无线局域网HCCA接入机制研究

分析IEEE802.11e的服务质量（QOS）调度机制,讨论了IEEE802.11e协议给出的简单调度算法（Simple Scheduler）及其支持可变比特速率业务（VBR）时存在的不足。对简单调度算法进行了改进,提出一种适用于实时VBR业务的调度算法,该算法可以动态地调整服务间隔、每个业务流的发送机会周期（TXOP）。在NS网络模拟器上实现了适用于VBR业务的HCCA调度算法,并进行模拟仿真,对两种调度算法的性能进行比较。仿真结果表明,该机制有效改善了延时和传输效率。     

VoWLAN中基于TDMA的QoS机制的研究

针对无线局域网语音应用缺乏可靠服务质量的情况,提出了一种基于时分多址接入的服务质量新机制。该机制采用了IEEE802.15.3无线个域网的中心调度方式,使用时分多址接入机制控制节点接入信道。并在平均接入时延、时延抖动和分组丢失率三方面对IEEE802.11b、IEEE802.11e和新方案三者的性能进行了对比。仿真结果表明,IEEE802.11b性能最差,新机制的性能在平均接入时延和分组丢失率方面比IEEE802.11e分别改善了20%和50%左右。     

有效的无线局域网接纳控制机制

针对IEEE802．11e混合协调功能框架没有给出实现接纳控制决策和具体计算接入时间方法的缺陷,提出一种支持实时业务的自适应接纳控制机制AAC。该机制考虑实时业务的突发特性与网络负载等因素,在保证系统现有业务丢包率与时延要求的前提下,通过动态分配发送机会提高系统的吞吐量。仿真实验以及与IEEE802．11e参考接纳控制机制的比较表明,该机制可以有效地降低实时业务的时延与丢包率,提高系统的吞吐量。     

无线局域网中TXOP动态分配方法

为解决802.11e网络的公平性问题,提出一种TXOP动态分配方法,在节点传输前,根据碰撞次数估计碰撞程度,根据估计得到的网络负载情况调整节点的TXOP,使不同优先级、不同传输速率的业务获得相应的TXOP值。理论分析和仿真结果表明,该方法在采用不同发送速率的业务间,能保证吞吐量的公平性,并减少分组重传次数,降低分组传输平均时延。     

IEEE 802.11e协议HCCA机制下的QoS研究

分析了IEEE 802.11e协议HCCA信道接入机制下的简单带宽调度算法对多媒体业务的QoS支持情况,指出其不足并在其基础上进行了改进,提出了一种基于业务等级的带宽调度算法E-HCCA（Enhanced HCCA）。E-HCCA对不同优先等级业务的数据在带宽分配上采用不同的策略,在优先保证各个节点CBR业务的基础上,根据节点的VBR流量动态平均分配剩余带宽。相比较简单调度算法,E-HCCA算法更好地支持了多用户下的语音业务流和视频业务流,降低了分组时延,增加了系统吞吐率。     

IEEE 802.16 Mesh模式下区分服务时隙调度算法

为了满足用户需求,IEEE802.16支持多种QoS。在IEEE802.16-2004定义的mesh模式的调度机制下,针对实时业务和非实时业务对时延具有不同敏感性的特点,提出了一种区分服务的时隙调度算法。该算法优先处理实时业务,并通过拥塞情况,自适应地调整实时业务的预留带宽。最后对算法进行了仿真,结果表明该算法下业务的延迟、服务请求拒绝率、时隙利用率均有所改善。     

AdHoc网络中考虑事件紧急度的跨层QoS保障机制

考虑Ad Hoc网络中事件紧急度,提出一种可以有效改善整体高优先级业务流时延和时延抖动性能的跨层QoS服务质量保障算法.根据时延要求及业务分组距离目的点的跳数信息,评估此业务的紧急度,并在调度传输时充分考虑各个事件的紧急度.在有实时要求的业务发生超时的情况下,通过合适的路由置换方法,改善业务流时延及网络吞吐量.本文重点讨论如何在IEEE802.11e协议基础上实现对指定优先级业务流的服务质量保障.GloMoSim仿真环境下的仿真结果显示该算法与原有IEEE802.11e协议相比,对于高优先级业务,在分组端到端时延、时延抖动以及网络吞吐量方面有较大的改善.     

