An Application Research Of The Ethernet In The CBTC System Based On The Time-triggered Mechanism

Communication-based train control system (CBTC) in China's urban rail transit development process, is occupying an increasingly important position. There are many issues to be overcome and to improve CBTC system for the study area.Data communications system of CBTC (Data Communicate System, DCS) includes not only wired LAN communications, and includes the vehicle-ground communications with the wireless LAN communication. Trains in the wireless local area network are often making a handover and confront with sudden drop of channel quality issues, which will increase the delay time. In addition, DCS is structured on the products of commercial off-the-shelf, the traditional event-triggered mechanism Ethernet when traffic communication increased will bring uncertainty. In contrast, time-triggered (Time-triggered, TT) mechanism can bring about more certain communication and improve the systems real-time property. Loosely time-triggered (Loosely time-triggered, LTT) mechanism is a weakened form of TT, suited for wireless local area network with real-time distributed control system. For safety-related control systems, to the extent possible, to reduce the uncertainty brought about by the impact of the communication. In this paper, how to solve the CBTC system for the uncertainty of communication problems, primarily the following research work:First of all, the paper made reference to seven OSI protocol model and the standard TCP / IP protocol four-level model, analysis and the established the location of the time-triggered protocol to study in standard TCP / IP protocol model in this paper. At the same time, this paper combined with the CBTC system the characteristics of vehicle-ground communications, made a analysis and gave the functional requirements the time-triggered protocol in order to achieve.Secondly, this paper has a research on the mechanism of LTT in-depth. From a detailed study of the LTT mechanism's principle, this paper focuses on vehicle-ground communications by Vehicle On Board Controller (VOBC) and Zone Controller (ZC) and designs the CBTC system time-triggered layer protocol, gives the realization of the process of time-triggered layer protocol. In accordance with the characteristics of communication between VOBC and ZC and using the master-slave architecture, it designed the scheduling table with which the time window can be dynamically assigned to. On this basis, the paper researched and analyzed of the performance of time-triggered protocol further from the perspective of qualitative.Then, this paper established Hierarchical timed colored Petri net (HTCPN) model for the vehicle-ground link communication, time-triggered layer protocol, as well as the wireless channel from top to bottom. In order to record the delay time for system performance analysis, we insert in the message a time stamp using the timed characteristic of modeling tools.Finally, we use the time-triggered layer protocol model to establish vehicle-ground communications with multi-vehicle model. The thesis, respectively, does a simulation about the multi-vehicle communication model with the time-triggered layer protocol model or not. The simulation results show that when the trains involved in the communication are increasing, compared to ET mechanism, LTT mechanism could make a more established system of communication and improve the system's communications real-time property.