【作者】 李健；

【导师】 王建新；

【作者基本信息】 中南大学 ， 通信与信息系统， 2007， 硕士

【摘要】 自组织网络是一种特殊的无线通信网络，所有的节点地位平等，无需设置任何中心控制节点，数据包通过多跳的方式传输，具有较高的灵活性和很强的抗毁性，多跳是自组织网络与其他移动通信网络最根本的区别。但是，由于无线信道的共享，多跳的方式往往加剧了信道的竞争，造成网络性能的降低。本文首先总结和分析了自组织网络中信道接入协议的研究现状，并着重介绍了目前广泛使用的802.11 DCF协议以及它在多跳环境下带来的问题。鉴于802.11 DCF产品的普遍性，本文主要就自组织网络中802.11 DCF协议改进展开了研究。首先，针对自组织网络中，多跳引发的隐／显接收终端问题造成节点信道接入率、吞吐量和公平性的降低，本文在802.11 DCF的基础上提出了一种基于信道状态显式通知的接入协议，该协议可以在信道空闲的时候及时通知发送节点进行通信，降低了退避节点盲目等待的时间，也减少了隐／显接收终端造成报文冲突丢弃的概率。仿真结果表明，基于信道状态显式通知的接入协议有效的提高了自组织网络的吞吐量、业务流的稳定性和公平性。其次，由于退避算法是解决无线信道冲突的主要方法，退避机制的研究对网络整体性能具有重要的意义。因此，本文通过模拟实现，分析和比较了现有的多种退避算法在自组织网络中的性能。由于结果显示现有算法并不能改善多跳环境下性能，因此本文又提出了一种基于源节点和转发节点区分机制的退避算法。这种算法能根据网络冲突状况，自适应的调节退避窗口，更适合多跳环境中流量分布不均的特点。仿真结果表明，这种区分退避机制能同时适应于低速和高速的流量环境，在提高网络吞吐量的同时，也提高了流与流之间的公平性，表现出了更好的网络性能。最后，本文给出了相关工作总结和进一步的研究方向。更多还原

【Abstract】 Wireless Ad Hoc network is a special peer-to-peer wireless networkwithout any infrastructure support. Multi-hop is a key character ofwireless Ad Hoc networks distinguished with other communicationnetworks and it provides flexibility and robustness. But with the fact ofchannel sharing, multi-hop also brings high collision and it decreasesnetwork performance. It makes Medium Access Control a criticalproblem in wireless Ad Hoc networks.The paper firstly introduced wireless ad hoc networks andsummarized its Mac protocols. Then, focused on the most popular 802.11DCF, we analyze and present several problems when it is used inmulti-hop ad hoc networks.Hidden/exposed receiver problems induced by multi-hop seriouslydamage network performance. In order to settle this problem, the paperfirstly proposed a channel state notification protocol. By sending a CCSframe to a deaf sender, it can reduce waste back-off time and packet lossdue to hidden/exposed receiver problems. Simulation showed that thenew protocol can effectively improve throughput, flow stability andfairness in various environments.As the back-off algorithm is important to resolve collision, we thensimulated and compared five different back-off algorithms in wireless adhoc networks. Based on these results, we proposed a DifferentiatedBack-off Algorithm. The algorithm can automatically adjust contentionwindow size according to collision status. Simulation studies show thatDBA performances well both in low-load and high-load networks. It alsoprovides much better fairness in multi-flow conditions.Finally, we make a conclusion of our work and give severalproposals for future work.更多还原