Research On Traffic Shaping And Handover Mechanism In 802.11 WLAN Based CBTC System

Communication Based Train Control (CBTC) system can effectively address the shortcomings of the traditional track circuit train control system, providing efficient, flexible and versatile characteristics in urban rail transit system to obtain a wide range of applications. At present, most of the CBTC systems are based on the use of mature IEEE 802.11 wireless LAN technology as part of its digital communications transmission systems to provide effective support for data transmission flow. However, as demand with the train control system for performance and diversified information services increases, there are still some outstanding issues in the CBTC system based on the 802.11 protocol.For example, the current communication system CBTC can only transmit low-bandwidth of the train control and signaling information. If real-time transmission of high-bandwidth multimedia applications data stream was supported, not only staff on the ground can grasp the situation on the runing trains on time, the provision of passenger information service diversification is also possible. This article discusses a number of issues in the transmission of video data in CBTC system and provides possible solutions.This article first proposed that in the video transmission system, two issues need to address: first, how to use limited resources in the system to guarantee the basic transmission of the control of information at the same time efficiently transmit video data streams; The second is how to solve the problem of frequent interruption of transmission of data flow caused by the frequent handover of mobile node in CBTC system (Train).In response to the first question, this article examined QoS guaranteeing and traffic shaping algorithm in current digital communications network and proposed a network interface layer solution which configure priority queue to use a wireless channel at the same time transferring control information and video stream. The program maintains a priority queue system according to the different attributes of the data stream (such as the purpose of the port) for packet classification to achieve the network interface layer traffic shaping. When congestion occurs on the network, high-priority control information need to be able to guarantee access to bandwidth, and low-priority video data queue adaptively adjust sending the rate to take full advantage of network bandwidth. The testing of platform based on network configuration tool iproute2 of the Linux system shows that the solution can effectively guarantee the high-priority traffic delay and bandwidth requirements, as well as low-priority network can take full advantage of the remaining bandwidth, so as to solve the above-mentioned problem of using limited resources to transmit a variety of network data.For the second question, this paper discussed the deficiencies of traditional handover algorithms such as IAPP protocol and the Mobile IP protocol in the CBTC system, pointing out that the one-way transmission of video streaming through the UDP connect from ground to the train makes these algorithms lose their power. Then the paper discussed the characteristics of the CBTC system as well as two important design decisions: the simplicity of movement of mobile nodes in CBTC system of mobile nodes make simple and effective motion estimation algorithm possible; in the same service area of CBTC system, Mobile nodes are sparsely distributed, which makes the use of two wireless network interfaces at the same time on one mobile nodes possible. In considering these two characteristics, this paper then proposed two-channel asynchronous handover algorithm. In this algorithm, a mobile node uses two separate wireless network interfaces at the same time to communicate with the ground control system. By configuring the handover algorithm parameters, two separate network interfaces perform "early" handover and "late" handover respectively, making the mobile node has at least a network interface is in normal working condition in the process of handover, in order to achieve uninterrupted video stream transmission. Simulation based on the NS2 simulation platform showed that, compared with the traditional handover algorithm, this two-channel asynchronous handover algorithm has good performance, validating the effectiveness of the algorithm. This paper also discussed the effect of algorithm parameters through analysis of the simulation results.