多信道无线Mesh网络的实现及其性能分析

通过搭建基于IEEE 802.11b/g协议的无线Mesh网络实验床,测试无线链路的吞吐率和延迟。实验结果表明,无线链路延迟随跳数增加而增加。利用多发射多信道的网络架构,在无线链路各跳间实现并行传输数据,保证了端到端的吞吐率,使网络性能不会因为无线跳数的增加而明显衰减。     

多跳无线与有线混合网络中TCP性能改进研究综述

多跳无线与有线混合网络中,数据流通过的传输媒体各不相同,这些差异会严重影响TCP性能.为了提高有线与无线混合网络中TCP协议的性能,研究者对混合网络TCP协议进行了大量的研究,主要是通过修改标准TCP协议,使之适合于混合网络环境,设计方法主要有分层设计与跨层设计两种.分析了多跳无线与有线混合网络的特点;探讨了多跳无线与有线混合网络中影响TCP性能的主要因素、TCP所要解决的主要问题,以及改进TCP性能面临的难点;对当前改进混合网络中TCP效率、公平性等方面的研究成果进行了分析和总结,并指出了多跳无线与有线混合网络中TCP进一步研究的重点.     

基于有序竞争的传感器网络MAC协议

在定向扩散路由无线传感器网络中,为减轻竞争型MAC协议CSMA/CD的碰撞冲突问题,以及减小MAC协议引入的时延,对CSMA/CD协议进行改进,提出基于定向扩散与有序竞争的传感器网络MAC协议,即CSMA/CD-DDOC,通过将同跳梯度层次内的转发节点进行分组后实施各分组对共享信道的有序竞争,减少同时争用信道的节点数目,以达到减少碰撞和减小时延的目的。理论分析和仿真结果表明,CSMA/CD-DDOC协议具有优于CSMA/CD协议的时延性能。     

无线传感器网络流量自适应混合MAC协议

提出一种应用于无线传感器网络的流量自适应混合介质访问控制协议(THMAC)。结合TDMA与CSMA的优势,通过预测机制跟踪网络负载和冲突变化情况,在不同负载下选择合适的信道访问方式,实现CSMA与TDMA的自适应混合,在降低数据冲突概率的同时尽力保持较高的信道利用率。仿真结果表明,THMAC在多竞争节点的情况下,能够有效降低数据冲突率,维持较高的吞吐率。     

基于链路状态加权的无线Mesh网络路由协议

由于无线Mesh网络还没有统一标准,ad hoc中的路由算法并不能完全适合无线Mesh网络.在分析ad hoc中经典路由协议AODV的基础上,结合无线Mesh网络的特点,提出了一种新颖的、基于链路状态加权的路由协议.该协议利用节点的可用带宽、吞吐性能和缓冲队列的饱和度等计算路由每一跳的代价(即权重),选择从信源到信宿累计权重最小的路径作为路由.仿真结果表明,提出的路由协议在数据包转发率、平均端到端延迟和标准化路由负栽等性能上均优于AODV,是一种适合无线Mesh网络的路由协议.     

基于LBS及网络编码的HWMP路由协议

在对混合无线Mesh路由协议(hybrid wireless Mesh protocol,HWMP)深入研究的基础上,针对目前HWMP协议中两个节点之间存在最短路径也要通过根节点进行数据转发的问题,以及当先验式生成树建立后网络中存在多个核心节点,在这些核心节点上会有多个数据流通过的特点,提出在原来HWMP协议的基础上加入本地链接库(local association base,LBS)及流间网络编码的改进方案。仿真结果表明,改进的HWMP协议降低了数据包的端到端时延,提高了网络吞吐量和数据包投递率。     

一种无线传感器网络MAC协议优化算法

在无线传感器网络中,各节点采集的信息以多跳的方式传送到汇聚点.从各节点到汇聚点形成一棵以汇聚点为根的传输树.文中在对无线传感器网络传输特点分析的基础上,剖析了基于CSMA/CA(载波多路监听/冲突避免)的MAC协议在树状结构无线传感器网络中的弊端,提出了一种基于CSMA/CA的MAC协议优化算法.算法基于节点在传输树中的位置信息调整其MAC信道接入分配,将CSMA/CA采用的各节点均等竞争信道的方法优化为各节点依据在传输树中的位置情况竞争信道的方式,这一优化提高了节点公平性,使MAC信道接入分配与树状结构的无线传感器网络传输特点相契合,解决了基于CSMA/CA的MAC协议与树状结构无线传感器网络不匹配的问题,从而减少了信道资源浪费,提高了网络传输效率,降低了能耗.实验结果表明该算法在网络丢包率、吞吐量和能耗方面的性能均有较大改进.     

