number of times of corrections matches with the above condition, the processing unit suspends the reception processing 60 for the other one repeater to shift to a communicable state with the repeater constituting the ad hoc network.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a relay device that is configured to receive a partner's route information and to be able to construct a relay route for the device itself and to form an ad hoc network.

  As this type of repeater, a repeater constituting an ad hoc network described in Patent Document 1 below is known. In order to avoid collision, this repeater adopts the CSMA / CA with Ack method and periodically receives route information of other repeaters as a routing protocol suitable for a wireless communication system. A proactive method for constructing route information (information indicating a relay route) is adopted. As a result, according to the ad hoc network equipped with this relay device, even if a failure occurs in one of the relay devices constituting the relay route, another relay route that does not include this relay device is automatically constructed. Therefore, it is possible to maintain the communication state continuously.

JP 2012-118655 A (page 8)

  However, there are the following problems to be improved in the repeater constituting the ad hoc network. That is, in this repeater, when the transmission operation is performed, the transmission waves can be prevented from colliding with each other (carrier collision) by performing transmission by the CSMA / CA method as described above. In two repeaters with a wave reception level less than the carrier sense level, one of the two repeaters performs a transmission operation with the third repeater located between the two repeaters as a counterpart. During execution, the other repeater of the two repeaters may perform a transmission operation (transmission operation to try the third repeater described above). In this case, in the third repeater, the reception level of the transmission wave from the one repeater that is already the communication partner (the partner in which the synchronization state is established and in the communication state) and the other The ratio (SNR (Signal-Noise Ratio)) with the reception level of the transmission wave from the repeater is less than a predetermined reference SNR (the reception level for the other repeater is higher than the reception level for one repeater) Interference may occur in some cases.

  Each repeater constituting an ad hoc network normally transmits a data part to be transmitted by error correction coding to the communication partner on the transmission side, and transmits the data part encoded with error correction from the communication partner on the reception side. A configuration is adopted in which an error correction process is performed on the data portion to convert it to the original data portion. For this reason, each repeater can normally receive the data part from the relay partner of the communication counterpart by error correction processing as long as the SNR does not reach the reference SNR. In addition, in a situation where each relay station cannot normally receive the data portion from the communication partner relay station even if error correction processing is performed in a situation where the SNR is less than or equal to the reference SNR, Then, for example, a retransmission request is transmitted to the communication partner's repeater to try to continue communication with the communication partner.

  For this reason, the state in which the other repeater continues the transmission operation to the third repeater as in the above-described one repeater, the other repeater, and the third repeater (that is, the third repeater In the state where interference continues in the machine), the third repeater cannot normally receive the data part retransmitted from one repeater, and therefore transmits a transmission request to one repeater. Continue useless operation. Also, in this state, when the reception level of the transmission wave from the other relay in the third relay is higher than the reception level of the transmission wave from one relay, the third relay The repeater may be selected as a new communication partner, but this is also difficult during the execution of an operation to continue communication with one repeater that is the current communication partner. Therefore, there is a problem to be improved in the above-described conventional repeater that configures the ad hoc network that a state where communication with any other repeater may not continue may continue.

  The present invention has been made to improve such a problem, and it is a main object of the present invention to provide a repeater that can reduce the occurrence of a situation where communication with any other repeater continues.

  In order to achieve the above object, the repeater according to claim 1 is configured to be able to execute a transmission process and a reception process for a frame having a synchronization signal part and a data part encoded with an error correction code to constitute an ad hoc network. In the reception process for the frame, which is a repeater and transmitted from another repeater constituting the ad hoc network, synchronization establishment processing based on the synchronization signal portion, error correction processing for the data portion, And a condition in which the number of corrections in the data part is defined in advance while performing a counting process in which the number of error corrections performed on the data part in the error correction process is counted as the number of corrections in the data part unit. A determination process is performed to determine whether or not the number of corrections is satisfied. To when it is determined that matches aborts the reception processing for the other one repeater, shifts to and can communicate with the relay device constituting the ad hoc network.

  The repeater according to claim 2 is the repeater according to claim 1, wherein the number of corrections is added and counted as the number of corrections in the counting process, and the number of corrections is a predetermined upper limit value. The determination process is executed under the condition that the value reaches the above.

