Ad hoc网络中动态阈值岭型函数退避算法*

无线自组织网络(Ad hoc)在MAC层中使用IEEE 802.11 DCF的接入机制来解决节点传输竞争问题,为了解决高负载环境下的网络拥塞状况问题,在退避算法中加入了动态阈值和岭型函数来降低数据传输的冲突概率并利用NS2对改进的退避算法进行网络仿真。仿真结果表明,相对于传统的退避算法,在负载较重的网络中改进后的退避算法大概能够提高网络性能20%左右,优化后网络的时延、吞吐量和公平性均得到明显改善。     

多跳无线 Ad hoc网络的分布式公平调度算法

为兼顾无线Ad hoc网络信道资源的公平利用及网络吞吐量,提出针对多跳无线Ad hoc网络的分布式公平调度算法。该算法采用链路竞争图说明网络中各子流的竞争关系,通过着色算法得到最大无竞争流的集合,通过邻居节点之间的信息交换得到局部(本地)流的信息,而无需全局流的信息交换。实验模拟结果表明,该算法可以在保证Ad hoc网络中各流公平性的同时,有效提高网络资源的利用率。     

Ad hoc无线网络负载自适应平衡跨层多径路由算法

提出了一种Ad hoc无线网络流量自适应平衡的多径路由选择算法LACLMRA。算法LACLMRA利用网络的跨层信息：传输层提供的数据端到端传输延迟、节点MAC层提供的最近一次传输成功前重传次数、网络层的缓存队列长度等信息,构建多条不相交的多径路由。在实际的数据传输过程中,根据路由的质量,在多条路径中自适应分配流量,缓解节点的拥塞,延长Ad hoc无线网络的生存时间。仿真结果也显示该算法与SMR、AOMDV等多径路由算法相比,在降低网络平均延迟、提高吞吐量、延长网络生存时间方面,有更好的表现。     

一种节能的Ad hoc网络路由协议

针对Ad hoc网络中移动节点能量有限的问题,综合考虑节点剩余能量和节点能量消耗速率两方因素,提出一种基于节点生存时间的路由算法MRL（Maximum Routing Life）。通过估算节点使用寿命,选择生存时间最长的路由,均衡移动网络中各节点的能量。引入NS能量模型,与Ad hoc网络中典型路由协议进行比较;仿真结果表明,与传统路由协议相比,新的路由协议有效地提高了Ad hoc网络性能。     

无线移动自组网络中的动态带宽分配

在已有的带宽分配算法的基础上,引入加权准则对竞争的数据流进行带宽分配,该算法工作紧密结合无限局域网的MAC层协议DCF,由此保证带宽分配的公平性和每条流的最低带宽需求。使用NS仿真进行性能评价,实验表明,该算法在满足数据流的最低带宽需求的同时,提高了系统吞吐量。该文提出的算法尤其适合单跳Ad hoc多媒体传输。     

一种基于跨层功率控制的Ad hoc网络路由算法*

在无线Ad Hoc网络路由协议中引入功率控制不但可以降低网络能量消耗,同时还能改善网络的吞吐量、投递率等性能,已成为当前Ad Hoc网络的一个研究热点.本文提出了一种基于跨层功率控制的按需路由算法CPC-AODV(Cross-layer Power Control Ad hoc On-demand Distance Vector).算法按需建立多个不同功率级的路由,节点选择到目的节点最小功率级的路由来传递分组,并对网络层的数据分组、路由分组和MAC层控制帧的传输采用不同功率控制策略来降低能量消耗.仿真结果表明:算法有利于降低通信能量开销,延长网络寿命,提高网络投递率及改善网络时延.     

Ad hoc网络中MAC不公平性的研究与改进

在Ad hoc网络中,节点间的不公平性会影响整个网络的性能。针对该问题,提出一种公平性算法——UTRVB。该算法用于有节点加入或移除的Ad hoc网络,通过节点发送量与参考值比较大小来调节退避时间。仿真结果证明,该算法在不导致系统吞吐量严重下降的前提下,有效地改善了DCF机制的公平性。     

一种新的Ad hoc网络中的QoS体系结构*

由于MAC机制本身具有带宽分配的能力,Ad hoc网中的区分服务不能实现理想的业务区分。针对该问题提出了一种新的Ad hoc网络的QoS体系结构Diff-EDCF,它通过把区分服务体系结构和IEEE 802．11e MAC层的EDCF机制相结合的方法来解决上述问题,并且进行了仿真验证。仿真结果表明,把此体系结构应用于Ad hos网络后,业务在任何情况下均能被区分,在特定拓扑下通过调节MAC层参数可以达到理想的吞吐量的比值。     

Ad hoc网络TDMA分布式动态时隙算法

随着GPS 技术的发展和应用, Ad hoc 网络的终端同步已经不成问题, 因此基于时隙的MAC 方案, 如TDMA 对Ad hoc 网络越来越有吸引力。提出一种新的分布式时隙动态分配算法, 其数据碰撞率低, 公平性好, 适于动态网络, 对于网络失步以及超负荷的数据发送都有较好的适应性。着重介绍了此时隙算法, 然后分析了该算法的性能, 并把它与时隙Aloha 的信道利用率进行了比较, 仿真表明此算法远远优于时隙Aloha 。     

Ad hoc网络中基于TDMA的定向发送和接收算法

近期研究结果表明利用有向天线实现定向发送和定向接收比用全向方式能显著提高系统吞吐量.为了支持多媒体和实时业务,在Ad hoc网络中提供服务质量(QoS)是一个关键部分.提出了一种利用有向天线且提供QoS支持适用于Ad hoc网络的分布式媒体接入控制(MAC)算法.仿真结果表明该算法与IEEE 802.11相比提高了系统吞吐量并降低了端-端延迟.     

