JP5748981B2 - Wireless communication system and wireless communication terminal station - Google Patents

Description

  The present invention relates to a wireless communication system and a wireless communication terminal station, and more particularly to the construction of a mesh network.

  As means for constructing a wireless network with low power, a system called a mesh network or an ad hoc network has been proposed as disclosed in Patent Document 1. In this mesh network, not only base stations (wireless stations) but also terminal stations participate (participate) in the form of relays to construct a mesh network. A feature of mesh networks is the use of multi-hop technology that performs multi-stage relays (hops). However, if the number of relays to the base station (hops) is large, the overall throughput decreases. In order to suppress this, the number of hops is made as small as possible.

  In smart meters that automatically measure and communicate the amount of power, gas, and gas used, it is considered to use the multi-hop relay communication function of the mesh network to cover the service area.

  Returning to the mesh network, the terminal station determines its own communication path according to the routing protocol when newly entering the mesh network. Various proposals have been made for this routing protocol. In any of the protocols, the communication path of the terminal station is acquired by performing broadcasting called flooding. Here, when multiple communication paths are acquired, the terminal station automatically selects the optimal communication path for its own communication from among them using an evaluation function related to the number of hops, reception level, etc. To do. Then, the terminal station enters the network via the selected communication path.

JP 2006-186565 A

  Now, in a wireless communication system that constructs a network that is generally deployed in a cellular form, station placement design is performed in consideration of the number of terminal stations accommodated in a base station. However, even if the terminal station is installed so as to be incorporated under the desired base station (higher-order apparatus) assumed in the design, the desired base station is related to the number of hops that is one of the elements of the evaluation function described above. When the number of other base stations happens to be smaller than that, the communication path to which the other base station belongs may be selected. As a result, the terminal station may not be able to be incorporated under the desired base station.

  Accordingly, the present invention has been made in view of the above-described problems, and an object of the present invention is to provide a technique that allows a terminal station to be incorporated under a desired base station in a mesh network. To do.

A radio communication system according to the present invention is a radio communication system in which a mesh network is constructed by a base station and a terminal station, and the terminal station is a position where terminal station position information for specifying a coordinate position of the own station is held in advance. An information holding unit is provided. When the terminal station intends to enter the mesh network blindly, (a) a candidate communication path that is a candidate for its own communication path among a plurality of communication paths in the mesh network based on a broadcast result (B) From the candidate base station that is the base station belonging to the candidate communication path, base station position information specifying the coordinate position of the candidate base station is received. Then, the terminal station calculates (c) a distance between the own station and each candidate base station based on the terminal station position information and the base station position information, and based on the distance, Regardless of the number of hops or reception level between each of the candidate base stations and the candidate base station, a base station that should establish the communication path of the own station is selected from the candidate base stations.

As another configuration, the wireless communication system according to the present invention is a wireless communication system in which a mesh network is constructed by a base station and a terminal station, and the terminal station tries to enter the mesh network blindly. when, based on the results of (a) broadcast, among the plurality of communication paths in the mesh network, obtains the candidate communication paths is a candidate communication path of the own station, to the mesh network via the candidate communication paths Temporary entry, (b) After (a), from the upper network of the mesh network, terminal station position information that identifies the coordinate position of the own station, and the candidate base station that is the base station belonging to the candidate communication path And the base station position information for specifying the coordinate position. Then, the terminal station calculates (c) a distance between the own station and each candidate base station based on the terminal station position information and the base station position information, and based on the distance, Regardless of the number of hops or reception level between each of the candidate base stations and the candidate base station, a base station that should establish the communication path of the own station is selected from the candidate base stations.

  According to the present invention, based on the distance between the base station and the terminal station, the base station that should establish the communication path of the terminal station is selected. Accordingly, the terminal station can be incorporated under the desired base station intended for the cell design.

3 is a functional block diagram illustrating configurations of a base station and a terminal station of the wireless communication system according to Embodiment 1. FIG. 3 is a schematic diagram showing an operation of the radio communication system according to Embodiment 1. FIG. 3 is a flowchart showing an operation of the radio communication system according to the first embodiment. 6 is a flowchart showing an operation of the radio communication system according to the second embodiment. 10 is a flowchart showing an operation of the wireless communication system according to the third embodiment. It is the schematic which shows a contrast communication system.

