Accumulating error-free frame blocks to improve throughput for IEEE 802.11-based WLAN

Improving channel throughput is significant in wireless local networks (WLANs). There exists a drawback in the distributed coordination function (DCF) of medium access control (MAC) protocol in IEEE 802.11 standard. That is, it may lead the sending node of a frame to blindly enlarge its contention window when the receiving node discards the received frame due to bit error, which decreases channel throughput and increases frame latency. To overcome this drawback, a novel scheme called gathering error-free block (GEB) scheme is proposed, which divides the payload of a frame into blocks, with each having a checksum, and lets the receiving node collect error-free blocks to assemble the original frame. The metric of channel throughput under the GEB scheme is formulated, which is based to develop the optimization model that is able to find the optimal number of blocks maximizing the throughput. Simulation results show that the GEB scheme is able to improve channel throughput and reduce channel occupancy duration per frame transmission in the WLAN with links prone to bit error.     

IEEE 802.11e无线局域网中VoIP容量分析及改进

IEEE 802.11e提供了对不同的业务流进行优先级区分的机制,提高了无线局域网（WLAN）内VoIP等实时业务相对于其他业务流的优先性,从而在一定程度上保障其QoS要求,但并没有对WLAN中移动节点（MH）和接入节点（AP）进行优先级的区分,导致AP处上下行业务流量的不平衡,并造成瓶颈效应,限制了VoIP容量。针对这一问题,在对WLAN中VoIP进行容量分析的基础上,提出一种改进AP信道接入能力的策略。通过对AP中队列长度的检测来判断网络拥塞程度,从而动态调整AP的仲裁帧间间隔（AIFS）,以达到根据网络拥塞状况来动态调整AP的优先级,弱化AP的瓶颈效应对VoIP容量制约的目的。仿真结果表明,所提出的改进方案能够有效地提高WLAN中的VoIP容量。     

802.11 WLAN的安全缺陷及其对策

近几年来,随着无线技术的迅猛发展,无线局域网（WLAN）得到了广泛的应用,但其安全性仍是个未决难题,该文深度分析了WLAN的主要安全缺陷,并进一步分析了当前安全解决方案的机制及存在的问题,最后,给出相应的安全改进措施。     

一种802.11a无线局域网均衡器设计实现

无线通信中,传输信号由于受信道时间选择性衰落和频率选择性衰落的影响容易出现传输失真,因此,通常在接收端通过均衡器进行校正.该文根据无线局域网的特点,结合OFDM技术和长训练序列的特性,提出了一种均衡器结构.系统在Simulink环境下根据802.11a定义进行了算法性能仿真并由FPGA完成硬件电路的开发,结果表明该均衡器结构简单,性能可靠且处理速度快,可作为802.11a无线局域网的一个模块,在实际运用中可获得较好的效果.     

802.11无线局域网中的智能AP切换策略

在802.11协议内,站点有权选择将要连接的接入点,因此接入点选择机制的实现就成为了负载平衡的一个关键因素.新提出的AP切换策略可应用到新的兼容802.11e的无线网络.由于无需对原有的802.11和802.11e作修改,无论从实现成本和兼容性两个方面来看,都是很实用的方案.仿真实验表明该策略既能充分利用无线通信资源,又可以实现无线局域网内的负载平衡.     

IEEE 802.11ax密集WLAN的干扰协调策略

无线局域网(WLAN)中设备数量的指数级增长,导致激烈的信道争用及严重的同频干扰。当运行在同一频段上的两个WLAN在彼此感知范围内时,使用先监听后发送的分布式信道接入方式会引起资源浪费,降低网络吞吐率。提出一种密集WLAN场景下的干扰协调策略。根据IEEE 802.11ax中的基本服务集颜色对站点进行分组,提出协调交替目标唤醒时间策略(CAT),安排不同组内站点在不同的时间接收数据,从而避免信道争用,最小化干扰,同时通过优化下行数据包大小提高网络吞吐率。仿真结果表明,当两个WLAN距离很近时,CAT方案能够保证网络吞吐率。此外,在考虑误码率的情况下,吞吐率并非随着数据包的增大单调递增,而是存在一个阈值,超过该阈值后网络吞吐率即呈下降趋势。     

IEEE 802.11无线局域网的区分服务机制研究

文中探讨了在IEEE 802．II无线局域网中提供区分服务的有关问题，分析了有线网络中提供区分服务的QoS机制不适用于无线网络的原因，介绍了IEEE 802．II媒体访问控制层上的一种基于初始竞争窗口CWmin的按节点区分的优先级机制，并在此基础上给出了基于流的区分方法。仿真实验表明，基于节点的优先级区分机制对UDP流已经可以达到比较理想的效果，而对于TCP流效果并不明显，但基于流的区分服务机制能够对TCP流进行很好的区分。     