  The repeater according to claim 3 is the repeater according to claim 1, wherein the number of times of error correction executed continuously for consecutive bits constituting the data part in the count process is the number of corrections. And the determination process is executed on the condition that the number of corrections has reached a predetermined upper limit value.

  According to the repeater according to claim 1, when another data is included in the frame by establishing synchronization with the other repeater, the other repeater transmits. Even if there is a situation in which interference occurs and the data part from another one of the repeaters cannot be received normally, the number of error corrections caused by the occurrence of interference (number of corrections ), The reception process for the data part from the other relay device is stopped (that is, communication with the other relay device is stopped), and transmission from another relay device is performed. It is possible to automatically shift to a state where a signal can be received. That is, according to this repeater, it is possible to avoid the occurrence of a situation where a state in which communication with any other repeater cannot be continued continues.

  According to the relay apparatus of claim 2, the number of corrections is added up in the counting process and counted as the number of corrections, and the above condition (stop condition) indicates that the number of corrections has reached a predetermined upper limit value. Therefore, even if a slight error occurs in the number of corrections, the reception process for the data part from the other relay is stopped and the transmission from the other relay is performed. It is possible to reliably shift to a state where a signal can be received. That is, according to this repeater, it is possible to reliably avoid the occurrence of a situation where a state in which communication with any other repeater cannot be continued.

  According to the repeater of claim 3, in the counting process, the number of times of error correction continuously performed on consecutive bits constituting the data part is counted as the number of corrections, and in the subsequent determination process, By making the above-mentioned condition (stop condition) that the number of corrections that have been counted reaches a predetermined upper limit value, error correction is continuously performed on consecutive bits constituting the data portion. In a situation (that is, a situation where an error has occurred continuously in consecutive bits constituting the data part. A situation in which interference is extremely severe), we quickly escaped from such a serious situation and made another relay. It is possible to shift to a state where a transmission signal from the machine can be received.

It is a block diagram of the ad hoc network 1 containing relay machine STR1, STR2, STR3. It is a block diagram of the relay machine STR. FIG. It is a flowchart for demonstrating the reception operation | movement of the relay station STR. It is explanatory drawing for demonstrating the reception operation | movement of relay STR3.

  Hereinafter, an embodiment of a repeater will be described with reference to the accompanying drawings.

  First, the configuration of the relay STR and the configuration of the ad hoc network 1 including a plurality of the relay STR will be described with reference to the drawings.

  As shown in FIG. 1, the ad hoc network 1 is configured to include a plurality of relay devices STR1 to STR3 (hereinafter, also referred to as “relay device STR” when not distinguished) in this example. . In this example, as an example, the repeater STR3 functions as an end point repeater (so-called master unit; hereinafter, also simply referred to as an end point device), and the other repeaters STR1 and STR2 are referred to as start point repeaters (hereinafter, simply referred to as start point devices). Say). Therefore, the relay devices STR1 and STR2 can directly communicate with the relay device STR3 (as shown in FIG. 1, the relay devices STR1 and STR3 are directly connected to each other, and the relay devices STR2 and STR3 are directly connected to each other. (As shown in FIG. 1, the two relays STR are both in the carrier sense range of the other party (the range indicated by the dashed-dotted circle)). Further, the distance between the relays STR1 and STR2 with respect to the relay STR3 is shorter in the relay STR2 than in the relay STR1 (that is, the received electric field strength for the relay STR2 in the relay STR3 is the same in the relay STR3). It is assumed that the received electric field strength is greater (larger) than the relay STR1.