<Embodiment 1>
First, before describing the radio communication system according to the first embodiment, a radio communication system (hereinafter referred to as “comparison communication system”) to be compared with the radio communication system will be described.

  FIG. 6 is a schematic diagram showing a contrast communication system. As shown in FIG. 6, the contrast communication system includes a plurality of base stations 1a and 1b and a plurality of terminal stations 2a, 2b, and 2c (a plurality of wireless communication terminal stations) that can perform wireless communication with each other. . The base stations 1a and 1b have substantially the same configuration, and the terminal stations 2a, 2b and 2c have substantially the same configuration. Therefore, in the following description, when the base stations 1a and 1b are not distinguished from each other, each is described as “base station 1”, and when the terminal stations 2a, 2b and 2c are not distinguished from each other, each is described as “terminal station 2”. . As shown in FIG. 6, the base stations 1a and 1b are connected to an upper network 3 that controls them.

  In the contrast communication system, not only the base station 1 but also the terminal station 2 can relay other terminal stations. Therefore, the plurality of base stations 1a, 1b and the plurality of terminal stations 2a, 2b, 2c can construct a mesh network. The construction of the mesh network, that is, the construction of the communication path of the terminal station 2 is performed autonomously. Thereby, the terminal station 2 is mutually connected centering on the base station 1 used as a gateway.

  Around each base station 1, a range in which the terminal station 2 should be accommodated is determined by design as a service area 4, and considering the number of terminal stations 2 accommodated by the base station 1, the terminal station 2 The service area 4 is distributed and installed. In the example shown in FIG. 6, since the terminal stations 2a and 2b are installed in the service area 4a of the base station 1a, they should be accommodated in the base station 1a. Similarly, since the terminal station 2c is installed in the service area 4b of the base station 1b, it should be accommodated in the base station 1b.

  In the following description, it is assumed that the base stations 1a and 1b are close to each other so that some of the service areas 4a and 4b overlap each other. In addition, the terminal station 2 is assumed to be provided in a smart meter that automatically reads the usage amount of power, gas, and water and communicates the usage amount. Such a terminal station 2 is installed at a fixed position without being moved.

  Next, the operation when the terminal station 2 tries to enter the mesh network blindly will be described. In this operation, the terminal station 2 first sends out a broadcast packet called flooding. Thereafter, the base station 1 and other terminal stations 2 that have received it repeat the broadcast in the same manner. By this operation, the terminal station 2 acquires a candidate for its own communication path. Hereinafter, the communication path candidate of the terminal station 2 may be referred to as “candidate communication path”, and the base station 1 belonging to the candidate communication path may be referred to as “candidate base station 1”.

  In addition, since the broadcast packet in flooding reaches only a certain distance from the terminal station 2, the base station 1 located around the terminal station 2 is acquired as the candidate base station 1.

  When a plurality of candidate communication paths are acquired due to the above-described flooding or the like, the terminal station 2 determines the number of hops (number of terminal stations 2 on the communication path) of each candidate communication path and Using an evaluation function related to the reception level and the like, a communication path that is optimal for the communication of the local station is selected from among a plurality of candidate communication paths. In the terminal station 2 that performs such a selection, a communication path with as few hops as possible is selected. Thereafter, the terminal station 2 enters the network via the selected communication path.

  Next, the problem that occurs in the network entry as described above will be described by taking the terminal station 2a shown in FIG. 6 as an example. However, in this description, it is assumed that, as a result of performing the above-described flooding, the terminal station 2a has acquired the communication path to which the base stations 1a and 1b belong as candidate communication paths. In addition, it is assumed that the reception level is so low that the terminal station 2a and the base station 1a cannot communicate directly with each other outside the line of sight.

  When the terminal station 2a acquires the candidate communication path (candidate base station 1) in the same manner as described above, the terminal station 2a selects the optimum communication path (base station 1) from among them using the evaluation function described above. Here, attention is paid to the number of hops, which is one element of the evaluation function. Since the terminal station 2a communicates with the base station 1a via the terminal station 2b because the terminal station 2a is in non-line-of-sight communication with the base station 1a, the communication path between the terminal station 2a and the base station 1a The number of hops is “1”. On the other hand, since the terminal station 2a communicates directly with the base station 1b without going through the relay station, the number of hops of the communication path between the terminal station 2a and the base station 1b is “0”.

