The Research Of Clock Synchronization Based On NTP For Metro Integrated Control System

With rapid development of economy and city construction, metro is more and more important for the city. To ensure the metro's safety and high-efficiency, it is very necessary to construct a control system for the metro. Recently, the technologies of computer and control system have made a rapid progress, the distributed and integrated metro control system has applied to metro control system. It is very strict for the synchronization of clocks for the integrated metro control system. Therefore, how to synchronize the clocks of the integrated metro control system is a quite important issue.The paper focuses mainly on research of clock synchronization in the integrated metro control system. Firstly, the paper introduces the traditional ways of clock synchronization and it points out the lack of traditional synchronization ways. Secondly, the paper expounds the theory of getting time from GPS. Through the analysis for several existent protocols in Internet, the paper adopts the Network Time Protocol (NTP) as the network synchronization protocol. According to the network topology of metro integrated control system and the underground characteristic of metro station's control room, the paper designs a project which integrates the NTP and GPS. The GPS supplies the UTC (Universal Time Coordinated) for the NTP Servers which locates the metro OCC(Operating Control Center). The paper also designs the NTP synchronization subnet for the metro integrated control system and the operation mode of NTP is client/server mode. Fatherly, the paper introduces the theory of NTP synchronization. The paper implements the NTP client and server program on the real-time operation system-QNX for the PSCADA(Power Supervision Control And Data Acquisition) sub-system. The client software implements the data filter algorithm, Intersection algorithm and Combining algorithm. Additionally, the paper designs two operation modes for the client software, one mode operates under single-server and another operates under multi-server, the two modes can switch each other automatically and it is due to the number of servers or the situation of network. Finally, the paper makes experiments in Internet and LAN to comparewith the traditional clock synchronization.