JP5930685B2 - Time synchronization apparatus, radio terminal and time synchronization system - Google Patents

Description

  The present invention relates to a time synchronization apparatus in which an apparatus serving as a time master of a system performs time synchronization via a network, and performs time synchronization of the entire system by relay via wireless communication.

  Cases of using wireless sensor networks with distributed arrangements for environmental monitoring of earthquakes and volcanic activity have been reported. In this wireless sensor network, a time accuracy of about 1 ms is required (for example, see Non-Patent Document 1).

  In recent years, the market for digital signage has been rapidly expanding, and in order to realize a reduction in price and an increase in the size of the display, a method of combining a plurality of inexpensive displays has been taken. In order not to detect a shift due to visual recognition, time synchronization with an error of about 10 ms or less is required.

  As a method of performing time synchronization with high accuracy, a method using a GPS (Global Positioning System) radio wave is known and widely used. Patent Document 1 discloses a method in which a gateway connected to an NTP (Network Time Protocol) server uses a wireless relay function to update time information of the entire network at low cost.

  A system using an NTP server will be described. This system is assumed to include an NTP client connected to an NTP server via a wide area network or the like and operating as a wireless time master and a plurality of wireless terminals operating as wireless time slaves. Further, it is assumed that the NTP client and the wireless terminal form a tree-type network with the NTP client as a vertex. In this case, the NTP client serves as a time master and distributes the time to each wireless terminal via wireless communication. The NTP client may be a general-purpose PC or an alternative, and it is assumed that the time is adjusted with the NTP server through the network. For example, in Windows (registered trademark) XP, the time can be automatically adjusted once a week by using an Internet time server. As a time distribution mechanism in a wireless section in which each wireless terminal is connected in a tree shape, an FTSP (Flooding Time Synchronization Protocol) known as a protocol for time synchronization in a wireless sensor network is used. A thing is assumed (refer nonpatent literature 2).

  As a time synchronization mechanism described in Non-Patent Document 2, first, the wireless time master periodically distributes its own time, and the wireless terminal that receives the distributed time information sets the time managed by itself. The time is synchronized with the time of the radio time master and the time is distributed in the same manner as the radio time master.

  In Patent Document 1, in order to avoid non-delivery by wireless communication, a terminal (unit power meter) that cannot receive time information distribution for a certain period (for example, several days) individually requests time information distribution. In some cases, the time is synchronized with the time of the wireless time master (gateway connected to the NTP server) by switching the communication path.

JP 2009-188929 A

Suzuki et al: “Research Trends of Time Synchronization Technology in Wireless Sensor Networks” Morikawa Laboratory Technical Research Report No.2008001, Apr.2008. M. Maroti, et al: “The Flooding Time Synchronization Protocol” Proceedings of the 2nd ACM Conference on Embedded Networked Sensor Systems (SenSys’04), Baltimore, Maryland (2004).

  A time synchronization system using GPS radio waves is expensive, and there is a problem that GPS radio waves cannot be used indoors. On the other hand, the technique described in Patent Document 1 assumes an application in which an error of several seconds to several tens of seconds may be included, and there is an NTP time skip, a radio propagation error, a delay in relaying, and the like. Not considered. That is, there is a problem that it can be easily realized and is low in cost, but cannot be applied to an application that requires highly accurate time synchronization. For example, if the clock source of each device (wireless time slave) has a frequency deviation of 100 ppm, for example, even if the time synchronization is taken at a certain time, a shift of 8.6 seconds will occur after 24 hours. Therefore, the accuracy of 10 ms cannot be satisfied. In addition, since it is assumed that the wireless time master in the assumed system configuration corrects the time once a day or a week in synchronization with an NTP server or the like, a time of about several seconds to several tens of seconds is required for the correction. When the jump occurs and the time is transmitted via relay, there is a problem that the time to be corrected becomes uneven.

  The present invention has been made in view of the above, and an object of the present invention is to obtain a time synchronization apparatus capable of realizing highly accurate time synchronization required in the wireless sensor network and digital signage fields with a simple configuration.

