The process of providing a unified time scale for distributed systems
Time synchronization is the process of providing a unified time scale for distributed systems through some operations on the local clock.In a centralized system, because all processes or modules can obtain time from the unique global clock of the system, any two events in the system have a clear priority relationship.In a distributed system, due to physical dispersion, the system cannot provide a unified global clock for modules that are independent of each other. Instead, each process or module maintains its own local clock.Because the timing rate and operating environment of these local clocks are inconsistent, even if all local clocks are calibrated at a certain time, these local clocks will be inconsistent after a period of time.In order for these local clocks to reach the same time value again, time synchronization must be performed.[1]
The so-called synchronization of all clocks in the system does not require all clocks to be fully aligned with the unified standard clock.It is only required to know the clock difference between each clock and the system standard clock at the comparison time and its drift correction parameters relative to the standard clock after comparison, without having to dial the clock.Skip step or leap second processing can only be performed when the accumulated clock difference of the clock is large.Because to align the clock faces of the two clocks at the time of comparison, a precise phase microstep regulator is required to adjust the phase of the clock's dynamic source. In addition, the drift rules of various driving sources are differentidenticalEven if the clocks at the two comparison times are completely aligned, there will be errors after the comparison. It is still necessary to observe the drift rule of the compared clock drive source relative to the standard clock, so this is generally not done.In the navigation system user equipment.exceptTime service receiverNeed to adjust 1 after positioningPPSOutside the time when the signal front edge appears (it requires that the time when the output second signal is consistent with the time when the clock second signal appears), the clock difference can generally be deducted by mathematical method.
Main categories
Announce
edit
radio waves
Another method of time synchronization is to useRadio wave propagationTime information.That is, the radio wave is used to transmit the time standard, and then the time service receiver restores the time signal to the corresponding time signal ratio of the local clock, deduces its delay on the propagation path and the influence of various error factors, and realizes the synchronization of the clock.
With theClock synchronizationWith the continuous improvement of accuracy requirements, the radio wave time service method was used to start time service (ms level accuracy). Because the short wave propagation path was affected by the ionospheric changes, there were once and many sky waves in the sky wave, and the ground wave propagation distance was close, so that the time service accuracy could only reach ms level.Later, it developed to use Omega station for timing with ultra long wave, and its timing accuracy is about 10 μ s. Later, it used Loran-C station chain for timing with long wave, and its timing accuracy can reach μ s. Even if Loran-C station chain is networked, it is difficult to achieve global coverage.Later, it developed to use satellite clocks as moving clocks.By using ultrashort wave to transmit time signals and receiving a satellite in common view by users, the accuracy of time service is better than that of moving clocks, and the accuracy can reach 10ns.
satellite
Seems to be usingSatellite time serviceIt is a good way to realize the precise synchronization of clocks around the world. Only by using satellites can time signals be transmitted with ultrashort waves around the world;Using ultrashort wave to transmit time signals not only has high transmission accuracy, but also can improve the clock comparison accuracy. By using the common view method, the satellite clock can be used as a carrier clock, and the timing accuracy can be higher than that of the direct carrier clock, which is difficult to make the clocks of two places look at it together.Common view can eliminate many system errors and errors that change slowly with time, and fast changing random errors can be eliminated by accumulation smoothing.
network
First, you should know what the NTP protocol is: the full name of the NTP protocolNetwork Time Protocol（Network Time Protocol）。Its purpose is to deliver uniform and standard time on the Internet.The specific implementation scheme is to specify severalClock sourceWebsites provide users with time service, and these websites should be able to compare with each other to improve accuracy.NTP was first designed and implemented by Professor Mills of Delaware University in the United States. It was first proposed in 1982 and has been developed for nearly 20 years. In 2001, the latest NTPv4 accuracy has reached 200 milliseconds.NTP simultaneous synchronization refers to the time calibration with the time source through the NTP protocol of the network.Preconditions: the time source output must pass through the network interface, and the data output format must conform to the NTP protocol.All PCs, servers and other devices in the LAN communicate withtime serverKeep synchronization and automatically judge NTP protocolNetwork Delay And make time compensation for the obtained data.Thus, the time of LAN devices can be kept uniform and accurate.