  Then, with respect to the number of hops, the communication path to which the base station 1b belongs is smaller than the communication path to which the base station 1a belongs. Therefore, in the comparative communication system, the communication path to which the base station 1b belongs may be selected. That is, in the comparative communication system, the terminal station 2a may actually be accommodated under the base station 1b even though it should be accommodated under the base station 1a.

  Therefore, in the radio communication system according to the present embodiment, it is possible to incorporate the terminal station 2 under the desired base station 1 regardless of the number of hops. Hereinafter, such a wireless communication system will be described. In the following description, the same components as those in the above-described comparison communication system are denoted by the same reference numerals, and description will be made focusing on differences from the comparison communication system.

  The wireless communication system according to the present embodiment constructs a mesh network by the base station 1 and the terminal station 2 as in the comparative communication system.

  FIG. 1 is a functional block diagram showing configurations of a base station 1 and a terminal station 2 included in the wireless communication system according to the present embodiment. Although the hardware configurations of the base station 1 and the terminal station 2 are exactly different, the functional blocks are the same.

  As shown in FIG. 1, the terminal station 2 includes a transmission / reception antenna 11, a transmission / reception unit 12, an external interface 13 used for connection to an external device 21 such as a personal computer (PC), and a position information holding unit 15. And a control unit 14 for comprehensively controlling them.

  The transmission / reception unit 12 modulates a carrier wave based on data from the control unit 14, and transmits a radio signal obtained thereby via the transmission / reception antenna 11. In addition, the transmission / reception unit 12 performs demodulation to acquire data included in the radio signal received by the transmission / reception antenna 11, and outputs the acquired data to the control unit 14.

  The position information holding unit 15 of the terminal station 2 holds terminal station position information that specifies the coordinate position of the terminal station 2 that is input when the terminal station 2 is installed. Similarly, the position information holding unit 15 of the base station 1 holds base station position information that specifies the coordinate position of the base station 1 that is input when the base station 1 is installed. The position information may be indicated by longitude / latitude coordinates per se, or may be indicated by plane rectangular coordinates that facilitate distance calculation. Since the base station 1 and the terminal station 2 are installed at fixed locations, accurate position information can be written in the position information holding unit 15 in advance.

  The terminal station 2 according to the present embodiment, as the first priority when selecting the communication path of the own station, the distance between the own station and the candidate base station 1 based on the terminal station position information and the base station position information And the base station 1 (higher level device) with a short distance is accessed. As will be described later, this operation allows the terminal station 2 located in the service area 4 shown in FIG. 1 to access the base station 1 corresponding to the service area 4. Can be made closer to the number of accommodations assumed at the time of cell design.

  FIG. 2 is a schematic diagram illustrating an operation when the terminal station 2 newly enters the mesh network in the wireless communication system according to the present embodiment, and FIG. 3 is a flowchart illustrating the operation. In FIG. 2, the position information of each of the base station 1 and the terminal station 2 is indicated by coordinates (x, y) in the plane rectangular coordinate system. Hereinafter, the operation when the terminal station 2 newly enters the network will be described using the terminal station 2a as an example with reference to FIG. 2 and FIG.

  When the network entry starts, in step s1, the terminal station 2a searches for a candidate (candidate communication path) of its own communication path from among a plurality of communication paths in the mesh network using a normal protocol. Specifically, the terminal station 2a transmits a broadcast packet called flooding as described above. Thereafter, the base station 1 and other terminal stations 2 that have received it repeat the broadcast in the same manner.

  With this operation, the terminal station 2a acquires a candidate communication path. Here, the terminal station 2a uses, as the above-described candidate communication paths, the communication path to which the base station 1a belongs (communication path passing through the terminal station 2a, the terminal station 2b, and the base station 1a) and the communication path to which the base station 1b belongs (terminal It is assumed that the communication path passing through the station 2 and the base station 1b) is acquired.

  In step s2, the terminal station 2a receives the base station position information transmitted from the candidate base stations 1a and 1b, respectively. Here, the terminal station 2a receives the base station position information indicating the coordinate position (3, 2) of the candidate base station 1a, and the base station position information indicating the coordinate position (9, 2) of the candidate base station 1b. Receive. In the example shown in FIG. 2, since the terminal station 2a is in non-line-of-sight communication in relation to the candidate base station 1a, the candidate base station 1a is connected to the candidate base station 1a via the terminal station 2b. Receive base station location information.