  In order to solve the above-described problems and achieve the object, the present invention includes a first device that manages a reference time and a plurality of second devices that operate in synchronization with the managed internal time. And a second device of the time synchronization system in which the first device, the second device, and the second device are wirelessly connected to each other, and a process of synchronizing the internal time with the reference time A time synchronization apparatus that performs time information indicating time managed by the host apparatus from the host apparatus, which is the first apparatus or another second apparatus of the host. The first time information and the second time information after the influence is removed from the first time information and the second time information indicating the time when the first time information is received. Based on the time information of The internal time correction means that estimates the frequency difference between the internal clock of the position device and the internal clock of the local apparatus, corrects the internal time using the estimation result, and synchronizes with the reference time, and the internal time correction means performs the correction. A time information distribution unit that distributes the internal time information indicating the internal time to the other lower time synchronization apparatus, and the internal time correction unit cannot acquire the first time information over a predetermined time. The internal time is corrected using the frequency deviation value estimated in the past.

  According to the present invention, it is possible to achieve highly accurate time synchronization with a simple configuration.

FIG. 1 is a diagram showing a configuration example of a time synchronization system including a time synchronization apparatus according to the present invention. FIG. 2 is a diagram illustrating an image of processing in which a wireless terminal synchronizes with an NTP client. FIG. 3 is a diagram illustrating an example of a time synchronization apparatus. FIG. 4 is a diagram showing an outline of the time synchronization control operation between the NTP client and the wireless terminal. FIG. 5 is a diagram showing an image of the time synchronization operation of the NTP client and the NTP server. FIG. 6 is a diagram showing an example of the relationship between the time managed by the NTP server and the internal time distributed by the NTP client. FIG. 7 is a diagram illustrating an example of the relationship between the time managed by the NTP server and the internal time distributed by the NTP client.

  Embodiments of a time synchronization apparatus, a wireless terminal, and a time synchronization system according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

(Overview of time synchronization system)
FIG. 1 is a diagram showing a configuration example of a time synchronization system including a time synchronization apparatus according to the present invention. This time synchronization system includes an NTP client 2 connected to an NTP server 1 via a wide area network, and wireless terminals 3 to 14 forming a tree-type network having the NTP client 2 as a vertex. The NTP client 2 operates as a wireless time master, and the wireless terminals 3 to 14 operate as wireless time slaves. The NTP client 2, the wireless terminal, and the wireless terminals are wirelessly connected. The wireless terminal includes a time synchronization device.

  The wireless time master (NTP client) distributes the time (reference time) managed by the wireless time master by the wireless communication function, and in order from the slave (wireless time slave that is a wireless terminal) close to the wireless time master, Perform the operation to match the clock. Here, all the slaves pass through a filter having a sufficient time constant to eliminate the influence of radio frame errors and jitter.

  In addition, each slave estimates the clock frequency deviation from the radio time master and corrects it internally, so that even if the time synchronization frame cannot be received for a certain period due to deterioration of the radio environment, the slave has the radio time master. Stay synchronized with the time.

  The wireless time master enables long-term and inexpensive operation by performing time synchronization with the NTP server via the network. The radio time master has a sufficient time constant when the time obtained from the NTP server jumps from the value held immediately before, considering that the slave has the above configuration. Deliver the smoothed time through the filter. With this configuration, the entire system can be time-synchronized with high accuracy even in an environment where errors due to radio, jitter, and delay due to relaying occur.

  In addition, the radio time master replaces the means for distributing the smoothed time through a filter having a sufficient time constant when the time is skipped by synchronizing the time with the NTP server, in advance for all slaves. Notify the time jump. On the other hand, in all slaves, time management is managed in two ways, and it operates independently before and after the time jump. After a sufficient time has passed after the time jump, or after notification from the master, By adjusting to the time, the entire system can be time-synchronized with high precision even in an environment in which errors, jitters, and delays due to radio occur.

  With the above configuration, it is possible to realize a time synchronization system that can be easily realized and can provide highly accurate time synchronization.

Embodiment 1 FIG.
Next, the first embodiment will be described. The configuration of the time synchronization system is as shown in FIG.

