DCF传输MPEG-4视频流的研究

IEEE 802.11无线局域网的基本接入方式是分布式协调功能(DCF)。当无线局域网中的负载变大时,采用DCF的站点可能在成功发送之前需要延迟几秒以上。这将严重影响MPEG-4视频流等实时应用的性能。该文针对实时应用提出了一种DCF的改进机制L-DCF,并介绍了在L-DCF下如何传输MPEG-4视频流。仿真结果表明,与DCF相比,采用L-DCF的MPEG-4视频流将经历较少的时延和抖动,同时又具有与采用DCF时相同的吞吐量与公平性,因此能够获得更好的播放质量。     

基于吞吐量比例公平的802.11MAC优化机制

802.11DCF MAC机制为单速率网络节点提供相同的信道接入机会,保证节点公平地使用信道。在多速率网络中依然沿用该接入机制,会导致低速率的节点长期占用信道,高速传输的优势得不到充分利用,使得多速率网络中所有节点的吞吐量都等于低速率节点的吞吐量,这就是著名的多速率网络"性能异常"问题。针对这一问题,通过分析建模,提出CCDa P(Change the CWmin according to Data rate and Packet-size)MAC优化机制以改善多速率网络中的节点的吞吐性能。仿真结果表明,该接入机制不仅可以保证节点公平性,还能提高网络整体吞吐量。特别地,在数据包长度不等的多速率WLAN中,该机制能够保证网络节点吞吐量的比例公平。     

IEEE802.11MAC协议中一种改进的退避解决算法

IEEE802.11无线局域网,在共享信道中经历碰撞的网络节点需要随机退避一段时间,这段时间是从竞争窗口中均匀选取,竞争窗口大小由BEB机制动态控制,一些文献研究表明,BEB机制在重负载的情况下,突现出公平性问题和低的吞吐量,本文基于MILD退避机制,提出一种适用于分布式协调功能改进算法。该算法通过修改802.11的MAC层中的DCF(DistributedCoordinationFunction)子协议,改善了IEEE802.11无线局域网在拥塞情况下的性能,提高了网络吞吐量。     

无线供电混合多址接入网络的资源分配

无线供电技术是延长无线网络节点寿命的有效方案。该文研究一个由基站和多个分簇用户组成的无线供电混合多址接入系统。系统的传输分为两个阶段。在第1阶段,基站向用户广播能量;在第2阶段,用户向基站传输信息。用户簇和用户簇之间采用时分多址接入,分时传输;同一簇内多个用户采用非正交多址接入,同时传输。该文研究联合优化系统的各阶段的传输时间、基站的发射功率、用户的发射功率等资源,分别以优化网络频谱效率和用户簇之间的公平性为目的,提出最大化网络吞吐量和最大化用户簇的最小吞吐量的联合资源分配算法。研究结果表明所提的两种算法分别能有效提高系统的频谱效率和保证用户簇之间的公平性。     

红外无线局域网MAC性能分析与优化

用红外线作为室内无线通信传输媒质的红外无线局域网,由于具有成本低、功耗小等特性,现已逐渐被广泛应用;分析了红外无线局域网媒质访问控制层协议及其所存在的问题;推导了红外无线局域网归一化吞吐量的计算公式,研究了竞争窗口的大小与调芍方式,在此基础上对网络的媒质接入性能进行了优化设计,提出了红外自适应接入算法,弥补了现有红外局域网的不足之处;利用OPNET仿真软件,对优化后的红外自适应接入性能进行了仿真验证。     

基于WLAN网络的视频流组播传输机制研究

本文针对基于IEEE802.11的无线局域网上组播视频流时终端异构问题,提出一种对客户端分组要：组播传输机制。该机制中,AP根据不同终端的接收能力和实时视频流的码率,将组播组内的站点进行分组,对划分的子组采用不同速率进行组播,以此来提高IEEE802.11无线局域网的传输性能,降低差站点对好站点视频接收效果的限制。NS-2仿真实验结果表明,分组组播机制有效地提高了无线局域网和好站点的性能。     