无线自组网络中TCP流公平性的分析与改进

研究了TCP(transmission control protocol)流在多跳无线自组网络中的公平性问题,发现IEEE802.11DCF协议在此环境下会导致严重的不公平性,即部分节点垄断了网络带宽而其他节点被饿死.首先,通过仿真分析了产生TCP流不公平性的原因,指出其根源在于MAC(media access and control)协议的不公平性,同时,TCP的超时机制加剧了不公平性的产生;然后,利用概率模型定量分析了TCP不公平性与MAC协议参数之间的关系,发现TCP流的公平性与TCP报文长度直接相关,并且增加MAC协议初始竞争窗口的大小能够有效提高公平性.据此,提出了一种根据TCP报文长度动态调节初始回退窗口大小的自适应回退MAC协议改进算法.理论分析和仿真表明,该算法在很大程度上可以有效缓解不公平性问题的产生,并且不会引起网络吞吐量的严重降低.     

Joint routing and flow rate optimization in multi-rate ad hoc networks

Typical radios in ad hoc networks can support multi-rate transmissions. However, traditional routing protocols do not use this feature well in multi-rate ad hoc networks and therefore, the network performance and resource utilization are not optimized. Some algorithms have been proposed to take advantage of the multi-rate transmission scheme, but their performance is not optimized either. In this paper, we show that a cross-layer optimization based approach can significantly improve the performance of multi-rate ad hoc networks over existing routing algorithms. For this, we consider link interference and propose joint routing and flow rate optimization for optimal performance in multi-rate ad hoc networks, i.e., a Cross-layer Optimization based Model for Multi-rate Ad hoc Networks (COMMAN). Considering the characteristics of multi-rate ad hoc networks, we design and implement a distributed heuristic of this centralized model. It is shown that the distributed heuristic algorithm can approximate the performance of COMMAN closely.     

多速率移动自组网中交叉层优化的性能分析

由于严格分层的协议无法很好地适应移动自组织网络（Mobile Ad hoe Network,MANET）的动态变化,交叉层设计与优化近年来成为MANET的一个研究热点。针对802．11b标准支持的多速率通信,分别在网络层和MAC层实现了两种多速率控制方案,并通过仿真实验详细分析了这两种速率控制方案的性能。实验结果表明,在网络层引入物理层提供的多速率信息作为路由选取标准,可有效提高网络吞吐率,降低端到端延迟;同时,在MAC层使用高速率转发中继,对网络层路由进行局部优化,可以进一步改善对拓扑动态变化的适应性。     

A QoS Estimation-Based Scheduler for Real-Time Traffic in IEEE 802.11e Mobile Ad hoc Networks

ABSTRACT

For mobile ad hoc networks, the IEEE 802.11e standard specification provides Quality of Service (QoS) facility support in the MAC layer by Enhanced Distribution Channel Access (EDCA), which provides differentiated and distributed access to the wireless medium with four access categories (AC). It works efficiently for constant bit rate (CBR) types of traffic; however, for the case of variable bit rate (VBR) types of traffic, it shows poor performance due to the static nature of computing the number of packets and the time required to transmit these packets. In this paper, we present an EDCA scheduling algorithm that allocates transmission opportunities (TXOP) for fluctuating VBR traffic depending on their queue length estimations for mobile ad hoc networks. We classify the channel state as good or bad based on the channel error conditions. Then the scheduler determines the mean application data rate and estimates the TXOP for the next interval on the same Traffic Stream (TS). By simulation results, we show that the proposed scheduling scheme achieves good throughput and fairness with reduced delay for the VBR traffic class.     

VANET中基于公平性的多信道MAC协议

随着车载无线通信需求的增加,车载自组织网络（VANET）将成为现代智能交通系统的一个重要组成部分。对VANET中多信道MAC层协议进行了研究,提出了一种提升网络公平性的退避算法。车辆通过比较自身已成功发送的业务量与平均业务量的大小,确定不同的退避方案,一定程度上实现网络中车辆的接入公平。考虑到该退避算法在提升公平性的同时,牺牲了一定的网络吞吐量,服务信道不饱和,进一步提出根据车辆密度调整服务预约时期长度的算法,提高服务信道的利用率,增大网络的吞吐量。仿真结果表明,提出的退避算法公平性指数相比于二进制退避算法,提升了约2.3倍,而相比于倍数增线性减退避算法,提升了约2.05倍,网络的吞吐量提高了约16%。     