  On the other hand, the relays STR1 and STR2 are in a positional relationship where they cannot communicate directly with each other (as shown in FIG. 1, the two relays STR are both out of the carrier sense range of the other party). And

  As shown in FIG. 2, each relay STR is configured by a transmission unit 2, a reception unit 3, and a computer, for example, and controls the operation of the transmission unit 2 and the reception unit 3. A processing unit 4 that executes various types of processing such as reception processing, and a storage unit 5 that stores holding information to be described later, identification information to be described later of the own device, a routing table to be described later, and the like are provided. In an ad hoc network, the number of relay stations STR is not limited, and may be comprised of several tens to several thousand. Each relay station STR that functions as a start point machine operates autonomously to reach an end point machine. The communication route is constructed and the route table is updated. However, each of the relays STR1 and STR2 functioning as the start machine in this example is arranged at a position where it can directly communicate with the end machine, and the next hop destination is fixed to the end machine (that is, the route) Since the table is not required to be updated), the description of the operation for constructing the communication path (route table update operation) will be omitted. The repeaters STR1 and STR2 include the retained information and the identification information of the own device in the data portion shown in FIG. 3, and error coding (for example, burst error correction such as Hagelberger code) for the information constituting the data portion. The data is encoded by a code, random error correction encoding, etc.) and transmitted via the transmission unit 2.

  The transmission unit 2 is controlled by the processing unit 4 in a CSMA / CA (with Carrier Sense Multiple Access / Collision Avoidance) with Ack method based on a predetermined carrier sense level CS, and is not illustrated via an antenna (not illustrated). Various types of information are wirelessly transmitted to the child device (mobile terminal) and other relay device STR.

  The receiving unit 3 constantly monitors the presence / absence of a transmission signal from the transmitting unit 2 of another relay STR by carrier sense. Specifically, the receiving unit 3 measures the reception electric field strength of the transmission signal, determines that there is a transmission signal when the reception electric field strength is equal to or higher than the carrier sense level, and determines the carrier sense signal as the processing unit 4. Is received, and information included in the transmission signal (information constituting the frame shown in FIG. 3 transmitted from another relay station STR or slave unit) is received and output to the processing unit 4 as reception information. .

  Regarding the processing unit 4, the operation contents are different between the processing unit 4 of the relay station STR3 which is an end point machine and the processing units 4 of the other relay machines STR1 and STR2. Therefore, first, the processing unit 4 of the repeaters STR1 and STR2 will be described. When the processing unit 4 detects the output of the carrier sense signal from the receiving unit 3, the processing unit 4 executes a receiving process 60 shown in FIG. In the reception process 60, as will be described in detail later, the processing unit 4 inputs the reception information (the frame having the configuration shown in FIG. 3) output from the reception unit 3 following the carrier sense signal, and is included in this frame. During the reception period of the synchronization signal portion (bit synchronization signal and frame synchronization signal), a synchronization establishment process is performed for establishing synchronization using this synchronization signal portion, and a data portion (included in the data portion) following this synchronization signal portion is executed. Data portion receiving processing (processing for acquiring information contained in the data portion and storing it in the storage portion 5) is executed. In this case, the processing unit 4 stores this information in the storage unit 5 as possessed information only when the information included in the acquired data unit is information addressed to itself.

  Further, when the holding information is newly created or updated, the processing unit 4 executes transmission processing to error-encode the data part including the holding information, and configures the frame as shown in FIG. It is transmitted to the hop destination stored in the route table (in this example, the relay station STR3 which is the end point relay station).

  Next, the processing unit 4 of the repeater STR3 that functions as a master unit will be described. The processing unit 4 performs the same operation (establishment of synchronization and acquisition of the data part) as the other relays STR1 and STR2 described above for the reception process, and information transmitted from the relays STR1 and STR2 Is received (acquired). On the other hand, the processing unit 4 executes the transmission process when new information is received from each of the relays STR1 and STR2, and transmits the received information (information held by each of the relays STR1 and STR2) to the transmission destination. (A server that is a hop destination in the other relays STR1 and STR2 and is connected via a communication infrastructure (not shown) in this example).

  Next, the operation of each relay STR configuring the ad hoc network 1 will be described. Note that the transmission process executed by the processing unit 4 of each of the repeaters STR1 to STR3 is the same as the transmission process in a known repeater that configures the ad hoc network, and therefore description thereof is omitted, and the reception process 60 is mainly illustrated in FIG. Will be described with reference to FIG.

  In each repeater STR, when another repeater STR within the carrier sense range executes transmission processing, the receiving unit 3 determines that there is a transmission signal from the other repeater STR and processes the carrier sense signal. In addition to outputting to the unit 4, information included in the transmission signal (information constituting the frame shown in FIG. 3) is received and output to the processing unit 4 as reception information.