基于TimeGAN和多臂老虎机的WMN接入模式选择方法

无线Mesh网络是一种多跳网络,与传统无线网络不同,它融合了无线局域网WLAN和Ad Hoc网络的优势,具有易组织、高速率和自愈性等优点。由于无线Mesh网络存在负载高、不均衡业务以及业务突发性高等特性,直接在无线Mesh网络中应用传统无线自组网的经典接入算法可能会导致网络性能下降,包括丢包率偏高、吞吐量不足、端到端时延无法得到保障等问题。基于此,提出了一种基于时间序列生成对抗网络(Time-series Generative Adversarial Networks, TimeGAN)与和多臂老虎机(Multi-Armed Bandits, MAB)的无线Mesh网络接入模式选择方法,该方法使用TimeGAN生成无线Mesh网络中Mesh节点的时间序列数据,并将其用于预测不同接入模式下的网络性能;然后,使用多臂老虎机算法选择最佳的接入模式,以实现最优的网络性能。在使用具体硬件搭建的无线Mesh网络中进行了实验,结果表明所提出的方法具有显著的优势和实用性,对于提高无线Mesh网络的性能具有重要意义。     

应用于航空Ad Hoc网络的高负载优先级均衡MAC协议

针对航空Ad Hoc网络在高负载下的低时延信道接入问题,提出一种优先级与公平性协作的多信道MAC协议(PBLL/HL)。在多信道检测统计基础上加以改进,结合优先级机制,在高负载网络中适时截流低优先级业务,优化网络流量,保证高优先级业务低时延发送;设计公平性优先级阈值与冲突退避窗口算法(PCA),减小低优先级业务接入时延。仿真结果表明,PBLL/HL能够在高负载网络有效控制信道接入,维持良好的网络流量,降低网络平均接入时延(低优先级业务时延过载时降低10%以上),稳定高负载网络吞吐量(最大吞吐量88.1%,过载时吞吐量下滑平缓),解决了航空数据链高业务量带来的高时延和网络拥塞。     

一种新型提供服务质量保证的自适应无线局域网媒体接入协议

在无线局域网中提供QoS保证近年来日益成为研究热点.本文基于自适应无线局域网MAC协议在一种新的建模分析的基础上提出了一种提供QoS的新算法QoS-NSAD.QoS-NSAD赋予高优先级站点和低优先级站点不同的自适应特性从而很好地保证高负载环境高优先级用户的QoS特性.同时由于继承了以前算法的自适应机制,系统吞吐量能够逼近最优.大量仿真结果验证了算法的有效性.     

新型随机多址接入无线传感器网络MAC控制协议与能量有效性分析

提出了一种新的随机多址接入无线传感器网络的MAC控制协议,在发送分组的时间1+a中,采用P概率检测与1-坚持的联合控制策略,并对多通道的随机多址接入无线传感器网络进行了分析,理论分析了系统的吞吐量和多通道中不同业务的吞吐量,以及信息分组的发送时延等参数,理论分析与仿真实验结果相一致.还结合无线传感器网络的能量有效性,通过对概率P值的选取,控制忙周期侦听信道的节点数和空闲期的休眠站点数,采用休眠技术实现了系统的节能效果.通过对2种无线传感器网络控制协议的能量有效性分析,证明了改进的控制协议其节点具有更长的生命周期,更适合作为无线传感器网络的MAC控制协议.     

