Network time synchronization that can be understood from the beginning

Chart 1 Fields of use of time synchronization

For example, a financial trading system needs to accurately manage the history of transaction amount and quantity together with the time when it was done. If this time is different between financial institutions, companies, and individuals, it will not be possible to place orders and orders for stock transactions accurately. Tracking fraud becomes impossible, and the transparency and credibility of financial transactions are lost. The broadcasting system also edits undisturbed video by synchronizing signals that match the phase and time of video and audio on the transmitting side and receiving side so that we can watch it. Occasionally, there are cases where the video and audio are out of sync with Internet video, but this is rarely seen on TV broadcasts. Also, familiarly, the PCs and smartphones we use every day also have a function to set the clock to the standard time. The basis for this time synchronization is Coordinated Universal Time (UTC). Adding 9 hours to UTC gives Japan Standard Time. This Japan Standard Time is decided and managed by the National Institute of Information and Communications Technology (NICT). Time synchronization is to match the time information of various devices / devices connected to the network to this UTC. How much error is allowed? The permissible time error in IT / network systems is much smaller than the minutes and seconds that are the unit of human life. Let's take the three examples of financial services, broadcasting networks, and 5G that I mentioned earlier. The requirements stipulated by the EU version of the Financial Instruments and Exchange Act, MiFID II (Financial Instruments and Exchange Directive, enforced in 2018), are microseconds (μs: one millionth of a second) or less. The same level of accuracy is required for time synchronization of shooting, video editing, distribution equipment, etc. in broadcasting systems. In the broadcasting industry, the transition from the video signal transmission standard called SDI to IP video transmission is progressing, and how to realize this high-precision synchronization on the IP network is an issue. And the 5G network is an infrastructure that is more familiar to our lives and requires extremely high-precision time synchronization. It is necessary to accurately match the timing of radio wave emission between 5G base stations at a level of μs or less. The reason why 5G is strict about "time" is that 5G uses the TDD (Time Division Duplex) method that uses the same frequency band for uplink and downlink communications. If the radio frame timing is not accurately matched between the base stations, radio wave interference will occur immediately. Specifically, the error in the start timing of the radio frame should be within ± 1.5 μs with respect to UTC. Time synchronization is also required for 4G TDDs, but 5G is expected to operate more base stations in a densely populated state, and more precise time synchronization is essential to continue to provide stable communication services. .. Unlike 4G, 5G does not have a guard band in the band allocated to each carrier (Chart 2), so timing must be adjusted between carriers as well. The same applies to local 5G, and synchronous control is required between local 5G base stations and with the 5G base stations of carriers.

Figure 2 Intermodulation Distortion Interference-In the Case of TDD

In addition, 5G, which features ultra-low latency and high-reliability communication, has a requirement to keep the relative time error between base stations within 65 nanoseconds (ns: one billionth of a second).

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