CN108650050A - A kind of distributed network clock synchronous method - Google Patents

Abstract

The present invention relates to a distributed network clock synchronization method, which is applied to a distributed network, including: determining a master device and a slave device through a clock selection algorithm; measuring the link delay between the master device and the slave device; The link delay acquires the time difference between the master device and the slave device; the clock synchronization of the slave device is completed through the master device and the time difference. The embodiments of the present invention measure the link delay, calculate the time difference through the link delay, and then realize clock synchronization, so that the clock synchronization accuracy of the distributed network reaches the nanosecond level, and the distributed network has strong stability , that is, when a device in the distributed network is abnormal, the distributed network can adaptively adjust the network structure in a short time, reselect the appropriate master clock and complete clock synchronization.

Description

A Distributed Network Clock Synchronization Method

technical field

The invention belongs to the field of electronics and communication, and in particular relates to a distributed network clock synchronization method.

Background technique

With the development of modern information technology, various application control devices are gradually becoming distributed, intelligent, and networked, and the accuracy requirements for network clock synchronization are getting higher and higher. Network clock synchronization can provide high-precision time and frequency synchronization signals for telecommunications, mobile communication base stations, PHS base stations, GSM network optimization and other systems. Therefore, its synchronization ability has been widely valued, and it is a key technology in the research and development of many network systems.

Traditional distributed network clock synchronization is realized by using NTP (Network Time Protocol) transmission mode, that is, to transmit unified and standard time on the Internet. Specifically, by specifying several clock source sites on the distributed network, for Users provide time service, and these sites should be able to compare with each other to improve accuracy. Synchronize time service through communication channels, such as computer networks and telephone networks.

However, this timing method can only meet the requirements of medium-precision time users due to the different delays of time signals transmitted to different terminals through channels, that is, it can only meet the requirements of time synchronization accuracy at the ms level. However, in the field of industrial control, some network systems that require very strict time synchronization, the current Internet Network Time Protocol (NTP), Simple Network Time Protocol (SNTP), etc. cannot achieve the required synchronization accuracy, especially for wireless The us-level time accuracy required by time synchronization base stations is far from enough.

Contents of the invention

In order to solve the above-mentioned problems in the prior art, the present invention provides a distributed network clock synchronization method. The technical problem to be solved in the present invention is realized through the following technical solutions:

An embodiment of the present invention provides a distributed network clock synchronization method, which is applied to a distributed network, including:

Determine the master device and slave device through the clock selection algorithm;

measuring the link delay between the master device and the slave device;

Obtaining the time difference between the master device and the slave device through the link delay;

Clock synchronization of the slave device is accomplished through the master device and the time difference.

In one embodiment of the present invention, before determining the master device and the slave device through the clock selection algorithm, it also includes:

Judging whether each device in the distributed network supports a clock synchronization protocol;

If so, run the clock selection algorithm;

If not, end clock synchronization.

In one embodiment of the present invention, the determining the master device and the slave device through the clock selection algorithm includes:

Each device in the distributed network runs the clock synchronization algorithm, wherein the clock synchronization algorithm is a BMCA algorithm;

Determine the main clock according to the running results;

The device corresponding to the master clock is the master device, and the remaining devices are the slave devices.

In an embodiment of the present invention, the measuring the link delay between the master device and the slave device includes:

Sending a delay request message to the master device through the slave device, and recording a first timestamp; wherein, the first timestamp is the sending time point of the delay request message;

receiving the delay request message through the master device, and recording a second time stamp; wherein the second time stamp is the receiving time point of the delay request message;

Sending a delayed response message to the slave device through the master device, and recording a third timestamp; wherein, the third timestamp is the sending time point of the delayed response message;

Sending a delay response following message to the slave device through the master device;

receiving the delayed response message through the slave device, and recording a fourth timestamp; wherein, the fourth timestamp is the receiving time point of the delayed response message;

receiving the delayed response following message through the slave device;

The link delay is calculated by using the first time stamp, the second time stamp, the third time stamp, and the fourth time stamp.

In an embodiment of the present invention, the delayed response packet includes the second timestamp, and the delayed response follow-up packet includes the third timestamp.

In an embodiment of the present invention, the link delay calculated by the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp satisfies:

Wherein, link_delay is the link delay, t1 is the first timestamp, t2 is the second timestamp, t3 is the third timestamp, and t4 is the fourth timestamp.