Temporal fairness guarantee in multi-rate wireless LANs for per-flow protection

Wireless LAN technologies such as IEEE 802.11a and 802.11b support high bandwidth and multi-rate data transmission to match the channel condition (i.e., signal to noise ratio). While some wireless packet fair queuing algorithms to achieve the per-flow throughput fairness have been proposed, they are not appropriate for guaranteeing QoS in multi-rate wireless LAN environments. We propose a wireless packet scheduling algorithm that uses the multi-state (multi-rate) wireless channel model and performs packet scheduling by taking into account the channel usage time of each flow. The proposed algorithm aims at per-flow protection by providing equal channel usage time for each flow. To achieve the per-flow protection, we propose a temporally fair scheduling algorithm called Contention-Aware Temporally fair Scheduling (CATS) which provides equal channel usage time for each flow. Channel usage time is defined as the sum of the packet transmission time and the contention overhead time due to the CSMA/CA mechanism. The CATS algorithm provides per-flow protection in wireless LAN environments where the channel qualities of mobile stations are dynamic over time, and where the packet sizes are application-dependent. We also extend CATS to Decentralized-CATS (D-CATS) to provide per-flow protection in the uplink transmission. Using an NS-2 simulation, we evaluate the fairness property of both CATS and D-CATS in various scenarios. Simulation results show that the throughput of mobile stations with stable link conditions is not degraded by the mobility (or link instability) of other stations or by packet size variations. D-CATS also shows less delay and less delay jitter than FIFO. In addition, since D-CATS can coordinate the number of contending mobile stations, the overall throughput is not degraded as the number of mobile stations increases. This work was supported in part by the Brain Korea 21 project of Ministry of Education and in part by the National Research Laboratory project of Ministry of Science and Technology, 2004, Korea.     

On Improving the Performance of IEEE 802.11 with Relay-Enabled PCF

Integrating wireless LAN (WLAN) techniques with the third generation cellular networks has become a promising way to improve the performance of wireless systems. As WLANs play an important role in such heterogeneous systems, the performance of WLANs becomes important to the whole system. It is well known that WLANs provide a physical layer multi-rate capability, and hence MAC layer mechanisms are needed to exploit this capability. In this paper, we propose a novel MAC layer relay-enabled point coordination function (PCF) protocol, called rPCF, to exploit the physical layer multi-rate capability. Since WLAN supports multiple data rates in response to different channel conditions, data packets may be delivered faster through a relay node than through the direct link if the direct link has low quality and low rate. To enable MAC layer relay, the access point needs to collect information about the channel conditions, and notify the mobile nodes which data rate to use and whether to transmit the data through a relay station. We design protocols to achieve this goal and refine these protocols to minimize the control overhead. Simulation results show that rPCF can significantly improve the system performance in terms of system throughput and transmission delay by adding only a negligible control overhead.     

CoopMAC: A Cooperative MAC for Wireless LANs

Due to the broadcast nature of wireless signals, a wireless transmission intended for a particular destination station can be overheard by other neighboring stations. A focus of recent research activities in cooperative communications is to achieve spatial diversity gains by requiring these neighboring stations to retransmit the overheard information to the final destination. In this paper we demonstrate that such cooperation among stations in a wireless LAN (WLAN) can achieve both higher throughput and lower interference. We present the design for a medium access control protocol called CoopMAC, in which high data rate stations assist low data rate stations in their transmission by forwarding their traffic. In our proposed protocol, using the overheard transmissions, each low data rate node maintains a table, called a CoopTable, of potential helper nodes that can assist in its transmissions. During transmission, each low data rate node selects either direct transmission or transmission through a helper node in order to minimize the total transmission time. Using analysis, simulation and testbed experimentation, we quantify the increase in the total network throughput, and the reduction in delay, if such cooperative transmissions are utilized. The CoopMAC protocol is simple and backward compatible with the legacy 802.11 system. In this paper, we also demonstrate a reduction in the signal-to-interference ratio in a dense deployment of 802.11 access points, which in some cases is a more important consequence of cooperation     

High throughput reactive routing in multi-rate ad hoc networks

A reactive routing algorithm for multi-rate ad hoc wireless networks is proposed. It enhances the AODV protocol and achieves higher throughput by utilising the multi-rate capability of the underlying wireless channel via the MAC delay routing metric. Simulations show that the protocol leads to a significant throughput increase over traditional ad hoc routing protocols.     

Channel-quality-based opportunistic scheduling with ARQ in multi-rate wireless networks: modeling and analysis

In this paper, we develop a novel framework for analyzing radio link level performance for opportunistic scheduling with automatic repeat request (ARQ)-based error control in multi-rate wireless networks. The multi-rate transmission is assumed to be achieved through adaptive modulation and coding (AMC) to adjust the transmission rate according to the channel condition. The residual error effect due to each AMC setting is counteracted by means of a limited persistence ARQ protocol. The novelty of the proposed analytical framework lies in the fact that we are able to derive complete statistics (in terms of probability mass function) for both short-term and long-term performance measures such as system throughput, per-flow throughput, inter-success delay under both uncorrelated and correlated wireless channels. These performance measures can also be obtained in case of non-identical channels for different users. Analytical results are validated through simulations and the impacts of channel behavior on the different radio link level performance metrics are investigated.     