p-RWBO:a novel low-collision and QoS-supported MAC for wireless ad hoc networks

QoS supported MAC mechanism is a key issue for supporting QoS in wireless ad hoc networks. A new backoff algorithm,named RWBO BEB,was proposed previously to decrease the packet collision probability significantly. In this paper,it is explored how to make RWBO BEB support service differentiation in wireless ad hoc networks,and a novel proportional service differentiation algorithm,named p-RWBO,is proposed to allocate the wireless bandwidth according to the bandwidth ratio of each station. In p-RWBO,station n's walking probability(pw,n) is selected according to its allocated bandwidth ratio. An analytical model is proposed to analyze how to choose pw,n according to the bandwidth ratios of station n. The simulation results indicate that p-RWBO can differentiate services in terms of both bandwidth and delay.     

一种无线Ad Hoc网络动态混淆匿名算法

无线Ad Hoc网络的特殊性决定了它要受到多种网络攻击的威胁,现有的加密和鉴别机制无法解决流量分析攻击.在比较了抗流量分析的匿名技术基础上,提出混淆技术可以满足无线Ad Hoc网络的匿名需求,但现有的混淆算法在Ad Hoc网络下却存在安全与效率的问题.提出了一种动态混淆的RM(pseudo-random mix)算法,该算法主要对混淆器的管理部分进行重新设计.RM算法根据混淆缓冲区的情况进行决策,当缓冲区未满时采用时延转发方式,缓冲区满后采用随机数转发方式,这样既保证了无线Ad Hoc节点的匿名性,同时又解决了停等算法的丢包现象.对RM算法的安全性和效率进行了分析,仿真结果与理论分析相一致,表明RM算法在无线Ad Hoc网络下具有较好的自适应性和实用价值.     

Differentiation, QoS guarantee, and optimization for real-time traffic over one-hop ad hoc networks

Nodes having a self-centrically broadcasting nature of communication form a wireless ad hoc network. Many issues are involved to provide quality of service (QoS) for ad hoc networks, including routing, medium access, resource reservation, mobility management, etc. Previous work mostly focuses on QoS routing with an assumption that the medium access control (MAC) layer can support QoS very well. However, contention-based MAC protocols are adopted in most ad hoc networks since there is no centralized control. QoS support in contention-based MAC layer is a very challenging issue. Carefully designed distributed medium access techniques must be used as foundations for most ad hoc networks. In this paper, we study and enhance distributed medium access techniques for real-time transmissions in the IEEE 802.11 single-hop ad hoc wireless networks. In the IEEE 802.11 MAC, error control adopts positive acknowledgement and retransmission to improve transmission reliability in the wireless medium (WM). However, for real-time multimedia traffic with sensitive delay requirements, retransmitted frames may be too late to be useful due to the fact that the delay of competing the WM is unpredictable. In this paper, we address several MAC issues and QoS issues for delay-sensitive real-time traffic. First, a priority scheme is proposed to differentiate the delay sensitive real-time traffic from the best-effort traffic. In the proposed priority scheme, retransmission is not used for the real-time traffic, and a smaller backoff window size is adopted. Second, we propose several schemes to guarantee QoS requirements. The first scheme is to guarantee frame-dropping probability for the real-time traffic. The second scheme is to guarantee throughput and delay. The last scheme is to guarantee throughput, delay, and frame-dropping probability simultaneously. Finally, we propose adaptive window backoff schemes to optimize throughput with and without QoS constraints.     

一种多速率移动自组网中的拓扑控制算法

提出了一种移动自组网中的拓扑控制算法MATC(multi-rate aware topology control),该算法充分考虑了无线环境中固有的多速率特性,在保证原有网络连接性的前提下删除部分低速链路,为上层的路由协议构造一个良好的拓扑,确保按需路由协议能够在一跳范围内发现最优路由.大量仿真结果表明,MATC对网络性能有较大的提高.     

Revealing the problems with 802.11 medium access control protocol in multi-hop wireless ad hoc networks

The IEEE 802.11 medium access control (MAC) protocol is a standard for wireless LANs, it is also widely used in almost all test beds and simulations for the research in wireless mobile multi-hop ad hoc networks. However, this protocol was not designed for multi-hop networks. Although it can support some ad hoc network architecture, it is not intended to support the wireless mobile ad hoc network, in which multi-hop connectivity is one of the most prominent features. In this paper, we focus on the following question: can IEEE 802.11 MAC protocol function well in multi-hop networks? By presenting several serious problems encountered in transmission control protocol (TCP) connections in an IEEE 802.11 based multi-hop network, we show that the current TCP protocol does not work well above the current 802.11 MAC layer. The relevant problems include the TCP instability problem found in this kind of network, the severe unfairness problem, and the incompatibility problem. We illustrate that all these problems are rooted in the MAC layer. Furthermore, by revealing the in-depth cause of these problems, we conclude that the current version of this wireless LAN protocol does not function well in multi-hop ad hoc networks. We thus doubt whether the current WaveLAN based system is workable as a mobile multi-hop ad hoc test bed. All the results shown in this paper are based on NS2 simulations, and are compatible with the results from the OPNET simulations.