  In the time synchronization system according to the present embodiment, the NTP server 1 distributes time information to the NTP client 2 that is the wireless time master (the NTP client 2 is distributed from the NTP server 1 through the wide area network. Based on the information, time synchronization is performed at a constant period, and time information indicating the time managed by itself is distributed to the wireless terminals 3 to 14 that are wireless time slaves. 2 is synchronized with the time of the NTP client 2 and relays the time information distributed from the NTP client 2 to other wireless terminals (wireless terminals that cannot directly communicate with the NTP client). If the wireless terminals 3 to 14 cannot directly communicate with the NTP client 2, By performing the synchronization with the relayed time by end performs synchronization with the time on the NTP client 2. Also, it relays the time information to other wireless terminals if necessary.

  The time synchronization processing of the wireless terminals 3 to 14 in the present embodiment is shown below. FIG. 2 is a diagram showing an image of processing in which the wireless terminal 3 (slave) synchronizes with the NTP client 2 (master). In FIG. 2, the horizontal axis indicates the time managed inside the wireless terminal 3, and the vertical axis indicates the internal time of the NTP client 2 described in the frame. ● represents a frame containing time information obtained from the NTP client 2.

  Here, when the internal clock frequencies of the NTP client 2 and the wireless terminal 3 are the same, the managed time is always equal to the arrival time, and thus a 45-degree direction (arrow 100) is indicated. Further, ◯ indicates a case where a frame containing time information cannot be received due to a radio error. The fact that ● and ○ do not lie on the straight line 200 represents jitter in communication processing or the like.

  A suitable method for processing in the wireless terminal 3 is to estimate and correct the arrow 200 from a large number of data, and the difference in the inclination of the arrow 100 and the arrow 200 is the internal clock frequency of the NTP client 2 and the wireless terminal 3. Means the difference.

  FIG. 3 is a diagram for the wireless terminal 3 to obtain the arrow 200 shown in FIG. 2 (the difference in frequency between the clock possessed by the NTP client 2 and the internal clock possessed by itself, hereinafter referred to as frequency deviation). It is a figure which shows an example of the time synchronizer which is a functional block structure. As shown in the figure, the wireless terminal 3 is a time synchronization device for obtaining a frequency deviation between the NTP client 2 and a demodulation / decoding processing unit 31, an address processing unit 32, a reception time generating unit 33, a time information extracting unit 34, A time calculation processing unit including a sender filter 35, a noise suppression filter 36, and a time correction calculation unit 37.

  The wireless terminal 3 configured as described above receives a wireless frame, performs demodulation / decoding processing in the demodulation / decoding processing unit 31, and inputs the received frame and reception timing signal obtained as a result to the time calculation processing unit. In the time calculation processing unit, the reception time generation unit 33 generates reception time information based on the reception timing signal, and the time information extraction unit 34 displays time information (hereinafter referred to as the time when this frame is transmitted from the reception frame). , Transmission time information). The time information extraction unit 34 also receives the reception time information generated by the reception time generation unit 33 and outputs it to the sender filter 35 together with the transmission time information. The address processing unit 32 extracts the sender identification information from the received frame and outputs it to the sender filter 35. The sender filter 35 confirms whether or not the input sender identification information indicates a desired partner (here, the NTP client 2), that is, whether or not the transmission time information is information received from the desired partner. If it is received from the other party, the transmission time information and the reception time information input from the time information extraction unit 34 are output to the noise suppression filter 36. If it is not received from the desired partner, the input transmission time information and reception time information are discarded. The noise suppression filter 36 removes the influence of jitter and packet loss from the transmission time information and the reception time information, and estimates a frequency shift. The time correction calculation unit 37 calculates a correction amount to be corrected with respect to the time in the wireless terminal, corrects it by reflecting it in the managed internal time, and outputs the corrected time information. In addition, the corrected time information is transmitted to the lower radio terminal.

  Further, the wireless terminal 3 can estimate the internal time of the wireless time master (NTP client 2) even when, for example, the time information cannot be received for a certain period due to the frequency deviation estimated by the time calculation processing unit. The estimated master time is notified to an application or a terminal connected to a lower level.

