【作者】 石军锋；

【导师】 钟先信；

【作者基本信息】 重庆大学 ， 仪器科学与技术， 2008， 博士

【摘要】 无线传感网络(Wireless Sensor Networks,WSN)是近年来随着嵌入式系统、无线通信、网络及微机电系统等技术的快速发展而出现的一种全新的信息获取技术。大量具有感知、计算和无线通信能力的传感节点形成自组织网络,协作地实时感知、采集和处理监测对象的信息,具有广阔的应用前景,引起了世界许多国家军事部门、工业界和学术界的极大关注。与传统无线网络相比,传感节点的能量非常有限,而且常常由于特殊的工作环境而无法得到补充,网络寿命直接受到节点能量的影响。如何使用有限的能量来最大限度地延长网络的生存时间,是无线传感网络需要解决的关键课题。在重庆市自然科学基金(No.2005BB2198)的资助下,论文研究了无线传感网络的节能机制,提出了一种低能耗、低时延、低占空比的动态休眠节能MAC协议——LD3-MAC(Low Dissipation, Low Delay, Low Duty cycle MAC),实现了节点的动态休眠协调;为其设计了相应的介质访问机制;并基于LD3-MAC提出了简单高效的跨层路由机制。主要工作如下:首先分析了传感节点各组成在不同工作状态的能耗情况,结果表明休眠是降低节点能耗的重要措施;阐述了现有节能机制的思想,分析比较了各种节能机制的特点;深入研究了休眠节能的原理、典型休眠节能协议的算法及思路。研究表明,休眠协议可以大大延长传感网络的寿命。但休眠导致多跳时延加大,重负载下吞吐量降低,因此必须解决节点的休眠协调问题。提出了LD3-MAC同步机制,实现了节点的休眠协调。休眠不仅大大降低了空闲监听能耗和不必要的“偷听”能耗,而且在一定程度上避免了冲突和干扰。然而节点关闭射频导致它不能感知周围的通信,因此必须对相邻节点的休眠进行同步。在分析现有同步机制原理、特点及低占空比MAC协议同步机制的基础上,结合无线传感网络静态数据采集的特点,提出了一种适用于休眠MAC的同步机制LD3-MAC同步机制。采用单向同步和全局计划,降低了计算复杂度和网络流量开销,提高了系统的节能效果;采用逐层扩散和相对同步,保证了同步精度,提高了同步的可扩展性,加速了同步过程的收敛;采用交错休眠和周期同步,降低了转发时延,使同步机制具有良好的拓扑自适应性。仿真及性能分析表明,LD3-MAC同步机制实现了全网休眠的协调,减少了不必要的空闲监听,减少了转发时延,降低了冲突概率,提高了数据发送的成功率。深入分析了无线传感网络MAC需要解决的关键技术问题,结合休眠MAC协议的特点,确定了LD3-MAC的介质访问机制。分析比较了RTS/CTS和虚拟载波侦听解决隐藏终端问题的效果,确定了LD3-MAC的握手机制。在分析数据链路重发机制的基础上,选择了LD3-MAC的重发机制,并确定了最佳帧长。深入研究了二进制指数退避算法BEB(Binary Exponential Backoff)及其改进算法的优缺点,确定了LD3-MAC的回退机制。利用马尔科夫模型分析了最大吞吐量与竞争窗口的关系,确定了LD3-MAC的竞争窗口。针对重负载下LD3-MAC同步机制性能较差的问题,提出了动态休眠机制。传感网络的不同应用数据流速率差异很大,而且传输具有突发性。采用固定的休眠计划,当数据负载突然增大时,大量数据包将在靠近基站的节点处由于休眠延迟而发生拥塞。如果不能有效处理拥塞,节点随后发送的数据将使拥塞更为严重并最终导致缓存溢出,大量数据被丢失,包丢失引起的重传反过来使网络时延急剧增大,吞吐量降低。针对这一问题,提出了动态监听和动态占空比机制。动态监听通过有限竞争和快速发送,降低了数据突发时的转发时延和冲突;动态占空比机制根据流量调整节点的常规监听周期,提高了信道的空间利用率。实现了仿真模型并对其进行了性能评估,结果表明,具有动态休眠机制的LD3-MAC在保证高效节能的同时,有效解决了重负载下休眠MAC协议时延大、吞吐量低的问题。休眠MAC协议引起的高时延也对上层的路由协议提出了特殊的要求。目前的许多节能路由协议是以持续监听的MAC作为基础研究,路由信息的收集、路由建立等过程的数据收发定时器设计都基于低时延。当负载较重时,休眠MAC的高时延会造成定时器触发紊乱从而导致路由协议失效。另外,现有的节能路由协议与节能MAC协议很少统一考虑,路由开销较大。论文在研究无线传感网络节能路由机制的基础上,提出了基于LD3-MAC的跨层路由协议。不需要发送专门的路由包,利用MAC层的同步信息建立、更新路由,路由算法简单,其存储、通信和计算开销都很小;采用随机方式确定下一跳节点,在一定程度上实现了负载均衡,降低了重负载下的时延。实现了仿真模型并对其进行了性能评估,结果表明,基于LD3-MAC的跨层路由协议在继承动态休眠LD3-MAC优异性能的基础上,实现了休眠节点的自组网功能。论文研究表明,休眠是降低节点能耗,延长网络寿命的有效措施;LD3-MAC将休眠同步与介质访问紧密结合起来,利用动态监听和动态占空比机制,实现了传感节点的动态休眠,在节能的同时降低了休眠协议的多跳时延,提高了系统的吞吐量;基于LD3-MAC的的跨层路由协议充分利用同步过程提供的信息选择路由,简单高效地实现了休眠节点的自组网功能。更多还原

【Abstract】 Wireless sensor networks(WSN) is a novel technology about acquiring and processing information, with the fast development of embedded system,wireless communication,network and micro-electro-mechanical system. A mass of sense nodes with the capacity of sensing, computation and wireless communication compose self-organized networks, they can be used for sensing, collecting and processing information of monitored objects,which promises wide application area, has attached military branches, industrial world and academic circles in many countries. Compares with the traditional wireless network, the power energy of sense node is extremely limited, and is often unable to obtain the supplement because of severe working conditions, the network life shows a strong dependence on battery energy. How uses the limited energy to maximize network life is an essential topic which WSN needs to solve at present. Under the funding of the natural science foundation of Chongqing Municipality of China (Grant No.CSTC 2005BB2198), the thesis researchs the energy-efficient mechanism based on node sleep for WSN, and proposes LD3-MAC, a dynamic sleep Medium Access Control protocol with low dissipation, low delay and low duty cycle for wireless sensor networks, which realizes nodes synchronization on sleep schedule; The medium access control mechanism of LD3-MAC is designed; and an efficient cross-layer route mechanism based on LD3-MAC is proposed. The main works include the following content.Firstly, the power dissipation characteristics of basic components of a sensor node are analyzed; the results show sleep is an important method of reducing energy cost of node. The existing energy conservation mechanism is elaborated, and the characteristic of each mechanism is analyzed. The energy conservation theory based on sleep and the characteristic of typical sleep protocol are researched, the research shows the sleep protocol can prolong the life of wireless sensor networks, but sleep cause significant multi-hop latency and low throughput under heavy load.The thesis proposes a synchronization mechanism designed for the LD3-MAC protocol which realized nodes synchronization on sleep schedule. Sleep reduce not only energy cost of idle listening and overhear, but also avoid collision and interference of neighboring nodes. If a node turns off radio, it will not realize surrounding communication; therefore nodes must coordinate on their sleep schedules. The thesis describes synchronization principle and algorism; analyze the characteristic of existing synchronization mechanism for the low duty cycle MAC, proposed a synchronization mechanism for the LD3-MAC. The synchronization mechanism uses the unidirectional synchronization and global schedule to reduce computation complexity and the network dataflow, enhance the whole energy conservation effect; Uses the cascade diffuseness and the relative synchronization to guarantee the synchronized precision, enhance the synchronized extendibility, and accelerate synchronized process convergence; Uses staggered sleep schedule and the cyclical synchronization to reduce the forward latency, enable the synchronization mechanism to be flexible and adaptive. Simulation results show that the synchronization mechanism for the LD3-MAC realizes nodes synchronization on sleep schedule, and reduces energy cost for idle listening, forward delay and collision probability, enhances the success ratio of data transmission.The thesis analyzes the key issue that MAC needs to solve, determines the medium access mechanism for LD3-MAC according to sleep MAC characteristic. Compares RTS/CTS with the virtual carrier sense on effect to avoid collision, definites the handshake mechanism for LD3-MAC; analyzes retransmission mechanism in data link layer, and definites the retransmission mechanism for LD3-MAC and the optimal frame length; make a deep study of the Binary Exponential Backoff algorism and redinement algorism, definites the backoff mechanism for LD3-MAC; deduces the relationship between throughput and contention window based on Markov model, and definites contention window for LD3-MAC.Aim at the poor performance of sleep MAC under heavy load, the thesis