Dynamic Rate and FEC Adaptation for Video Multicast in Multi-rate Wireless Networks

Video multicast over Wireless Local Area Networks (WLANs) faces many challenges due to varying channel conditions and limited bandwidth. A promising solution to this problem is the use of packet level Forward Error Correction (FEC) mechanisms. However, the adjustment of the FEC rate is not a trivial issue due to the dynamic wireless environment. This decision becomes more complicated if we consider the multi-rate capability of the existing wireless LAN technology. In this paper, we propose a novel method which dynamically adapts the transmission rate and FEC for video multicast over multi-rate wireless networks. In order to evaluate the system experimentally, we implemented a prototype using open source drivers and socket programming. Our experimental results show that the proposed system significantly improves the multicast system performance.     

无线Ad hoc网络速率自适应MAC协议研究

无线Ad hoc网络多采用IEEE802.11系列标准,在物理层为数据传输提供了多速率能力。为了充分利用多速率能力提高系统的吞吐量,速率自适应MAC协议根据对当前信道质量的估计,选择最佳的数据传输速率。在分析了速率自适应MAC协议的基本工作过程之后,重点综述了目前存在的速率自适应MAC协议,并讨论了进一步研究的方向。     

Contention-Based Airtime Usage Control in Multirate IEEE 802.11 Wireless LANs

In a multirate wireless LAN, wireless/mobile stations usually adapt their transmission rates to the channel condition. It is difficult to control each station's usage of network resources since the shared channel can be overused by low transmission-rate stations. To solve this problem, we propose a distributed control of stations' airtime usage which 1) always guarantees each station to receive a specified share of airtime, and 2) keeps service for individual stations unaffected by other stations' transmission rates. Such airtime control enables service differentiation or quality of service (QoS) support. Moreover, it can achieve a higher overall system throughput. The proposed airtime usage control exploits the Enhanced Distributed Channel Access (EDCA) of the IEEE 802.11e standard . Two control mechanisms are proposed: one based on controlling the station's arbitration inter-frame space (AIFS) and the other based on the contention window size. We show how the stations' airtime usage is related to the AIFS and contention window size parameters. Using this relation, two analytical models are developed to determine the optimal control parameters. Unlike the other heuristic controls or analytical models, our model provides handles or parameters for quantitative control of stations' airtime usage. Our evaluation results show that a precise airtime usage control can be achieved in a multirate wireless LAN     

Ad Hoc网络基于TCP吞吐量的速率自适应技术

目前的IEEE 802.11标准在物理层支持多速率传输,但在媒体接入控制(MAC,Media AccessControl)层却没有相应的速率自适应方法。针对目前Ad Hoc网络自适应速率控制方法的不足和在无线通信环境下TCP性能会大幅度恶化,提出了一种改进的MAC层速率自适应协议,该协议实现简单,与传统标准兼容性好,在提高TCP吞吐量的基础上能适应快速变化的无线信道,且能实现分段数据包的速率自适应传输。仿真结果表明,该协议比RBAR协议有更好的吞吐量。     

An Efficient IEEE 802.11 ESS Mesh Network Supporting Quality-of-Service

IEEE 802.11 infrastructure basic service set (BSS) wireless local area networks (WLANs) are widely used, each comprising an access point (AP) and its associated stations. There is a need to interconnect BSSs wirelessly to create an extended service set (ESS) mesh network. We propose a solution for the architecture and protocols of a mesh distributed coordination function (MDCF) to interconnect a large number of APs in order to form an efficient ESS mesh network under distributed control. MDCF applies time-division multiple access to share the radio medium and is able to run on a single frequency channel on top of the IEEE 802.11a/b/g physical layers, concurrently to legacy stations. MDCF is capable to efficiently exploit channel capacity, fairly distribute bandwidth among the mesh points and support multihop relaying of a large number of concurrent traffic flows strictly observing specific quality-of-service requirements. Example simulation results proof the outstanding performance of the new concept proposed