In an embodiment of the present invention, the acquiring the time difference between the master device and the slave device through the link delay includes:

Sending a synchronization message to the slave device through the master device, and recording a fifth timestamp; wherein, the fifth timestamp is the sending time point of the synchronization message;

sending a follow-up message to the slave device through the master device, wherein the follow-up message includes the fifth timestamp;

Receive the synchronization message through the slave device, and record a sixth time stamp; wherein, the sixth time stamp is the receiving time point of the synchronization message;

receiving the following message through the slave device;

Calculate the time difference between the master device and the slave device according to the fifth time stamp and the sixth time stamp.

In an embodiment of the present invention, the time difference calculated by the fifth timestamp and the sixth timestamp satisfies:

D=link_delay+t6-t5

Wherein, D is the time difference, link_delay is the link delay, t5 is the fifth timestamp, and t6 is the sixth timestamp.

In an embodiment of the present invention, when the slave device has a next-level device, the synchronization method is used to complete the clock synchronization of the next-level device.

In an embodiment of the present invention, both the clock selection algorithm and the distributed network clock synchronization run periodically.

Compared with prior art, the beneficial effect of the present invention:

1) The clock synchronization method provided by the present invention provides a more reliable time synchronization solution, so that the time synchronization accuracy can reach the nanosecond level. Compared with the previous micro-level synchronization accuracy, it can meet some distributed networks that have strict clock synchronization requirements needs;

2) The clock synchronization method provided by the present invention makes the distributed network have strong robustness. When the super master clock or other devices in the network are abnormal, the distributed network can self-adaptively adjust the network structure in a short time , reselect an appropriate clock source and complete the clock synchronization task.

Description of drawings

Fig. 1 is a flow chart of a distributed network clock synchronization method provided by an embodiment of the present invention;

Fig. 2 is a schematic diagram of the implementation flow of a distributed network clock synchronization method provided by an embodiment of the present invention;

3 is a schematic diagram of the principle of measuring link delay of a distributed network clock synchronization method provided by an embodiment of the present invention;

FIG. 4 is a schematic diagram of a clock synchronization principle of another distributed network clock synchronization method provided by an embodiment of the present invention.

Detailed ways

The present invention will be described in further detail below in conjunction with specific examples, but the embodiments of the present invention are not limited thereto.

Embodiment one

Please refer to FIG. 1 . FIG. 1 is a flowchart of a distributed network clock synchronization method provided by an embodiment of the present invention. The clock synchronization method of the present invention can be used for clock synchronization of a distributed network, and is applied to a distributed network. Specifically, the method includes the following steps:

Step 1. Determine the master device and the slave device through the clock selection algorithm;

Step 2, measuring the link delay between the master device and the slave device;

Step 3, acquiring the time difference between the master device and the slave device through the link delay;

Step 4. Complete the clock synchronization of the slave device through the master device and the time difference.

Among them, before step 1, it may also include:

Step X1, judging whether each device in the distributed network supports the clock synchronization protocol;

If so, run the clock selection algorithm;

If not, end clock synchronization.

Wherein, for step 1, may include:

Step 11, running each device in the distributed network with the clock synchronization algorithm, wherein the clock synchronization algorithm is a BMCA algorithm;

Step 12, determine the main clock according to the operation result;

Step 13, the device corresponding to the master clock is the master device, and the remaining devices are the slave devices.

Among them, for step 2, may include:

Step 21, sending a delay request message to the master device through the slave device, and recording a first timestamp; wherein, the first timestamp is the sending time point of the delay request message;

Step 22, receiving the delay request message through the master device, and recording a second timestamp; wherein, the second timestamp is the receiving time point of the delay request message;

Step 23, sending a delayed response message to the slave device through the master device, and recording a third timestamp; wherein, the third timestamp is the sending time point of the delayed response message;

Step 24, sending a delay response following message to the slave device through the master device;

Step 25, receiving the delayed response message through the slave device, and recording a fourth timestamp; wherein, the fourth timestamp is the receiving time point of the delayed response message;

Step 26, receiving the delayed response following message through the slave device;

Step 27. Calculate the link delay according to the first time stamp, the second time stamp, the third time stamp, and the fourth time stamp.

