基于邻居表的滑动平均值切换策略研究

针对IEEE802.11标准中有关无线切换规定的不足,提出一种基于无线接入点(AP)邻居表的滑动平均值预测策略.通过运用背景扫描机制获取AP信号强度,该策略动态地计算信号强度平均值以决策移动终端(MH)发起切换的时间点.为降低切换过程中数据传输的丢包率,该策略还结合了邻居表将数据路由至候选AP,使得MH与候选AP建立连接后接收到数据.实验结果表明,该策略有效地减少了MH切换过程所消耗的时间,同时保证了切换过程数据传输的可靠性.     

基于虚拟接口的802.11网络切换技术

802.11网络现有的切换方案可以解决域内切换问题,域间切换涉及网络层切换操作,导致较大的网络延迟和大量的分组丢失。该文提出一种基于虚拟接口的802.11网络切换方法,通过对一个物理接口设备分时使用,使得无线终端在保证现有通信质量的情况下,平滑地完成与新无线访问点AP建立无线链路的过程。该方法具有实现简单、不用修改现有的Mobile IP协议等优点,在NS系统上的模拟表明,该方法能够满足实时交互式应用对移动切换过程中的网络质量需求。     

IEEE802.11网络中过后注册切换机制实现及性能分析

首先对移动切换过程和过后注册切换机制进行分析.在IEEE802.11网络上,对管理帧的信息单元进行扩展来构造二层触发消息,并在Linux上实现了过后注册切换机制.最后从丢包和切换时延两方面对过后注册切换机制进行性能分析.分析结果表明,过后注册切换机制明显降低了切换时延,减少了数据包的丢失.     

IEEE 802.11宽带无线局域网负载均衡优化研究*

在基于IEEE 802.11的宽带无线局域网中,随着终端STA的接入、移动和无线信道的时变性,各个接入点（AP）的负载将会产生差异,需要负载均衡优化机制平衡各个AP的差异,以达到网络资源的最大利用率。详细讨论了无线局域网中用到的几种负载均衡机制,分析了各自的优缺点,结合这些特点给出一种新的负载均衡机制。该机制能较准确地统计终端业务信息,降低切换失败风险,快速达到APs之间的负载均衡,优化提高网络整体性能。     

基于AP预先转发的802.11无线局域网切换机制研究

IEEE802．11无线局域网(WLAN)规范包括MAC子层和物理层(PHY)两个协议层，支持结点的移动通信，但它并没有提供移动主机(MH)的无缝切换算法．当MH从当前的接入点(AP)覆盖小区移动到另一个AP小区时，由于通信的暂时中断会对系统的性能造成一定的影响．目前的切换算法包括硬切换(hard-handoff)和平滑切换(smooth-handoff)等，这些算法要么丢包率大，要么切换延迟长，对切换的性能改善程度并不是很高．从切换延迟和丢包率两方面人手，提出了一种AP预先转发的WLAN切换算法，并对其进行了仿真实验研究，实验结果表明，该切换算法的整体性能优于其他算法．     

基于IEEE 802.11ah的井下安全监测传感网性能分析

针对煤矿井下特殊通信环境对安全监测无线传感网的需求及现有安全监测无线传感网存在接入设备数量和传输距离受限等不足,提出了将IEEE 802.11ah应用于煤矿安全监测无线传感网,并利用NS-3仿真平台对其传输距离、吞吐量、时延、能耗等性能进行了分析。分析结果表明,IEEE 802.11ah的传输距离远大于传统WiFi通信技术,其传输距离为600m时丢包率仍为0;当节点数较少、网络未饱和时,节点竞争信道冲突和碰撞情况并不严重,IEEE 802.11ah在吞吐量、时延、能耗等方面的性能优势并不明显,但在接入节点数众多时,IEEE 802.11ah能很好保证网络的吞吐量、时延、能耗等性能,网络饱和后随着接入节点数的增加,相比传统WiFi通信技术网络性能的持续大幅下降,其网络性能下降幅度更小且渐趋稳定;IEEE 802.11ah网络上行链路数据传输性能与受限访问窗口参数选取有关,准确选取受限访问窗口参数可以更好地发挥基于IEEE 802.11ah的煤矿安全监测无线传感网的性能优势。     

