Research On Wireless Communication System Between Vehicles Based On OpenWRT

At present,the construction of high-speed railway in China is in a golden period,which promotes the research capacity of China's rail transit.Further research on the core technology of train communication network can promote the continuous improvement and innovation of the core technology in the field of rail transportation in China.At present,the physical media of wire train bus system in the train body is cable,and cable should be exposed to the unstable external environment.This state is always with extra work on the maintenance of the wire train bus system and restructuring physical communication link when split or link carriages.With the rapid development of Internet and Wireless technology,Wlan(Wireless Local Area Network,Wlan)with the advantages of convenient has been popularized rapidly and widely used.Accompanied by the continuous development of IEEE 802.11 series standard and technologies about the physical layer and MAC layer,WLAN technology in such aspects as bandwidth,data transmission rate and reliability have been greatly improved.The paper work with Openwrt as main development platform,on the basis of the existing IEEE802.11 agreement,to implement two wi-fi devices within close range via a wireless link for data transmission through the redesign of the MAC layer protocol access rules,which could avoid the link aging and a series of problems by replacing the cable with high data rate and low access latency.Main work of this thesis from thr following seral aspects:first,build Openwrt develo-pment platform under the Linux system,make the program compiled by Openwrt system can run on the router,analyze the implementation of IEEE802.11 MAC layer protocol in the Linux kernel,and based on the program to analyze the generation,sending and receiving,extraction process of 802.11 MAC layer signaling.According to the system requirements of one-to-one communication,redesigned part of the signal structure,thus implementing one-on-one communication on function.Secondly,on the premise of single user access,to analyze the main factors influencing the efficiency of MAC layer access in tail perspective from the handshake mechanism,mechanism of collision avoidance and Inter Frame Spacing,then optimizes and realized by programming;Under the condition of the data frame shorter length,joining the RTS/CTS confirmation mechanism will increase the system overhead,the size of the collision window and Inter Frame Spacing which waiting before data frame sending will affect the probability of success that system takes up channel.Loading optimized protocol driver into the laboratory equipment,the test results show that on the premise of using binary index retreat algorithm and EDCA mechanism,the communication delay using twice handshake mechanism is less than using quartic handshake mechanism in one-on-one communication network environment;And on the premise of using twice handshake mechanism,reducing the fallback waiting time and shortening the data frame interval also can effectively reduce the communication time delay,reduce the communication time delay fluctuation,so as to improve the system reliability.The organizational structure of the thesis is divided into four chapters,the first chapter tells the background,research status,the basic structure and main work papers.Chapter 2 illustrates the technology related to MAC layer including DCF mechanism operating principle/procedure and EDCA mechanism working principle.Chapter 3 is discussing the ideas about programming implement of one-on-one access from the perspective of function.The fourth chapter redesigns the access process of the agreement and implement in the driver from the perspective of performance,and tests the system performance under different optimization strategy.At last,the thesis is concluded and the directions of further work are presented.