Further, the delayed response message in steps 23 and 25 includes the second timestamp, and the delayed response follow-up message in steps 24 and 26 includes the third timestamp.

Further, in step 27, calculating the link delay through the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp satisfies:

Wherein, link_delay is the link delay, t1 is the first timestamp, t2 is the second timestamp, t3 is the third timestamp, and t4 is the fourth timestamp.

Among them, for step 3, may include:

Step 31. Send a synchronization message to the slave device through the master device, and record a fifth timestamp; wherein, the fifth timestamp is the sending time point of the synchronization message;

Step 32. Send a follow-up message to the slave device through the master device, wherein the follow-up message includes the fifth timestamp;

Step 33: Receive the synchronization message through the slave device, and record a sixth time stamp; wherein, the sixth time stamp is the receiving time point of the synchronization message;

Step 34, receiving the following message through the slave device;

Step 35. Calculate the time difference between the master device and the slave device according to the fifth time stamp and the sixth time stamp.

Further, the time difference in step 35 satisfies:

D=link_delay+t6-t5

Wherein, D is the time difference, link_delay is the link delay, t5 is the fifth timestamp, and t6 is the sixth timestamp.

Wherein, after step 4, it may also include:

When the slave device has a next-level device, the clock synchronization of the next-level device is completed through the synchronization method

Further, the clock selection algorithm and the distributed network clock synchronization in step 1, step 2, step 3 and step 4 all run periodically.

In this embodiment, the super master clock is determined by the clock algorithm, and the link delay is measured, and the link delay is added when the clock is synchronized, so that the clock synchronization accuracy reaches the nanosecond level, which can meet some clock synchronization requirements Very strict requirements for distributed network systems.

Embodiment two

Please refer to Fig. 2, Fig. 2 is a schematic diagram of the implementation flow of a distributed network clock synchronization method provided by an embodiment of the present invention; Fig. 3 is a measurement link delay of a distributed network clock synchronization method provided by an embodiment of the present invention Schematic diagram of the principle; FIG. 4 is a schematic diagram of the clock synchronization principle of a distributed network clock synchronization method provided by an embodiment of the present invention. This embodiment further describes the clock synchronization method in detail on the basis of the foregoing embodiments. Specifically, the method includes the following steps:

Step 1. Send and receive messages

Each node in the distributed network is a time-aware system, and each time-aware system corresponds to a device, each device. When each node in the distributed network is powered on, that is, after each device in the distributed network is powered on and running, the two connected devices send messages to each other to determine whether each device supports the 802.1AS protocol, and if so, run the clock selection Algorithm, if not supported, end clock synchronization.

Preferably, the clock selection algorithm is the BMC algorithm, and the BMC algorithm is the best master clock algorithm, which is one of the most important core technologies of IEEE1588 (accurate clock synchronization protocol). The system that performs clock synchronization according to the IEEE1588 protocol runs the best master clock algorithm to select the master clock in the system. All other clocks use the master clock as a reference for clock synchronization. Among them, IEEE1588 is a time protocol for network measurement and control systems, which can achieve high network time synchronization accuracy and high-precision time synchronization.

Step 2. Run the BMC algorithm to determine the master device

Announce messages (announcement messages) are sent between devices in the distributed network. Each device receives and processes message information of other devices, interacts through Announce messages, compares the information of each clock, and selects the master clock , where the device where the master clock is located is the master device, and the device connected to the master device is the slave device, and the synchronization path between the master device and other devices in the distributed network is determined.

Step 3. Measure the link delay between the master device and the slave device

As shown in Figure 3, after the master device is determined, the slave device sends a delay_Req message (delay request message) to the master device. After the message is sent, the slave device records the sending timestamp of the message, and sends the message The time point is recorded as the first timestamp;

Preferably, after the master device receives the delay_Req message sent from the device, the master device records the time point of receiving the message, and records the time point of receiving the message as the second timestamp;

Preferably, the master device sends a delay_Resp message (delayed response message) to the slave device, wherein the delay_Resp message includes a second timestamp, and after the delay_Resp message is successfully sent, the master device records the sending time point, and will send the message The time point of the document is recorded as the third time stamp. Immediately afterwards, the master device sends a delay_Resp_Follow_Up message (delayed response follow message) to the slave device, wherein the delay_Resp_Follow_Up message includes a third timestamp;

Preferably, the delay_Resp message is received from the device, and the second time stamp is obtained through the message, then, the time point of receiving the message is recorded, and the time point is recorded as the fourth time stamp, and finally, the delay_Resp_Follow_Up message is received Text acquisition timestamp third timestamp,;

Preferably, the master device calculates the link delay from the master device to the slave device through a formula, then the calculation formula is:

Wherein, link_delay is a link delay, t1 is a first timestamp, t2 is a second timestamp, t3 is a third timestamp, and t4 is a fourth timestamp.