基于Openwrt的802.11r无缝漫游系统的实现

标准的IEEE 802.11协议在设计之初没有考虑用户移动性,所以在用户切换AP时会出现比较大的延时,严重影响一些对网络实时性要求很高的业务开展,而基于IEEE 802.11r协议的无缝漫游可以大大缩短切换延时,非常有利于开展目前热门的实时语音、视屏直播等业务。笔者利用开源的Openwrt系统,让普通的消费级无线路由也可以享受只有企业级无线路由器才可以支持的基于IEEE 802.11r无缝漫游技术,并通过iNetTools软件统计切换延时,结果表明该系统能够满足普通家庭用户对无缝漫游的需求,降低了无缝漫游系统的架设成本。     

地铁环境中双网卡软切换机制的研究与应用

高速移动的无线节点在接入点间切换时,切换延迟较大、丢包率较高.在单网卡切换环境中,不可避免地存在网络中断的现象.该文结合地铁无线通信环境对切换延迟和丢包问题进行研究,为移动节点配备两块无线网卡,控制两块网卡协同工作,共同完成无线切换和数据传输.测试结果表明,双网卡软切换机制无需修改网络层及上层协议栈,在特定环境中可以实现低延迟和零丢包.该切换机制已经在地铁信号系统国产化预研项目中得到应用.     

基于IEEE 802.11无线局域网的VoIP性能分析

对无线局域网的两种标准IEEE 802.11b和IEEE 802.11g的差异进行了研究,后者在前者的基础上采用了正交频分复用(OFDM)技术,通过设计一个针对这两种标准的无线局域网上的VoIP性能测试的试验,使用Chariot网络工具进行仿真,对延迟、抖动、丢包率进行对比分析,结果表明IEEE 802.11g标准比IEEE 802.11b标准具有明显的性能优势.     

校园无线局域网性能优化途径之初探

系统吞吐率是衡量网络性能的最重要的指标之一。无线局域网不同标准的选择、无线网络标准的单一IEEE802.11g模式和IEEE802.11g/b双模式、接入点AP设备的数量以及定位原则直接影响吞吐率的提高,围绕上述三个因素概括性的阐述了校园无线网络性能优化的措施。     

Design and evaluation of a novel MAC layer handoff protocol for IEEE 802.11 wireless networks

In recent years, the IEEE 802.11 wireless network family has become one of the most important set of standards in the wireless communications industry. IEEE 802.11 compliant devices are inexpensive and easier to configure and deploy than other wireless technologies. In an IEEE 802.11 wireless network, wireless terminals can move freely. As a result, when the wireless terminal moves away from its current access point, it must switch to another access point to maintain the active connection. This is known as the MAC layer handoff process. MAC layer handoff latency should be minimized to support real-time applications and to provide mobile devices with seamless roaming in IEEE 802.11 wireless networks. This paper proposes a novel MAC layer handoff protocol over IEEE 802.11 wireless networks by introducing advertisement messages sent from other mobile nodes and from which wireless terminals are able to receive the information of access points in their neighborhood. A mobile node can try to associate with access points based on the prediction before starting the probe process. The experimental results demonstrate that our solution can reduce MAC layer handoff latency to meet the requirements of real-time applications.     

Fast Directional Handoff and lightweight retransmission protocol for enhancing multimedia quality in indoor WLANs

More and more mobile devices such as smartphones are being used with IEEE 802.11 wireless LANs (WLANs or Wi-Fi). However, mobile users are still experiencing poor service quality on the move due to the large handoff delay and packet loss problem. In order to reduce the delay, a new handoff scheme using the geomagnetic sensor embedded in mobile devices is proposed in this paper. The proposed scheme predicts the movement direction of a Mobile Station (MS) from the currently associated Access Point (AP) and performs active scanning with a reduced number of channels. In terms of the packet loss, a lightweight retransmission protocol is also proposed to minimize lost packets on Wi-Fi without producing a lot of acknowledgement packets. The proposed approaches are implemented on Android smartphones, and their performance is evaluated in a real indoor WLAN environment. The evaluation results demonstrate that the proposed schemes maintain seamless quality for real-time video even in an environment with frequent handoffs. Note that the proposed schemes are a client-only solution and do not require modification of the existing APs, which renders them very practical.     