  In step s3, the terminal station 2a determines between the own station and the candidate base stations 1a and 1b based on the terminal station position information held in the position information holding unit 15 of the own station and the received base station position information. Are calculated respectively. When the terminal station 2a calculates the distance between the terminal station and the candidate base station 1a, the coordinates (2, 5) indicated by the terminal station position information of the terminal station and the base station position information of the candidate base station 1a are calculated. Is applied to the coordinates (3, 2) indicated by, and the linear distance between them is calculated (here, about 3.2). Similarly, the terminal station 2a calculates a straight line distance (here, about 7.6) between the own station and the candidate base station 1b.

  In step s3, the terminal station 2a selects the base station 1 for which the communication path of the terminal station should be constructed from the candidate base stations 1a and 1b based on the calculated distance. Here, the terminal station 2a selects the candidate base station 1a corresponding to the shortest distance as the base station 1 that should construct its own communication path. Thus, if the distance between the base station 1 and the terminal station 2a is used as a parameter for selecting the accommodation destination of the terminal station 2a, the base station 1a in the service area 4a in which the terminal station 2a is installed, that is, the cell The desired base station 1a intended in the design can be selected.

  In step s4, the terminal station 2a enters the network through the communication path to which the selected base station 1a belongs, and then performs normal operation. As a result, the terminal station 2a is incorporated under the desired base station 1a.

  According to the wireless communication system and the wireless communication terminal station according to the present embodiment as described above, based on the distance between the base station 1 and the terminal station 2 (wireless communication terminal station), the terminal station 2 A base station 1 for which a communication path is to be constructed is selected. Therefore, the terminal station 2 can be incorporated under the desired base station 1 intended in the cell design. That is, since it is possible to know which base station 1 accommodates the terminal station 2, it is possible to construct a mesh network according to the cell design. Therefore, the actual accommodation number of the base station 1 can be brought close to the accommodation number assumed at the time of cell design. Moreover, since unnecessary access can be reduced, it is possible to suppress the amount of traffic in the network, and as a result, it can be expected to suppress an increase in energy required for constructing the mesh network.

  In addition, for example, even if the terminal station 2a shown in FIG. 2 has established a communication path with the service area 4 other than the service areas 4a and 4b, the necessity of switching the base station after that occurs. Moreover, a communication path can be established with the base station 1a by performing the same operation as described above.

<Embodiment 2>
In the radio communication system according to Embodiment 1, the terminal station position information is held in the position information holding unit 15 of the terminal station 2, and the base station position information is transmitted from the base station 1 to the terminal station 2. In the radio communication system according to the second embodiment, the terminal station position information and the base station position information are transmitted from the upper network 3 to the terminal station 2. Hereinafter, in the description of the radio communication system according to the present embodiment, the same components as those in the radio communication system according to the first embodiment are denoted by the same reference numerals, and different parts from the radio communication system according to the first embodiment. The explanation will be focused on.

  FIG. 4 is a flowchart showing an operation when the terminal station 2 newly enters the mesh network in the wireless communication system according to the present embodiment. Hereinafter, the operation when the terminal station 2 newly enters the network will be described with reference to FIG.

  When the network entry starts, in step s11, the terminal station 2 performs flooding using a normal protocol in the same manner as in step s1 described above, and searches for candidate communication paths from among a plurality of communication paths in the mesh network. get. Then, the terminal station 2 according to the present embodiment temporarily enters the mesh network blindly through any one of the acquired candidate communication paths. Thereby, the terminal station 2 can communicate with the base station 1 and the upper network 3. Then, the terminal station 2 acquires, from the candidate base station 1, unique base station identification information that identifies the candidate base station 1 belonging to the candidate communication path.

  In step s12, the terminal station 2 transmits the base station identification information and the user information of the own station to the upper network 3 via the base station 1 or the like. The host network 3 holds in advance terminal station position information corresponding to user information and base station position information corresponding to base station identification information. Upon receiving user information and base station identification information from the terminal station 2, the upper network 3 transmits terminal station position information and base station position information corresponding to the user information and base station identification information to the terminal station 2. Thus, in step s12, the terminal station 2 receives the terminal station position information of the own station and the base station position information of the candidate base station 1 from the upper network 3 of the mesh network. In the present embodiment, the terminal station 2 holds the received terminal station position information and base station position information in the position information holding unit 15 of the own station.