  In this case, the processing unit 4 of the relay STR detects the output of the carrier sense signal from the receiving unit 3 and executes the receiving process 60 shown in FIG. In this reception process 60, the processing unit 4 first inputs the reception information (the frame having the configuration shown in FIG. 3) output from the reception unit 3 following the carrier sense signal and executes the synchronization establishment process (step 61). ). In this synchronization establishment process, the processing unit 4 performs transmission processing using this synchronization signal unit during the reception period of the synchronization signal unit (bit synchronization signal and frame synchronization signal) included in the received frame. Is established with the relay STR (the relay STR of the communication partner).

  Next, the processing unit 4 executes a data part reception process. In this data part reception process, the processing part 4 first executes an error correction process for each bit constituting the data part received following the synchronization signal part. In this error correction process, each time a new bit constituting the data part is acquired, the processing unit 4 executes an error detection process for the new bit (step 62), and based on the detection result in this error detection process. Then, it is determined whether or not an error is detected in the new bit (step 63). Further, when it is determined that an error has been detected as a result of the determination, the processing unit 4 executes a correction process for correcting the erroneous bit (step 64) and completes the error correction process. On the other hand, when determining that no error is detected in step 63 (that is, when determining that the new bit is correct), the processing unit 4 completes the error correction process without executing step 64, The process proceeds to step 67 described later, and the correct new bit (information) is stored in the storage unit 5.

  In addition, when the processing unit 4 executes a correction process for correcting a bit having an error in the error correction process in the above-described step 64 and completes the error correction process for this bit, the process for counting the number of corrections is then performed. Is executed (step 65). In this counting process, the processing unit 4 resets the count value to the initial value (zero) at the start of reception of the data part for one frame, and then performs error correction processing on the bits constituting this data part. Each time is executed, the count value is incremented to count (accumulate) the number of error corrections (the number of corrections in units of data parts) executed for the data part.

  Further, when the processing unit 4 executes this counting process (that is, when the count value is incremented), the new count value (the integrated value of the number of corrections) is then set in advance under a predetermined condition (stop condition). ) Is executed (step 66). In this determination processing, the fact that this new count value has reached a predetermined upper limit value is used as the above condition (stop condition), and in the count processing, it continues for consecutive bits constituting the data portion. The number of error corrections performed in this way is counted as the number of corrections, and the above condition (stop condition) can be that the number of corrections thus counted has reached a predetermined upper limit value. . In this example, as an example, the processing unit 4 adopts the former condition of the above-described two conditions (stop condition) and executes the determination process.

  In addition, when the result of the determination process in step 66 does not match the above condition (stop condition), the processing unit 4 stores the correct bit corrected in step 64 in the storage unit 5 (step 67). ). In addition, when the processing unit 4 determines that no error is detected in the determination in step 63, the processing unit 4 proceeds to step 67 and stores the bit (correct bit) determined to have no error in step 63. Store in unit 5.

  After execution of this step 67, the processing unit 4 detects whether or not the reception of the data part is completed (that is, detects whether or not reception of all the bits constituting the data part is completed normally ( Step 68)) When it is determined that the processing has not been completed, the process proceeds to Step 62, and the next new bit constituting the data part (the bit next to the bit stored in the storage unit 5 in Step 67). Error correction processing for each of the processes (steps 62 to 67).

  In this way, the processing unit 4 executes this reception processing 60 until it is detected in step 68 that reception of the data part is completed, so that all the bits constituting the received data part are correct (no error). Even when there is an error, when the correct content is corrected in the correction process in step 64, and the number of corrections (correction number) does not match the above condition (stop condition) (in this example, correction) When the number of times has not reached the upper limit value), all the bits constituting the data part are stored in the storage part 5 without error.

  On the other hand, before the processing unit 4 stores all the bits constituting the data unit in the storage unit 5 in an error-free state in the step 67, the number of corrections in the determination process in the step 66 is the above condition (stop condition). When it is determined that the number of corrections has been reached (the number of corrections has reached the upper limit), the reception process 60 for receiving transmission information from the relay STR of the communication partner with which synchronization is currently established is stopped (preferably immediately (Stop immediately) and shift to a state where communication with a new relay STR is possible.