proposes a dynamic sleep mechanism. For different application, dataflow difference is very big; moreover traffic in the network can be extremely bursty. If MAC uses the fixed sleep schedule, when the data load increases suddenly, mass data package will lead to jam at nodes near the base station because of the sleep delay. If the network cannot process the jam effectively, the succeeding data will cause the jam to be more serious and finally cause the buffer overflow, the mass datas are lost, in turn the retransmission by reason of lost lead to rapid rise of end to end delay, and lower throughput. To solve the problem, the thesis proposes the dynamic listening and dynamic duty cycle mechanism. The dynamic listening mechanism adopts the finite competition and the fast transmission, reduces forward latency and the conflict when the traffic in the network arises suddenly; The dynamic duty cycle mechanism adjusts the regular listening period according to the traffic load, raises spatial use factor of wireless channel. The thesis implements LD3-MAC on NS2 and evaluates its performance; the result show LD3-MAC with dynamic sleep mechanism has outstanding energy-saving effect, effectively solves the prominent delay and low throughput problem of the sleep MAC under heavy load.The high latency associated with sleep MAC also set the special request to the high-level route protocol. Many energy effient route protocols assume the MAC keep listening the channel; the transmission timers for route information collection and route establishment are designed based on low latency. Using sleep MAC, the high latency enable the timer triggering disorder to cause the route agreement expiration when the load is heavy. Moreover, the existing energy efficient route protocols are very little considered integrating with energy efficient MAC protocols, and the route overhead is significant. The thesis analyzes energy-costing route strategies of wireless sensor network,proposes a cross layer route protocol based on LD3-MAC. Does not need to transmit the special route package, the routing protocol uses the synchronization information provided by MAC to establish and update route, therefore its memory, communication and computation cost are very small; uses the stochastic method to select the next hop node, which realizes the load equalization to a certain extent, reduces latency under the heavy load. The simulation model is implemented in NS2 and the performance is evaluated, the results show the cross layer route protocol inherits the LD3-MAC outstanding performance,simultaneously implements self-organization of sleep nodes.The study shows that sleep is an effective method of reducing energy consumption and prolongs network life; LD3-MAC integrates media access control mechanism with node synchronization on sleep schedule, utilizes dynamic listening and dynamic duty cycle mechanism, resulting in dynamic sleep of nodes, which provides significant latency reduction and throughput enhancement while ensuring energy savings. The cross-layer route protocol based on LD3-MAC utilizes the information provided by synchronization mechanism to choose route, achieves the self-organization of sleep nodes.更多还原