【作者】 邹复民；

【导师】 蒋新华；

【作者基本信息】 中南大学 ， 交通信息工程及控制， 2009， 博士

【摘要】 我国每年有上十亿旅客在列车上度过数百亿小时与Internet隔离的时光,旅客列车宽带Internet应用研究不仅是提升铁路服务质量的需要,同时也是一个潜力巨大的市场。为此,论文首先提出采用WiFi技术具有最合理的技术与经济可行性,但必须先解决铁路无线覆盖和车-地宽带互联两个关键问题。然后,论文设计了基于集中接入模式的旅客列车宽带Internet应用体系的四层结构模型,从而确定出这两个问题实质上所包含的4项关键技术,即1)车载通信网关在同一AP覆盖范围内如何实现宽带的车-地互联、2)在不同AP间如何实现无缝的切换、3)如何实现铁路沿线的合理的无线宽带网络覆盖、4)针对用户大都是快速移动的特点,如何实现高效的数据通信。由于WiFi通常不是针对移动应用而设计的,为此论文首先建立了802.11a移动应用的信道模型及其仿真系统,对802.11a在不同移动环境下的工作性能进行深入的仿真分析,并着重研究了不同数据帧长和不同发送速率对其移动应用性能的影响,最后针对旅客列车车-地宽带互联,提出了WiFi移动应用方案TGI@11a。TGI@11a不仅将802.11a移动应用的误码率由5‰下降到0.4‰,同时将有效发送速率由最高6Mbps提高到20Mbps。为描述车载通信网关自主地从一个AP切换到下一个AP的完整过程,论文首先提出了狭义和广义AP间切换的概念,并建立了车-地宽带互联的广义切换模型。基于该模型,进一步设计了基于RSM的切换判决模型和基于多无线模块的快速AP间切换算法,不仅可将网络中断时间减少至毫秒级,并且能保证切换前后的通信质量。为实现数百甚至上千公里铁路线的无线覆盖,论文首先提出了榕树型拓扑、多跳无线链、节点失效危险度、破坏因子等概念,设计了榕树型拓扑的动态最小生成树算法,并基于榕树型拓扑设计了一种称为多出口总线的铁路无线Mesh网络结构和多模Mesh节点结构,最后对一段300公里长度的铁路线进行了实例部署规划。理论分析和实例部署规划表明,榕树型拓扑具有较低的传输延迟、较高的传输带宽和较强的网络健壮性,适合铁路无线宽带覆盖,并且只要求在铁路沿线部署若干个有线网络出口,结构调整灵活,具有良好的可实施性。由于铁路无线Mesh网络的用户大都具有快速移动的特点,论文首先提出了一种支持用户快速移动的二层数据包转发策略,以较低的网络开销实现切换后的转发路径实时更新。然后根据铁路无线Mesh网络的结构特点,设计了邻居节点状态报告机制和尾节点数据包转发策略,实现节点故障信息的动态检测与预测,并自动调整数据包转发路径,使铁路无线Mesh网络可持续地保持高效运行。为验证上述理论研究成果,论文基于X86和嵌入式硬件平台,采用Linux开源代码对本文的理论研究成果进行了部分的具体实现,先后研制了开放性实验平台M-Gateway、M-Router和S-Center,并集成这些实验平台组建了铁路无线Mesh网络实验床。实验结果表明,本文提出的铁路无线Mesh网络实现了以较低的传输延迟和较高的传输带宽将宽带Internet延伸到铁路沿线,并不间断、较稳定地进一步延伸到快速移动的旅客列车上,基本解决了铁路无线覆盖和车-地宽带互联两个关键问题。更多还原

【Abstract】 More than 100 million passengers have to be isolated from the Internet while traveling on train up to billions of hours in China, which shows that it’s not only the demand to promote the railway service quality, but also a great potential market for providing the broadband Internet services on passenger trains. Then, the state of the art is surveyed for the Internet applications on passenger train firstly and it is concluded that WiFi is the most appropriate technology which is technically and economically feasible, but the following two questions must be resolved in advance, that is the railway wireless coverage and train-ground internetworking. Hence, an integrated access mode based 4-layer architecture model is proposed for the broadband Internet applications on passenger train, and the two questions are decomposed into 4 key technologies, those are 1) how to realize the broadband train-ground internetworking within a single AP’s overlay; 2) how to realize the seamless handoff among the APs; 3) how to realize the broadband wireless overlay along the railway and 4) how to realize the efficient communication for the fast moving users.To realize the stable and broadband train-ground internetworking with WiFi, which is always not designed for the mobile scenario, a mobile communication model and a simulation system are built up for IEEE 802.11a. Following, its performance under various mobile conditions is investigated, especially for the influence of different configurations, e.g., frame duration and bit rate. According to the simulation results, a new scheme for the broadband train-ground internetworking with WiFi is presented, which called TGI@11a. With TGI@11a, its bit error rate (BER) is decreased from 5‰to 0.4‰, and its effective bit rate is increased from 6Mbps to 20Mbps.To analysis the complete process of the mobile node which handoff from one AP to another, a special and a general inter-AP handoff concepts are presented firstly, and then a general inter-AP handoff model for the broadband train-ground internetworking is built up. Based on this model, a RSM (Radio-Signal-Measurement) based handoff decision model and a fast handoff algorithm are designed, which can decrease the network disrupt time to several milliseconds and can maintain the link quality during the pre-and-post handoff process, while the mobile node is moving along the serial APs with the passenger train.To realize the wireless overlay along the railway, which is always more than hundreds of kilometers, a number of new concepts such as banyan-tree topology, multi-hop wireless link, invalid risk degree and influence factor are presented firstly, and a dynamical minimum spanning tree algorithm is proposed too. Based on the banyan-tree topology, a multi-portal bus structure and a multi-radio Mesh node structure are designed for the railway wireless mesh network. Finally, an illustrative implementation proposal is designed for a railway with 300Km distance. Theoretical analysis and illustrative implementation proposal show that the banyan-tree topology is suitable for the implementation of railway broadband wireless overlay with low latency, high throughput and robust characters, and it is feasible to deploy because it only requires fewer portals to the wired network and is more flexible to adjust.Because most users of the railway wireless Mesh network are fast moving, a layer-2 forwarding policy of MAC bridge for fast moving users is proposed firstly, which can realize the forwarding path update in real-time after the frequent handoff with low bandwidth cost. Following, a state reporting mechanism and a tail-node forwarding policy are proposed, which support the network status detecting and predicating, and adjust the forwarding path automatically. Theoretical analysis shows that the railway wireless Mesh network can work efficiently and continuously.To verify the theoretical research results, several open experimental platforms are developed utilizing with the open source codes, based on the X86/arm hardware and Linux operation system, e.g., M-Gateway, M-Router and S-Center. Furthermore, the railway wireless Mesh network testbed is set up with these open platforms. Experimental results show that, the railway wireless Mesh network can expand the broadband Internet along the railway with low latency, high throughput and robust characters, and extend to the fast moving passenger train continuously and rather stably.更多还原