  Accordingly, in each of the relays STR1, STR2, STR3 arranged in the positional relationship shown in FIG. 1, for example, when the relays STR2, STR3 are not executing transmission processing, the relay STR1 performs transmission processing on the relay STR3. In the relay STR3, the receiving unit 3 determines that there is a transmission signal from the relay STR1 as another relay STR and outputs a carrier sense signal to the processing unit 4 and is included in the transmission signal Information (information constituting the frame shown in FIG. 3) is received and output to the processing unit 4 as reception information.

  In a state where the relay station STR2 is not executing transmission processing, the transmission signal transmitted from the relay station STR1 to the relay station STR3 is normally received by the relay station STR3. For this reason, in the relay unit STR3, the processing unit 4 executes the reception process 60 to establish synchronization with the relay unit STR1 by time t0 as shown in FIG. 5, and from time t0, from the relay unit STR1. Each bit constituting the data part to be transmitted is received without error and stored in the storage unit 5. As a result, when the state in which only the relay STR1 transmits to the relay STR3 continues until the end of the data part, the processing unit 4 of the relay STR3 sets all the bits constituting the data part transmitted from the relay STR1. The data is received without error and stored in the storage unit 5.

  On the other hand, as shown in FIG. 5, when the repeater STR2 starts transmission processing from the time t1 to the repeater STR3 that is continuing the reception operation for the data part (outside the carrier sense range of the repeater STR2). Since the relay device STR1 is located, the relay device STR2 can start transmission even during transmission of the relay device STR1). In the relay device STR3, the transmission signal from the communication partner relay device STR1 and the relay device Interference occurs with the transmission signal from STR2. In particular, in this example, since the relay STR2 is closer to the relay STR3 than the relay STR1, the reception field strength of the transmission signal from the relay STR2 at the relay STR3 is the reception of the transmission signal from the relay STR1. Stronger (greater) than field strength. For this reason, in the relay unit STR3, the processing unit 4 detects errors at a high frequency in step 63 in the reception processing 60 with respect to the bits of the data part from the relay unit STR1 received after this time t1, and in step 64 The correction processing is executed with high frequency for this bit, and the number of corrections is incremented with high frequency in step 65. Therefore, in the relay station STR3, the number of corrections counted by the processing unit 4 reaches the upper limit value that is a condition (stop condition) in a short time after the elapse of time t1 (for example, at time t2). As a result, the processing unit 4 of the relay STR3 detects this in step 66, stops the reception processing 60 (immediately stops) as described above, and communication with the new relay STR is performed. Transition to a possible state.

  In this case, in the receiving unit 3 of the relay STR3, the transmission signal from the relay STR2 located closer is stronger than the transmission signal from the relay STR1 that has established synchronization as a communication partner. Received with electric field strength. Therefore, in the relay STR3, immediately after the time t2 when the processing unit 4 shifts to a state in which communication with the new relay STR is possible, the receiving unit 3 transmits the transmission signal from the relay STR2 immediately and automatically. The carrier sense signal is output to the processing unit 4 and the information (information constituting the frame shown in FIG. 3) included in the transmission signal is received and output to the processing unit 4 as reception information. To do. In this case, the difference between the received electric field strength (large) for the transmission signal from the relay STR2 in the relay STR3 and the received electric field strength (small) for the transmission signal from the relay STR1 is based on a predetermined reference SNR. Is larger, the receiving unit 3 of the repeater STR3 receives the bits constituting the synchronization signal unit and the data unit included in the frame with very few errors and outputs them to the processing unit 4.

  Next, the processing unit 4 of the relay STR3 detects the output of the carrier sense signal from the receiving unit 3, and executes the receiving process 60 shown in FIG. As described above, in the state where each bit constituting the synchronization signal unit and the data unit output from the reception unit 3 has very few errors, the processing unit 4 of the repeater STR3 performs the synchronization signal unit (bit synchronization signal and frame synchronization). By executing the synchronization establishment process (step 61) during the reception period of the signal), the synchronization with the relay STR2 is reliably established by the end of the synchronization signal section (time t3), and after time t3 The data part (data part from the relay STR2) output from the receiving part 3 is accurately received and stored in the storage part 5 in a state where almost no errors occur in the bits in the data part receiving process.