改进的WiMAX上行链路调度机制

基于IEEE 802.16e协议的实时业务调度机制,以及对VoIP业务特性的分析,提出了一种改进的上行链路调度机制,该调度机制自适应地调整数据带宽分配周期。从理论上分析了所提出的调度机制和已有调度机制的VoIP用户容量。系统仿真结果表明,在保证系统容量的前提下,所提出的调度机制可以有效地降低用户的上行时延,从而提高了服务质量。     

802.11e无线局域网QoS技术研究

本文分析了IEEE802.11中DCF、PCF和IEEE802.11e中的EDCF、HCF四种接入机制。研究了这四种机制对实时业务的QoS支持能力。通过NS-2软件仿真得出,相比于802.11中的DCF、PCF,802.11e中的EDCF、HCF机制提高了实时业务的QoS。对当前WLAN网络中实时业务的传输有着重要的指导意义。     

802.11e无线局域网QoS技术研究

本文分析了IEEE 802.11中DCF、PCF和IEEE 802.11e中的EDCF、HCF四种接入机制.研究了这四种机制对实时业务的QoS支持能力.通过NS-2软件仿真得出,相比于802.11中的DCF、PCF,802.11e中的EDCF、HCF机制提高了实时业务的QoS.对当前WLAN网络中实时业务的传输有着重要的指导意义.     

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.     

Providing MAC QoS for multimedia traffic in 802.11e based multi-hop ad hoc wireless networks

Ad hoc wireless networks with their widespread deployment, now need to support applications that generate multimedia and real-time traffic. Video, audio, real-time voice over IP, and other multimedia applications require the network to provide guarantees on the Quality of Service (QoS) of the connection. The 802.11e Medium Access Control (MAC) protocol was proposed with the aim of providing QoS support at the MAC layer. The 802.11e performs well in wireless LANs due to the presence of Access Points (APs), but in ad hoc networks, especially multi-hop ones, it is still incapable of supporting multimedia traffic.One of the most important QoS parameters for multimedia and real-time traffic is delay. Our primary goal is to reduce the end-to-end delay, thereby improving the Packet Delivery Ratio of multimedia traffic, that is, the proportion of packets that reach the destination within the deadline, in 802.11e based multi-hop ad hoc wireless networks.Our contribution is threefold: first we propose dynamic ReAllocative Priority (ReAP) scheme, wherein the priorities of packets in the MAC queues are not fixed, but keep changing dynamically. We use the laxity and the hop length information to decide the priority of the packet. ReAP improves the PDR by over 28% in comparison with 802.11e, especially under heavy loads. Second, we introduce Adaptive-TXOP (A-TXOP), where transmission opportunity (TXOP) is the time interval during which a node has the right to initiate transmissions. This scheme reduces the delay of video traffic by reducing the number of channel accesses required to transmit large video frames. It involves modifying the TXOP interval dynamically based on the packets in the queue, so that fragments of the same packet are sent in the same TXOP interval. A-TXOP is implemented over ReAP to further improve the performance of video traffic. ReAP with A-TXOP helps in reducing the delay of video traffic by over 27% and further improves the quality of video in comparison with ReAP without A-TXOP. Finally, we have TXOP-sharing, which is aimed at reducing the delay of voice traffic. It involves using the TXOP to transmit to multiple receivers, in order to utilize the TXOP interval fully. It reduces the number of contentions to the channel and thereby reduces the delay of voice traffic by over 14%. A-TXOP is implemented over ReAP to further improve the performance of voice traffic. The three schemes (ReAP, A-TXOP, and TXOP-sharing) work together to improve the performance of multimedia traffic in 802.11e based multi-hop ad hoc wireless networks.     