  In step s 13, the terminal station 2 calculates the distance between the own station and the candidate base station 1 based on the terminal station position information and the base station position information received from the upper network 3. Then, the terminal station 2 determines whether the candidate base station 1 connected to the own station from the temporary entry in step s11 to the present is the shortest with respect to the distance. If it is determined that the terminal station 2 is shortest, the process proceeds to step s16. If it is determined that the terminal station 2 is not shortest, the process proceeds to step s14.

  In step s14, the terminal station 2 searches for a communication path to the candidate base station 1 in which the calculated distance is the shortest. In step s15, the terminal station 2 changes the currently used communication path to the searched communication path. By performing the above steps s13 to s15, the terminal station 2 selects the candidate base station 1 corresponding to the shortest distance as the base station 1 that should construct its own communication path.

  In step s16, the terminal station 2 performs normal operation after entering the network through the communication path to which the selected base station 1 belongs. As a result, the terminal station 2 is incorporated under the desired base station 1 intended in the cell design.

  According to the wireless communication system and the wireless communication terminal station according to the present embodiment as described above, the distance between the base station 1 and the terminal station 2 (wireless communication terminal station) is the same as in the first embodiment. Based on this, the base station 1 that constructs the communication path of the terminal station 2 is selected, so that the same effect as in the first embodiment can be obtained. Further, according to the wireless communication system and the wireless communication terminal station according to the present embodiment, the terminal station position information and the base station position information are transmitted from the upper network 3 to the terminal station 2. Therefore, when the base station 1 is installed, the base station position information is input to the position information holding unit 15 of the base station 1 or when the terminal station 2 is installed, the position information holding unit 15 of the terminal station 2 is Since it is not necessary to perform the work of inputting the terminal station position information, the installation work of the base station 1 and the terminal station 2 can be simplified.

<Embodiment 3>
In the base station 1 as described above, in order to increase the number of terminal stations 2 to be accommodated, another base station 1 using a different frequency channel may be additionally installed at the same position as a certain base station 1. is there. In such a case, the radio communication system according to Embodiment 3 can appropriately select the base station 1 for which a communication path is to be constructed. Hereinafter, in the description of the radio communication system according to the present embodiment, the same components as those in the radio communication system according to the first embodiment are denoted by the same reference numerals, and different parts from the radio communication system according to the first embodiment. The explanation will be focused on.

  FIG. 5 is a flowchart showing an operation when the terminal station 2 newly enters the mesh network in the wireless communication system according to the present embodiment. Hereinafter, the operation when the terminal station 2 newly enters the network will be described with reference to FIG.

  When the network entry starts, in steps s21 to s23, the terminal station 2 performs the same operation as the above-described steps s1 to s3. As described above, when the plurality of base stations 1 are provided at the same position, the distances between the terminal station 2 and the plurality of base stations 1 are the same. Then, after acquiring a plurality of base stations 1 having the same distance as candidate base stations in step s21, the terminal station 2 establishes the communication path of its own station for the candidate base stations 1 having the same distance in step s23. You may choose as

  In such a case, in step s24, the terminal station 2 receives the number of terminal stations 2 accommodated by the candidate base station 1 from the candidate base station 1. Then, in step s25, the base station 1 sets the candidate base station 1 having a small number of terminal stations 2 among the candidate base stations 1 having the same distance to the base station 1 that should establish the communication path of the base station 1. Choose as. Then, in step s26, the terminal station 2 performs normal operation after entering the network through the communication path to which the selected base station 1 belongs.

  According to the radio communication system and the radio communication terminal station according to the present embodiment as described above, among the base stations 1 at the same position, those having a small number of terminal stations 2 accommodated are Select as the base station 1 that constructs the communication path. Therefore, the accommodation numbers of the base stations 1 at the same position can be made equal, and the traffic amount in these base stations 1 can be made uniform. In the above description, the case where the present embodiment is applied to the first embodiment has been described. However, the present embodiment may be applied to the second embodiment.

  1, 1a, 1b Base station, 2, 2a, 2b, 2c Terminal station, 3 Upper network.