A link layer adaptive pacing scheme for improving throughput of transport protocols in wireless mesh networks

In this paper, we study the performance of a static multihop wireless network, specifically that of the backhaul network of a two-tier Wireless Mesh Network (WMN) operating on IEEE 802.11 Medium Access Control (MAC) protocol. The performance of an IEEE 802.11 based backhaul network is greatly affected by the MAC contention and congestion in the network. If the sources pump data into the network than can be supported, loss rate increases due to MAC contention and congestion in the network. This also leads to the problem of unfairness among flows. In this paper, we propose a Link Layer Adaptive Pacing (LLAP) scheme that adaptively controls the offered load into the network. This improves the performance of higher layer protocols without any modifications to them. Our LLAP scheme estimates the four hop transmission delay in the network path without incurring any additional overhead (Control packets) and accordingly paces the packet transmissions to reduce MAC contentions in the network. We implement the LLAP scheme in ns-2.29 network simulator and extensively study its performance for both User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) traffic in different network scenarios. In all the cases, our scheme shows a significant improvement in the performance of both UDP and TCP traffic.     

无线局域网中一种自适应RTS门限调整算法

媒体访问控制是无线局域网的重要部分,决定了具有受限通信带宽的无线信道的共享效率.IEEE 802.11系列标准基于现有以太网技术,具有良好的操作性和兼容性,已发展成为WLAN的主要标准.IEEE 802.11 DCF中有两种接入方式:基本方式和RTS/CTS方式.RTS/CTS方式是IEEE 802.11无线网络的可选握手过程,用于减小碰撞的可能性.决定使用RTS/CTS握手方式的RTS门限值是研究的重要参数,不同的取值对数据传送会产生不同的性能特点.文章通过分析IEEE 802.11 DCF分组发送成功和碰撞的持续时间,获得了RTS和基本方式下分组传输时间开销与分组长度的关系.给出了以最小化分组传输时间代价为优化目标的最优RTS门限的计算公式.通过对当前信道分组发送的冲突概率的预测实现了最优RTS门限的计算;给出了一个具体的RTS门限自适应调整算法,使终端能自动调整其RTS门限以达到或接近最优值.仿真表明RTS门限自适应调整算法明显减小了分组传送时收发器发送单位比特数据的时间开销.     

一种基于IEEE802.11的无线自组网MAC协议

基于IEEE802.11DCF提出了一种应用于无线自组网的媒质接入控制协议，该协议包括网络适应性退避算法和拥塞反馈两个关键机制。协议的主要思想是根据节点周围实际竞争状况和网络拥塞情况进行包调度。仿真结果表明，提出的机制有效地降低了平均端到端时延和数据丢包率,提高了信道接入公平性。     

Runtime optimization of IEEE 802.11 wireless LANs performance

IEEE 802.11 is the standard for wireless local area networks (WLANs) promoted by the Institute of Electrical and Electronics Engineers. Wireless technologies in the LAN environment are becoming increasingly important and the IEEE 802.11 is the most mature technology to date. Previous works have pointed out that the standard protocol can be very inefficient and that an appropriate tuning of its congestion control mechanism (i.e., the backoff algorithm) can drive the IEEE 802.11 protocol close to its optimal behavior. To perform this tuning, a station must have exact knowledge of the network contention level; unfortunately, in a real case, a station cannot have exact knowledge of the network contention level (i.e., number of active stations and length of the message transmitted on the channel), but it, at most, can estimate it. We present and evaluate a distributed mechanism for contention control in IEEE 802.11 wireless LANs. Our mechanism, named asymptotically optimal backoff (AOB), dynamically adapts the backoff window size to the current network contention level and guarantees that an IEEE 802.11 WLAN asymptotically achieves its optimal channel utilization. The AOB mechanism measures the network contention level by using two simple estimates: the slot utilization and the average size of transmitted frames. These estimates are simple and can be obtained by exploiting information that is already available in the standard protocol. AOB can be used to extend the standard 802.11 access mechanism without requiring any additional hardware. The performance of the IEEE 802.11 protocol, with and without the AOB mechanism, is investigated through simulation. Simulation results indicate that our mechanism is very effective, robust, and has traffic differentiation potentialities.     

存在隐藏终端的IEEE802.11e多跳无线网络模型分析

首次将802.11e的接入机制放入多跳环境中进行仿真分析和定量研究。提出一种新的多跳环境下802.11e网络模型,结合M/G/1/K排队模型,定量分析在隐藏终端影响下802.11e网络的MAC层吞吐率、MAC时延和帧丢失率,研究802.11e在多跳环境下性能表现的内在原因。经过仿真实验结果与数值分析结果的对比,验证了分析模型的准确性;通过有(无)隐藏终端影响下MAC层性能的对比分析,指出了802.11e在多跳无线网络中支持QoS的局限性:受隐藏终端的影响,802.11e对不同接入等级的业务提供QoS区分的性能明显降级。因而有必要研究隐藏终端问题的解决方案,提高802.11e在多跳环境下的性能。