一种新的队列管理机制在无线Mesh网络中的应用

在无线Mesh网络中,对于传输距离不同的数据流存在严重的不公平性问题,同时物理位置不同的AP之间也存在着同样问题。为保证网络公平性,提出改进MAC层机制和引入缓存管理的联合解决方案。在分析了无线Mesh网络空间不公平性的基础上,通过改进MAC层中竞争窗口,引入缓存管理算法,得到一种新的队列管理方案IQMA（Improving Queue Management Algorithm）。仿真结果表明,新的方案改善了节点间的不公平性问题和长跳流歧视问题,提高了网络资源利用率。     

基于分组交换网络的B-Reno性能研究*

通过仿真实验的方法,研究了B-Reno协议在分组交换网络中的性能。实验使用了不同的网络拓扑和链路参数,全面考察了B-Reno的吞吐率和TCP友好性,并将实验结果与其他TCP协议版本相比较。实验结果显示,B-Reno在分组交换网络中能取得高出Reno和New-Reno10%以上的吞吐率,达到与Sack相当的水平;同时,当与Reno（传统TCP）竞争共享信道时,B-Reno也具有良好的TCP友好性。实验结果表明B-Reno在分组交换网络中也能取得良好的性能。     

无线Mesh网络下嵌入式视频监控系统的设计

结合无线网络视频监控的新要求,深入分析了无线网状网的网状拓扑、多跳链路、动态路由协议等关键技术和发展现状,将无线自组织网络按需距离矢量路由协议（AODV）开发应用到嵌入式Linux平台上,利用TCP协议进行呼叫连接控制,并在该平台上提出了利用多个单独视频终端节点构建基于无线Mesh网络技术的视频监控网络应用方案。实验证明这种方案是可行的。     

Throughput-coverage tradeoff in a scalable wireless mesh network

The wireless mesh network (WMN) is an economical and low-power solution to support ubiquitous broadband services. However, mesh networks face scalability and throughput bottleneck issues as the coverage and the number of users increase. Specifically, if the coverage is extended by multiple hops, the repeatedly relayed traffic will exhaust the radio resource and degrade user throughput. Meanwhile, as the traffic increases because of more users, the throughput bottleneck will occur at the users close to the gateway. The contention collisions among these busy users near the gateway will further reduce user throughput. In this paper, a newly proposed scalable multi-channel ring-based WMN is employed. Under the ring-based cell structure, multi-channel frequency planning is used to reduce the number of contending users at each hop and overcome the throughput bottleneck issue, thereby making the system more scalable to accommodate more users and facilitate coverage extension. This paper mainly focuses on investigating the overall tradeoffs between user throughput and cell coverage in the ring-based WMN. An analytical throughput model is developed for the ring-based WMN using the carrier sense multiple access (CSMA) medium access control (MAC) protocol. In the analysis, we also develop a bulk-arrival semi-Markov queueing model to describe user behavior in a non-saturation condition. On top of the developed analytical model, a mixed-integer nonlinear optimization problem is formulated, aiming to maximize cell coverage and capacity. Applying this optimization approach, we can obtain the optimal number of rings and the associated ring widths of the ring-based WMN.     

一种新的竞争与分配相结合的MAC协议

针对IEEE 802.11在高竞争下吞吐量低,以及TDMA协议在低负载下吞吐量低的问题,提出了一种新的TDMA MAC协议：Q-TDMA。结合TDMA协议可以保证节点在每个周期内有一个发送机会,而同步CSMA/CA机制可以解决隐藏终端和暴露终端的问题,Q-TDMA协议要求节点根据本身的队列长度来竞争复用时隙。仿真实验结果表明,Q-TDMA提高了网络吞吐量,降低了端到端的延迟。     

An accurate closed-form formula for the throughput of long-lived TCP connections in IEEE 802.11 WLANs