Step 4. Complete the clock synchronization of the slave device.

As shown in Figure 4, after measuring the link delay, the master device sends a Sync message (synchronization message) to the slave device, records the sending time point after sending the message, and records this time point as the fifth time stamp. Immediately afterwards, the master clock sends a Follow_Up message (following message) to the slave device, wherein the Follow_Up message includes a fifth time stamp.

Preferably, after receiving the Sync message, the slave device records the time point of receiving the message, and records the time point as the sixth time stamp, and obtains the fifth time stamp after receiving the Follow_Up message from the device.

Preferably, the slave device calculates the time difference between the slave device and the master device, then the calculation formula of the time difference is:

D=link_delay+t6-t5

Wherein, D is the time difference, t5 is the fifth time stamp, and t6 is the sixth time stamp.

Preferably, the slave device corrects the synchronization clock of the slave clock by passing the time difference D on the basis of the master device, and completes the clock synchronization of the slave device.

Step 5, step-by-step synchronization, to complete the clock synchronization of the distributed network

After the slave device completes clock synchronization, similarly, the link delay and time difference are sequentially measured to the next-level device connected to the slave device, where the link delay is the link from the slave device to the next-level device connected to it. Road delay, the time difference is the time difference from the slave device to the next-level device it is connected to, and is synchronized with the clock of the slave device. And so on, until the clock synchronization of the distributed network is completed.

Preferably, the BMC algorithm runs periodically, and the distribution is that the network clock synchronization is also a periodic operation. Wherein, the time period of the BMC algorithm and clock synchronization is obtained through a Signaling message (signaling message), and the Signaling message is passed through the Signaling message of the device. Configure interface settings.

The embodiments of the present invention measure the link delay, calculate the time difference through the link delay, and then realize clock synchronization, so that the clock synchronization accuracy of the distributed network reaches the nanosecond level, and the distributed network has strong stability , that is, when a device in the distributed network is abnormal, the distributed network can adaptively adjust the network structure in a short time, reselect the appropriate master clock and complete clock synchronization.

Embodiment Three

A distributed network is formed by the interconnection of node machines distributed in different locations and having multiple terminals. Any point in the network is connected to at least two lines. When any line fails, the communication can be completed through other links, which has high reliability. At the same time, the network is easy to expand.

Preferably, all nodes in the distributed network constitute a clock synchronization system domain, and all nodes in the domain include clock sources, bridges and end stations respectively, wherein the number of clock sources is one, and the number of end stations is at least one, The number of bridges is at least one. Each node in the distributed network is a time-aware system, that is, clock sources, bridges and end stations are all time-aware systems.

Preferably, the time awareness system includes: a synchronous site synchronous site module (Sitesync), a first port (portsync 1), a second port (portsync 2), a data transfer module (Md), a master clock module (ClockMaster) and a slave clock module (ClockSlave), wherein, the synchronous site module determines the data flow direction according to the result selected by the clock selection algorithm, and calls the relevant functional modules; the first port is used to fill the message with the information sent by the synchronous site module, and send the message; the second port is used to receive and parse the message, and send the message to the synchronization station module; the data transfer module is connected to the outside, obtains the message information, and completes the Message forwarding; the master clock module is connected to the clock source and is used to provide standard time information and send the standard time information to the synchronization site module; the slave clock module receives and sends information to the synchronization site A module that calculates the link delay based on the information, calculates the time difference with the upper node according to the link delay, and corrects the local clock based on the time difference.

Preferably, the clock selection algorithm is to send an Announce message (announcement message) between each node of the distributed network where the time-aware system is located, wherein each node receives and processes message information of other nodes, and passes the Announce message It interacts with the text, compares the information of each clock, and selects the master clock, and the clock synchronization path of the distributed network. Among them, the node where the master clock is located is the master node, and the node connected to the master node is the slave node, and the master node and the distributed network are determined. Synchronization paths to other nodes in the network.