  As described above, in the relay STR configuring the ad hoc network 1, the processing unit 4 performs a synchronization establishment process (steps) based on the synchronization signal unit in the reception process 60 for the frame transmitted from the other relay STR. 61), error correction processing for the data portion (steps 62 to 64), and count processing (step 65) for counting the number of error corrections performed on the data portion in the error correction processing as the number of corrections in units of data However, a determination process (step 66) is performed to determine whether or not the number of corrections in the data portion matches a predetermined condition (stop condition), and the number of corrections in this determination process matches this condition. If it is determined, the reception process 60 for the other relay STR is stopped (preferably immediately stopped), and It moves to a communicable state with the new repeater STR constituting the hook network 1.

  Therefore, according to the relay STR, when the data part included in the frame is received by establishing synchronization with the other relay STR1 as in the relay STR3 described above. When another relay device STR2 starts transmission and interference occurs in this relay device STR3, the data part from the relay device STR1 cannot be normally received (time t1 to time in FIG. 5 described above) Even if a situation during the period of t2 occurs, reception of the data part from another relay STR1 based on an increase in the number of times of error correction (correction count) caused by the occurrence of interference The process 60 can be stopped (that is, communication with the other relay STR1 can be stopped), and the process can be automatically shifted to a state where a transmission signal from another relay STR2 can be received. That is, according to this relay STR, it is possible to avoid a situation in which a state in which communication with any other relay STR continues is impossible.

  Further, according to this relay STR, the processing unit 4 adds up the number of corrections in the counting process and counts it as the number of corrections, and that the number of corrections has reached a predetermined upper limit value (the above condition ( Since the discriminating process is executed as the (stop condition), even if a slight error occurs in the count of the correction times, the reception process 60 for the data part from the other relay STR1 is stopped and another process is performed. It is possible to reliably shift to a state where a transmission signal from the relay STR2 can be received. That is, according to this relay STR, it is possible to reliably avoid the occurrence of a situation in which a state in which communication with any other relay STR continues is impossible.

  In the relay STR, as described above, the processing unit 4 counts the number of error corrections continuously performed on consecutive bits constituting the data part as the number of corrections in the counting process, and then In the determination process, it can be set as the above condition (stop condition) that the number of corrections counted in this way has reached a predetermined upper limit value. In this case, when the number of corrections when the number of corrections described above is simply added to obtain the number of corrections is, for example, about 10 times, “continuous” here is a number smaller than the number of corrections ( Several (for example, about 3 to 5). The situation where error correction is continuously performed on consecutive bits constituting the data part means that errors have occurred continuously in the consecutive bits constituting the data part. It seems that the situation is extremely serious. Therefore, by adopting this condition, it is possible to quickly escape from such a serious situation and shift to a state in which a transmission signal from another relay device can be received.

1 Ad hoc network
60 Reception processing
61 Synchronization establishment process
62-64 Error correction processing
65 Count processing
66 Discriminating process STR1, STR2, STR3 Repeater

Claims (3)

A repeater configured to execute a transmission process and a reception process for a frame having a synchronization signal part and an error correction encoded data part, and constituting an ad hoc network,
In the reception process for the frame transmitted from another relay station constituting the ad hoc network, in the synchronization establishment process based on the synchronization signal part, the error correction process for the data part, and the error correction process Whether or not the number of corrections in the data part satisfies a predetermined condition while executing a counting process for counting the number of error corrections performed on the data part as the number of corrections in the data part unit When the determination process determines that the number of corrections matches the condition, the reception process for the other relay device is stopped and the relay that configures the ad hoc network is performed. A relay that shifts to a state where it can communicate with the machine.   2. The relay according to claim 1, wherein the number of corrections is added and counted as the number of corrections in the counting process, and the determination process is executed on the condition that the number of corrections reaches a predetermined upper limit value. Machine.   In the counting process, the number of times of error correction continuously performed on consecutive bits constituting the data part is counted as the number of corrections, and the number of corrections has reached a predetermined upper limit value. The repeater according to claim 1, wherein the determination process is executed as the condition.

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