  The wireless terminals 5 to 7 estimate the time of the NTP client 2 and estimate the internal time of the NTP client 2 based on the time information received from the wireless terminal 3 that periodically distributes the time information. The operation in which the wireless terminals 5 to 7 estimate the internal time of the NTP client 2 is the same as the operation of the wireless terminal 3 described above. The wireless terminal 6 periodically distributes the time information in the same manner as the wireless terminal 3, and the wireless terminal 11 that has received the time information receives the received time information and uses the same method as the wireless terminal 3 and the wireless terminals 5 to 7 based on the received time information. Thus, the internal time of the NTP client 2 is estimated. FIG. 4 is a diagram showing an overview of the time synchronization control operation between the NTP client 2 and the wireless terminals 3, 5 to 7 and 11 shown in FIG. The NTP client 2 that is a wireless time master periodically distributes packets that store time information, and the wireless terminal 3 that is a wireless time slave that requires packet relay processing receives this information when time information is distributed. To adjust the managed internal time. Further, the packet storing the time information is periodically distributed to the lower radio terminals 5 to 7. Similarly to the wireless terminal 3, the wireless terminals 5 to 7 adjust the internal time and distribute packets storing time information. The wireless terminals 3 and 5 to 7 deliver packets storing time information only when other wireless terminals exist, when it is possible to recognize whether or not there are other wireless terminals below. You may make it do.

  Although the operations of the wireless terminal 3 and the wireless terminals 5 to 7 and 11 positioned below the wireless terminal 3 have been described, the operations of the other wireless terminals 4, 8 to 10 and 12 to 14 are the same.

  Next, time synchronization processing in the NTP client 2 in the present embodiment will be described.

  FIG. 5 shows the NTP client 2 and the NTP server 1 via a general wide area network when the NTP client 2 periodically adjusts the time according to the time of the NTP server 1 once a day or once a week. It is a figure which shows the image of this time synchronous operation | movement. FIG. 6 shows a state where the NTP client 2 delivers the internal time as it is to a portion corresponding to this time jump. In the case of FIG. 6, the time information of the NTP client 2 is transmitted from t1 to t10 in a constant cycle, but the value jumps suddenly at the time correction timing by NTP. When such distribution is performed, if the time synchronization processing flow (operation of the time calculation processing unit shown in FIG. 3) in the wireless terminal 3 described above is performed, the influence of the noise suppression filter and t6 are propagated. The delay difference causes a large time shift in the system. Therefore, the NTP client 2 does not send the time-synchronized value from the NTP server as it is at t1 to t10 in FIG. 6, but distributes the time through a time constant filter that takes into account the time constant of the filter of the wireless terminal. (FIG. 7). As a result, there is no time jump and a gradual change can be caused. Therefore, even in an environment where errors due to radio, jitter, and delay due to relaying occur, each device (radio time master and radio Time slave) can be synchronized with high accuracy.

  As described above, in the time synchronization system according to the present embodiment, a wireless terminal that is a wireless time slave is periodically distributed from an upper apparatus (a wireless time master (NTP client) or another wireless terminal). When receiving a packet in which information on the managed internal time is stored, the time synchronization device provided is based on the received internal time information and the received time of the packet, While estimating the frequency shift of the internal clock, the internal time is corrected and synchronized with the internal time of the host device. In addition, a packet storing the corrected internal time information is generated and distributed to other lower wireless terminals. Furthermore, when estimating the frequency difference between the internal clock of the host device and the internal clock of its own device, the effects of jitter and packet loss are removed from each time information (the host device's internal time information and packet reception time information). Then, it was decided to estimate the frequency shift using these time information. Thereby, highly accurate time synchronization can be realized with a simple configuration.

Embodiment 2. FIG.
Next, the second embodiment will be described. It is assumed that the system configuration is the same as in the first embodiment. In the present embodiment, points different from the first embodiment will be described.

  In the time synchronization system of this embodiment, the wireless terminal manages two independent planes as time calculation processing. These are time calculation process A and time calculation process B, respectively.