There is a vast literature on the performance modeling of the 802.11 MAC protocol, but the interplay between the TCP dynamics and the self-regulating behavior of the 802.11 CSMA/CA access method has not been sufficiently investigated. In addition, it has been observed that TCP stations in a WLAN are sporadically active due to the TCP flow control mechanisms. This makes traditional saturated models of the network capacity of 802.11 WLANs not very accurate. We presented a rigorous analytical model to calculate the distribution of the number of active TCP stations in a WLAN with multiple long-lived TCP flows [R. Bruno, M. Conti, E. Gregori, Performance modelling and measurements of TCP transfer throughput in 802.11-based WLAN, in: Proceedings of the IEEE MsWiM 2006, Torremolinos, Malaga, Spain, 2006, pp. 4–11, [1]]. Starting from this analysis, in this paper we derive a simple but accurate closed-form expression of the per-connection TCP throughout as a function of the average duration of collisions, the average backoff period and the TCP packet size. We validate this formula through performance tests carried out on a real WLAN. Then, we use our model to mathematically study the optimal minimum contention window size to maximize the TCP throughput. Our analytical results indicate that the initial window size specified in the 802.11b standard is about two times larger than the optimal one. However, we also show that the TCP throughput performance is not very sensitive to changes of the minimum contention window size, and that the TCP flows significantly underutilize the channel bandwidth also in the optimal MAC operating state.     

基于泛洪协议的无线Mesh网络路由研究

在分析无线Mesh网络结构特点的基础上,针对无线Mesh网中客户端网络节点的移动性、拓扑结构变化较大等特点,提出了一种基于泛洪协议改进的路由算法.该算法设计了以序列号形式存在的路由表,同时通过调节最大、最小序列号的大小,泛洪消息的生命周期TTL等形式提高算法的性能.仿真结果显示,提出的算法在网络吞吐量和平均传输速率等方面要好于AODV算法,算法在拓扑变换频繁的无线Mesh网络中是可行的.     

认知无线电网络分布式多信道MAC协议*

在认知无线电网络中,MAC协议用于信道感知、选择和接入控制。以单网卡多信道MAC（MMAC）协议为基础,依据IEEE 802.22标准定义的静默期管理的两阶段感知策略,提出一种认知无线电网络分布式多信道MAC（CR-MMAC）协议。将两阶段感知机制和分布式协商融入MMAC协议,利用空闲频谱进行数据传输,并在MMAC协议ATIM窗的数据信道协商阶段引入预约机制,以避免选择同一信道的认知节点对在后续数据传输阶段再利用CSMA/CA造成的竞争碰撞。仿真对比分析表明,CR-MMAC协议能提高网络吞吐量、降低通信时     

改进的动态p-坚持CSMA协议

通过研究p-坚持CSMA协议发送概率的动态调整算法,提出一种动态p-坚持CSMA协议。在该协议中,发生冲突重传时的发送概率并非固定不变,而是关于冲突重传次数的函数。通过建立二维马尔可夫链模型,进行理论推导并分析归一化系统饱和吞吐量的性能,结果表明,与p-坚持CSMA相比,动态协议的性能更优。     

基于ECN阶跃标记的TFRC改进协议

针对无线网络中TCP友好速率控制(TFRC)协议无法准确判断分组丢弃原因而导致的效率低下问题,提出一种基于显式拥塞反馈的阶跃标记方法,设计新的无线流媒体传输协议。仿真实验表明,新协议能在无线信道下准确区分丢包原因,并提供准确的拥塞通告信息,在保证TCP友好性的同时提高吞吐率。     

Performance analysis of CSMA and BTMA protocols in multihop networks (I). Single channel case

Busy tone multiple access protocols have been used in multihop networks to reduce the effect of the hidden terminal problem. Due to complexity, the performance of these protocols for large networks has not been analyzed. In this paper, using a Markov chain model and an approximation, we are able to analyze and evaluate the throughput performance of the non-persistent CSMA protocol, the conservative busy tone multiple access (C-BTMA) protocol and the ideal destination-based busy tone multiple access (ID-BTMA) protocol for large networks. The throughput comparison of the protocols is given. The results show that in a large multihop network, the BTMA protocols have a better performance than the non-persistent CSMA protocol and the ID-BTMA protocol has a better performance than the C-BTMA protocol at light channel loads.     

Protocol performance measurements in a heterogeneous network environment

Reported are comparisons of TCP-, XTP¹- and UDP throughout measurements over an ATM inhouse network, the STM based part of the BERKOM^{, 2} B-ISDN and a CSMA/CD LAN. The impact of concurrent processes on the throughput performance was observed. XTP has been found to perform as well as and in some situations better than TCP/IP. Both XTP and TCP achieved about 8 Mb/s over Ethernet (ftp ˜ 8.7 Mb/s) and more than 17 Mb/s over a 140 Mb/s STM-channel of the BERKOM B-ISDN network (ftp up to 16 Mb/s). TCP/IP throughput performance on the basis of the latest Fore ATM system release available (SBA200 series) revealed a substantial increase in performance compared to previous releases of the Fore ATM network adapter. Experiments have shown that average throughput rates of up to 59 Mb/s are achievable with a SPARCstation 10 based ATM endsystem.