Preferably, when the time-aware system is selected as the master clock through the clock selection algorithm, the node where the time-aware system is located is the master node, the device where the master node is located is the master device, and other nodes connected to it are slave nodes, and the slave node device As a slave device, the synchronization station module calls the main clock module, the first port and the data transfer module according to the clock selection algorithm, wherein the main clock module is connected to the clock source, and the main clock provides the standard time for clock synchronization. When the time-aware system is a slave clock, the node where the time-aware system is located is a slave node, and the synchronization station module calls the data transfer module, the second port, and the slave clock module according to the clock selection algorithm, wherein the slave clock calculates the obtained master The time difference between the clock information and the local clock, and correct the local clock. When the slave node still has a next-level node, the synchronization station module also calls the first port, and sends the corrected time to the next-level node through the first port.

Preferably, when this node sends a message to the next-level node, first select the call module through the node synchronization station module, select the called master clock module or the slave clock module to send information to the synchronization station module, and the synchronization station module sends the information For the first port, the sent information is filled with the message of the first port and then sent to the data transfer module in the form of a message. The data transfer module sends the message to the next level point by connecting with the network card interface.

Preferably, when the node receives the message from the upper node, it first receives the message through the data transfer module, and the data transfer module can obtain the timestamp information, and then send the message and the timestamp information to the second port, the second The section port parses the message, and sends the parsed message information and time stamp information to the synchronization station module, and finally the synchronization station module sends the information to the slave clock module.

Preferably, when two connected time-aware systems with a clock synchronization master-slave relationship are to complete clock synchronization, the prime minister needs to measure link delay, as shown in Figure 3, the measurement of link delay includes:

Send the delay request message to the upstream connected node through the distributed network node, and record the first timestamp, where the first timestamp is the time point when the delay request message is sent; receive the delay request message through the upstream connected clock node, and record The second time stamp, wherein the second time stamp is the receiving time point of the delayed request message; the upstream connected clock node sends the delayed response message to the node, and records the third time stamp of the sending time, wherein the delayed response message contains The second timestamp; the node receives the delayed response message, obtains the second timestamp, and records the fourth timestamp of the receiving time; the clock node connected to the upstream sends a delayed response follow message to the node, wherein the delayed response follow message contains There is a third timestamp; the node receives the delayed response follow-up message and obtains the third timestamp; finally, the link delay is calculated by the formula, and the formula for calculating the link delay is:

Wherein, link_delay is a link delay, t1 is a first timestamp, t2 is a second timestamp, t3 is a third timestamp, and t4 is a fourth timestamp.

Preferably, as shown in Figure 4, measuring the time difference includes: sending a synchronization message to this node through an upstream connected node, and recording the fifth timestamp; wherein, the fifth timestamp is the sending time point of the synchronization message; The node sends a following message to the node, wherein the following message contains the fifth timestamp; the synchronization message is received by the node, and the sixth timestamp is recorded; the sixth timestamp is the receiving time point of the synchronization message; The node receives the following message; calculates the time difference between the upstream connected node and the current node according to the fifth timestamp and the sixth timestamp. The formula for calculating the time difference is:

D=link_delay+t6-t5

Wherein, link_delay is the link delay, D is the time difference, t5 is the fifth timestamp, and t6 is the sixth timestamp.

Preferably, after this node completes the clock synchronization, similarly, it sequentially completes the measurement of link delay and time difference to the next-level node connected to it, and completes time synchronization, wherein the link delay is from the current node to the next node connected to it. The link delay of the upper-level equipment is synchronized with the local node clock. And so on, until the clock synchronization of the distributed network is completed.

Preferably, the delay_Req message, the delay_Resp message, and the Sync message are event messages; the Pdelay_Resp_Follow_Up message and the Follow_Up message are general messages, wherein the event message needs to record accurate timestamps when sending and receiving, and the general Messages do not need to record timestamps.

Preferably, after calculating the time difference with the master clock, the slave clock calls the clock adjustment function through the clock source interface function to correct the time difference with the master clock, thereby completing the clock synchronization with the master clock.

