In the field of communication, the concept of "synchronization" refers to the synchronization of frequency, that is, the clock frequency and phase synchronization of each node in the network, and its error should comply with the provisions of relevant standards. At present, in the communication network, the problem of frequency and phase synchronization has been basically solved, but the time synchronization has not been well solved. Time synchronization means that the clocks of all nodes in the network and the clocks of all application interfaces connected through the network are synchronized with Coordinated Universal Time (UTC), at least with Beijing time in local area network or metropolitan area network. Time synchronization network is the basis of ensuring time synchronization, which can be composed of wired or wireless mode. This paper mainly introduces the internet time synchronization technology and products, that is, the network time synchronization is realized through the network time server (DNTS series of Sino-Singapore Branch) supporting NTP protocol.

The basic unit of time is the second, which is one of the seven basic units of the International System of Units (SI). CGDM (1967) gives a new definition of second: "One second is the duration of 9 192 63 1 770 radiation cycles corresponding to the transition of cesium133 cs atoms between two ultra-fine energy levels in the ground state at 0 K. The commonly used Coordinated Universal Time (UTC) is actually the atomic seconds adjusted by leap second.

At present, cesium atomic clock is mainly used in international and national standards. The frequency reproducibility of cesium atomic clock in cold atomic fountain established by China Institute of Metrology is 5× 10- 15, which is close to the international advanced level. In fact, at the application level, the time and frequency accuracy as high as the national benchmark is not needed. Different applications have different requirements for accuracy. Table 1 lists the requirements of some typical applications for time accuracy (the error of application interface time relative to UTC time).

Application of some typical applications in time accuracy

Application time accuracy requirements

Computers and servers for banking, securities, stocks and futures trading 1 sec.

Power line fault diagnosis 1 microsecond

Switch and billing system 1 sec

CDMA2000 and TD-SCDMA 10 ms

Network management system 500 milliseconds

No.7 signaling monitoring system 1ms