Claims (8)

A wireless communication system for constructing a mesh network by a base station and a terminal station,
The terminal station includes a position information holding unit in which terminal station position information for specifying the coordinate position of the terminal station is held in advance.
When the terminal station tries to enter the mesh network blindly,
(A) obtaining a candidate communication path that is a candidate for the communication path of the own station among a plurality of communication paths in the mesh network based on the result of the broadcast ;
(B) receiving from the candidate base station that is the base station belonging to the candidate communication path, base station position information specifying the coordinate position of the candidate base station;
(C) calculating a distance between the own station and each candidate base station based on the terminal station position information and the base station position information, and based on the distance, the own station and each candidate base station A wireless communication system that selects a base station that should establish the communication path of its own station from the candidate base stations regardless of the number of hops or reception level between them. A wireless communication system for constructing a mesh network by a base station and a terminal station,
When the terminal station tries to enter the mesh network blindly,
(A) Based on the result of the broadcast , a candidate communication path that is a candidate for the communication path of the local station is acquired from a plurality of communication paths in the mesh network, and temporarily enters the mesh network via the candidate communication path And
(B) After (a), from the upper network of the mesh network, terminal station position information for specifying the coordinate position of the own station, and the coordinate position of the candidate base station that is the base station belonging to the candidate communication path Receiving base station location information to be identified,
(C) calculating a distance between the own station and each candidate base station based on the terminal station position information and the base station position information, and based on the distance, the own station and each candidate base station A wireless communication system that selects a base station that should establish the communication path of its own station from the candidate base stations regardless of the number of hops or reception level between them. A wireless communication system according to claim 1 or claim 2, wherein
The terminal station
The wireless communication system in (c), wherein the candidate base station corresponding to the shortest distance is selected as a base station that should establish the communication path of the local station. A wireless communication system according to any one of claims 1 to 3,
The terminal station
In (c), when the candidate base stations having the same distance are selected as base stations that should construct the communication path of the local station, the number of terminal stations accommodated by the candidate base station is determined as the candidate. A radio communication system that selects a candidate base station, which is received from a base station and has a small number of terminal stations, as a base station on which the communication path of the local station is to be constructed. A wireless communication terminal station used as the terminal station in a wireless communication system for constructing a mesh network between a base station and a terminal station,
A position information holding unit in which terminal station position information for specifying the coordinate position of the own station is held in advance;
When trying to enter the mesh network blindly, based on the result of the broadcast , among a plurality of communication paths in the mesh network, a means for acquiring a candidate communication path that is a candidate for the local communication path;
Means for receiving base station position information identifying the coordinate position of the candidate base station from the candidate base station that is the base station belonging to the candidate communication path;
Based on the terminal station location information and the base station location information, calculate the distance between the own station and each candidate base station, and based on the distance, between the own station and each candidate base station A wireless communication terminal station comprising: means for selecting a base station that should establish the communication path of the own station from the candidate base stations regardless of the number of hops or reception level. A wireless communication terminal station used as the terminal station in a wireless communication system for constructing a mesh network between a base station and a terminal station,
When trying to enter the mesh network in a blind manner, a candidate communication path that is a candidate for the communication path of the local station is acquired from a plurality of communication paths in the mesh network based on a broadcast result, and the candidate communication path Means for temporarily entering the mesh network via
From the upper network of the mesh network that has temporarily entered, terminal station position information that identifies the coordinate position of its own station, and base station position information that identifies the coordinate position of the candidate base station that is the base station belonging to the candidate communication path; Means for receiving
Based on the terminal station location information and the base station location information, calculate the distance between the own station and each candidate base station, and based on the distance, between the own station and each candidate base station A wireless communication terminal station comprising: means for selecting a base station that should establish the communication path of the own station from the candidate base stations regardless of the number of hops or reception level. The wireless communication terminal station according to claim 5 or 6,
A wireless communication terminal station, comprising: means for selecting the candidate base station corresponding to the shortest distance as a base station that should establish the communication path of the local station. A wireless communication terminal station according to any one of claims 5 to 7,
When the candidate base station having the same distance is selected as a base station that should establish the communication path of the own station, the number of terminal stations accommodated by the candidate base station is received from the candidate base station, A wireless communication terminal station comprising means for selecting the candidate base station having a small number of the terminal stations as a base station for establishing the communication path of the own station.

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