Preferably, the clock selection algorithm is a periodic operation, and the distribution is that the network clock synchronization is also a periodic operation, wherein the clock selection algorithm and the time period of clock synchronization are obtained through a Signaling message (signaling message), and the Signaling message is passed through the device The Signal configuration interface settings.

In this embodiment, the time difference is further calculated by measuring the link delay between connected devices, and the synchronous clock is corrected by the time difference, and the clock synchronization of the distributed network is completed step by step. Using this method, the distributed network can reach the nanosecond level The clock synchronization accuracy is high, and the distributed network clock synchronization is more stable, which can better meet the needs of more devices.

The above content is a further detailed description of the present invention in conjunction with specific preferred embodiments, and it cannot be assumed that the specific implementation of the present invention is limited to these descriptions. For those of ordinary skill in the technical field of the present invention, without departing from the concept of the present invention, some simple deduction or replacement can be made, which should be regarded as belonging to the protection scope of the present invention.

Claims (10)

1. A distributed network clock synchronization method, characterized in that it is applied to a distributed network, comprising: Determine the master device and slave device through the clock selection algorithm; measuring the link delay between the master device and the slave device; Obtaining the time difference between the master device and the slave device through the link delay; Clock synchronization of the slave device is accomplished through the master device and the time difference. 2. The synchronization method according to claim 1, characterized in that, before determining the master device and the slave device by the clock selection algorithm, it also includes: Judging whether each device in the distributed network supports a clock synchronization protocol; If so, run the clock selection algorithm; If not, end clock synchronization. 3. The synchronization method according to claim 1, wherein said determining the master device and the slave device through a clock selection algorithm comprises: Running the clock synchronization algorithm on each device in the distributed network, wherein the clock synchronization algorithm is a BMCA algorithm; Determine the main clock according to the running results; The device corresponding to the master clock is the master device, and the remaining devices are the slave devices. 4. The synchronization method according to claim 1, wherein the measuring the link delay between the master device and the slave device comprises: Sending a delay request message to the master device through the slave device, and recording a first timestamp; wherein, the first timestamp is the sending time point of the delay request message; receiving the delay request message through the master device, and recording a second time stamp; wherein the second time stamp is the receiving time point of the delay request message; Sending a delayed response message to the slave device through the master device, and recording a third timestamp; wherein, the third timestamp is the sending time point of the delayed response message; Sending a delay response following message to the slave device through the master device; receiving the delayed response message through the slave device, and recording a fourth timestamp; wherein, the fourth timestamp is the receiving time point of the delayed response message; receiving the delayed response following message through the slave device; The link delay is calculated by using the first time stamp, the second time stamp, the third time stamp, and the fourth time stamp. 5. The synchronization method according to claim 4, wherein the delayed response packet includes the second timestamp, and the delayed response follow-up packet includes the third timestamp. 6. The synchronization method according to claim 4, wherein the link delay is calculated by using the first timestamp, the second timestamp, the third timestamp, and the fourth timestamp is satisfied when: Wherein, link_delay is the link delay, t1 is the first timestamp, t2 is the second timestamp, t3 is the third timestamp, and t4 is the fourth timestamp. 7. The synchronization method according to claim 6, wherein said obtaining the time difference between the master device and the slave device through the link delay comprises: Sending a synchronization message to the slave device through the master device, and recording a fifth timestamp; wherein, the fifth timestamp is the sending time point of the synchronization message; sending a follow-up message to the slave device through the master device, wherein the follow-up message includes the fifth timestamp; Receive the synchronization message through the slave device, and record a sixth time stamp; wherein, the sixth time stamp is the receiving time point of the synchronization message; receiving the following message through the slave device; Calculate the time difference between the master device and the slave device according to the fifth time stamp and the sixth time stamp. 8. The synchronization method according to claim 7, wherein the time difference calculated by the fifth timestamp and the sixth timestamp satisfies: D=link_delay+t6-t5 Wherein, D is the time difference, link_delay is the link delay, t5 is the fifth timestamp, and t6 is the sixth timestamp. 9. The synchronization method according to claim 1, further comprising, when the slave device has a next-level device, completing the clock synchronization of the next-level device through the synchronization method. 10. The synchronization method according to claim 1, wherein both the clock selection algorithm and the distributed network clock synchronization run periodically.