Performance analysis of the TXOP burst transmission scheme in single-hop ad hoc networks with unbalanced stations

Transmission Opportunity (TXOP) is a promising burst transmission scheme defined in the IEEE 802.11e Medium Access Control (MAC) protocol to achieve differentiated Quality-of-Service (QoS) and improve the utilization of the scarce wireless bandwidth. Although performance modelling of the TXOP scheme has attracted tremendous research efforts from both the academia and industry, most existing analytical models have been developed under the unrealistic assumption that stations in wireless networks generate identical traffic loads. However, this assumption fails to capture the behaviour of wireless stations under the realistic working environment. To fill this gap, this paper proposes a new analytical model for the TXOP service differentiation scheme in single-hop ad hoc networks in the presence of unbalanced stations with different traffic loads. The QoS performance metrics including throughput, end-to-end delay, frame dropping probability, and energy consumption are derived. Extensive NS-2 simulation experiments are conducted to validate the accuracy of the developed model. The analytical results demonstrate the efficiency of the TXOP scheme for QoS differentiation and performance enhancement. Moreover, the analytical model is adopted as a cost-effective tool to investigate the impact of the buffer size and the number of stations on the performance of the TXOP scheme. The performance results show that the desirable throughput differentiation for different stations can be achieved by setting the appropriate TXOP limits.     

Channel time allocation scheme based on feedback information in IEEE 802.11e wireless LANs

The IEEE 802.11e standard defines a set of quality of service (QoS) enhancements for wireless local area network (WLAN) applications such as voice and streaming multimedia traffic. In the standard, a new medium access control (MAC) protocol is called the hybrid coordinator function (HCF), and also a channel access scheme to transmit multimedia traffic is called the HCF controlled channel access (HCCA). In the IEEE 802.11e WLANs, to satisfy the stringent real-time constraints of multimedia services it is very important to provide an efficient method of allocating network resources. In this regard, we propose a feedback-assisted dynamic channel time allocation scheme considering the application layer information in order to achieve better performance of multimedia traffic over IEEE 802.11e HCCA under the constrained QoS requirements. Performance of the proposed scheme is investigated by simulation. Our results show that under typical channel error conditions, the proposed scheme is very effective regardless of the variation of station numbers and service intervals. Also, it yields high performances while guaranteeing the delay bound.     

无线Mesh网中具有QoS保障的MAC算法

(中南民族大学电子信息工程学院,武汉 430074)     

An 802.11e HCCA scheduler with an end-to-end quality aware territory method

In this paper we present a solution for the IEEE 802.11e HCCA (Hybrid coordination function Controlled Channel Access) mechanism which aims both at supporting strict real-time traffic requirements and, simultaneously, at handling TCP applications efficiently. Our proposal combines a packet scheduler and a dynamic resource allocation algorithm. The scheduling discipline is based on the Monolithic Shaper-Scheduler, which is a modification of a General Processor Sharing (GPS) related scheduler. It supports minimum-bandwidth and delay guarantees and, at the same time, it achieves the optimal latency of all the GPS-related schedulers. In addition, our innovative resource allocation procedure, called the territory method, aims at prioritizing real time services and at improving the performance of TCP applications. For this purpose, it splits the wireless channel capacity (in terms of transmission opportunities) into different territories for the different types of traffic, taking into account the end-to-end network dynamics. In order to give support to the desired applications, we consider the following traffic classes: conversational, streaming, interactive and best-effort. The so called territories shrink or expand depending on the current quality experienced by the corresponding traffic class. We evaluated the performance of our solution through extensive simulations in a heterogeneous wired-cum-wireless scenario under different traffic conditions. Additionally, we compare our proposal to other HCCA scheduling algorithms, the HCCA reference scheduler and Fair Hybrid Coordination Function (FHCF). The results show that the combination of the MSS and the territory method obtains higher system capacity for VoIP traffic (up to 32 users) in the simulated scenario, compared to FHCF and the HCCA reference scheduler (13 users). In addition, the MSS with the territory method also improves the throughput of TCP sources (one FTP application achieves between 6.1 Mbps without VoIP traffic and 2.1 Mbps with 20 VoIP users) compared to the reference scheduler (at most 388 kbps) and FHCF (with a maximum FTP throughput of 4.8 Mbps).     

WiMAX拓展实时轮询调度机制分析

为了提高无线资源的利用率,使WiMAX系统更好地支持语音业务,基于IEEE 802.16e协议的QoS调度体系,提出了针对带有静默压缩语音业务的拓展实时轮询调度机制的具体实现方案.而且通过对方案中关键参数的分析,给出了一种QoS保证方法.方法根据用户时延对数据带宽分配进行补偿,并按照用户负载情况动态地调整轮询请求带宽的分配周期.使用OPNET软件进行了系统级性能评估,结果表明所提出的方法可有效地降低用户上行接入时延和上行丢包率,从而提高了通信质量.