  The NTP client 2 notifies the wireless terminal that a time jump has occurred (time fluctuation amount has exceeded a certain value) before new time distribution (before distribution after t6 in FIG. 7). Since it can be assumed that the wireless terminal uses some of the subsequent data to cause variations in the system, in the time calculation process A, the time information (time information distributed from the NTP client 2) input thereafter is set for a certain period of time. , Do not take in the inside, and self-run at the time estimated from the past data. In parallel, in the time calculation process B, the internal time data is reset, and an operation for fetching data after t6 is performed. After a sufficient time that is considered to be updated throughout the entire system has elapsed, a weighted average of 1 (x) is applied to the result of time calculation process A and x (t) is applied to the result of time calculation process B. x (t) is a function of a monotonously decreasing time and takes a value between 0 and 1 and smoothly connects to prevent variations in the entire system.

  According to the present embodiment, it is possible to time-synchronize the entire system with high accuracy even in an environment where errors due to radio, jitter, and delay due to relay occur.

  As described above, according to the present invention, it is possible to achieve an effect that can be easily realized and can provide highly accurate time synchronization required in the field of wireless sensor networks and digital signage.

  The time synchronization apparatus according to the present invention is useful when realizing a time synchronization system in which a master apparatus that manages a reference time and a slave apparatus that synchronizes the internal time with the reference time are wirelessly connected.

1 NTP server 2 NTP client (wireless time master)
3-14 Wireless terminal (wireless time slave)
31 Demodulation / Decoding Processing Unit 32 Address Processing Unit 33 Reception Time Generation Unit 34 Time Information Extraction Unit 35 Sender Filter 36 Noise Suppression Filter 37 Time Correction Operation Unit

Claims (4)

A first device that manages a reference time; and a plurality of second devices that operate in synchronization with the managed internal time in synchronization with the reference time. The first device, the second device, and the second device In each second device of the time synchronization system connected by radio, the time synchronization device performs processing for synchronizing the internal time with the reference time,
When time information indicating the time managed by the host device is received from the host device, which is the first device or another second device, the first time information as the time information and Based on the first time information and the second time information after removing the influence due to jitter and packet loss from the second time information indicating the time when the first time information is received, An internal time correction unit that estimates a frequency shift between the internal clock of the host device and the internal clock of the host device, corrects the internal time using the estimation result, and synchronizes with the reference time;
Time information distribution means for distributing internal time information indicating the internal time after the internal time correction means performs correction to other lower time synchronization devices;
With
The internal time correction means estimates the frequency shift between the internal clock of the host device and the internal clock of the host device based on the first time information and the second time information after removing the influence, While obtaining the estimated value, based on the value of the frequency deviation estimated in the past, calculating the frequency deviation between the internal clock of the host device and the internal clock of its own device as a second estimated value, from the first device, When it is notified that the amount of time variation, which is the difference between the time managed by the time synchronization destination device of the first device and the reference time, exceeds a certain value, a predetermined time elapses. Until this time, the internal time is corrected using the second estimated value.   A wireless terminal comprising the time synchronization apparatus according to claim 1. A management device for managing the reference time is provided, and a plurality of wireless terminals according to claim 2 are provided,
Each of the wireless terminals synchronizes an internal time with a reference time managed by the management device. A first device that manages a reference time; and a plurality of second devices that operate in synchronization with the managed internal time in synchronization with the reference time. The first device, the second device, and the second device In the first device of the time synchronization system that is wirelessly connected, the time synchronization device that synchronizes the internal time with the host device and distributes the internal time to the second device,
When time information indicating the time managed by the host device is received, the time is larger than the first time information as the time information and the second time information indicating the time when the first time information is received. Internal time correction means for synchronizing the internal time with the reference time after removing the effect of jump;
Time information distribution means for distributing internal time information indicating the internal time after the internal time correction means performs correction to another time synchronization apparatus provided in the plurality of second devices;
With
The time information distribution means determines that when the amount of time fluctuation, which is the difference between the time managed by the host device and the internal time, exceeds a certain value, the time variation exceeds a certain value. The plurality of second devices based on the reception time of the internal time information distributed in the past and the internal time indicated by the internal time information distributed in the past , notified to other time synchronization devices included in the second device To correct the internal time of
A time synchronizer characterized by that.

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