802．11 WLAN区分服务机制及其性能分析

针对无线Ad Hoc网络不能区分数据流优先级的问题,提出一种基于802.11DCF退避算法的改进机制,该机制通过减少高优先级数据流的MAC层重传次数,使发送高优先级数据流（如实时数据流）的节点更容易连接信道,高优先级数据流占有更多带宽资源,从而减少传输时延。分析结果表明,该算法能有效提高高优先级数据流的传输性能。     

基于802．11a／b MAC的WLAN用户定位技术研究

本文研究了包括IEEE 802，11a／b结构特征在内的IEEE 802，11无线局域网(WLAN)的特性，提出了基于IEEE 802．11a／b MAC的WLAN用户室内室外定位技术方案。     

A survey of energy efficient MAC protocols for IEEE 802.11 WLAN

In recent years, IEEE 802.11 wireless local area networks (WLANs) have been widely deployed, and more and more mobile devices have built-in WLAN interfaces. However, WLAN employs the carrier sense multiple access with collision avoidance (CSMA/CA) medium access control (MAC) protocol, which consumes a significant portion of the energy resources of a mobile device. Hence, minimizing the energy consumption of the WLAN interface in mobile devices has recently attracted considerable interest in both academia and industry. This article provides a survey and an experimental study of the energy consumption issues and energy-efficient technologies of the MAC protocol in IEEE 802.11 WLAN.     

802.11b无线局域网嗅探的实现与防范

嗅探无线局域网直接威胁网络安全。本文分析了无线局域网嗅探实现的可能性、讲述了原理和过程，并介绍了一种实现无线网络嗅探的具体方法，最后提出了相应的防范策略。     

Balancing energy efficiency and throughput fairness in IEEE 802.11 WLANs

The proliferation of wireless networks based on IEEE 802.11 has resulted in a heterogenous set of devices using a variety of applications to compete for the desired service performance. Most notably, the class of highly mobile and energy constrained devices is showing high growth rates. Yet, fairness of resource allocation is still only considered in terms of achievable throughput and without considering energy efficiency. In this paper we first show that performing an energy efficient and fair resource allocation in current IEEE 802.11 WLANs is challenging, given the diversity of power consumption figures among mobile devices. We then propose a criterion to objectively balance between the most energy-efficient configuration (where all resources are given to one station) and the throughput-fair allocation (where the power consumption is not considered). We derive a closed-form expression for the optimal configuration of 802.11 with respect to this criterion. Our analysis is validated through simulations, showing that our approach betters the prevalent allocation schemes discussed in literature in terms of energy efficiency, while maintaining the notion of fairness among devices. Experimental results obtained in a real-world testbed confirm the main results derived from our analysis and simulations.     

Analyzing the effective throughput in multi-hop IEEE 802.11n networks

In this paper we characterize the effective throughput for multi-hop paths in IEEE 802.11n based wireless mesh networks. We derive an analytical model capturing the effects of frame aggregation and block acknowledgements, features found in the new IEEE 802.11n standard. We describe the throughput at MAC layer as a function of physical data rate, error rate, aggregation level and path length. While being mathematically tractable, the proposed model is flexible enough to account for complex and realistic error characteristics of the wireless channel, such as long-term fluctuations and burstiness. We further show how to integrate the well-known Gilbert-Elliot channel model into our model and evaluate both models in our indoor wireless testbed.     

Evaluation of throughput estimation models and algorithms for WLAN frequency planning

Frequency optimization is required to maximize the WLAN throughput in environments where several networks coexist. This paper evaluates four throughput estimation models and two optimization algorithms. Throughput is selected as the optimization criteria for channel assignment. Thus, the result of a throughput estimation model is used as an input for an optimization algorithm. The target is a frequency plan that maximizes multi-cell WLAN throughput. The throughput estimation models are based on radio spectrum usage, practical throughput measurements, WLAN protocol behavior, and theoretical coverage estimations. The models use separate functions for defining the minimum channel distance. In the evaluation, Genetic Algorithm (GA) and a distributed optimization algorithm produce the final frequency plan. A dedicated simulator has been implemented for comparing the throughput estimation models. Usage of a throughput estimation model for frequency optimization in a real WLAN implementation has also been evaluated with a hardware prototype. As overall simulation results, the evaluated throughput estimation models did not produce significant WLAN throughput improvements compared to each other. Still, the selection of the throughput estimation model and optimization algorithm pair is significant, since certain combinations cause poor results.     

Throughput analysis and bandwidth allocation for IEEE 802.11 WLAN with hidden terminals

Motivated by observations from real world wireless local area network (WLAN) deployments, we develop in this paper a novel analytical model to characterize the saturation throughput of an IEEE 802.11-based access point (AP) and stations under the influence of hidden terminals. Unlike existing models, our model can accommodate different numbers of hidden nodes without increasing the model complexity. Given any number of hidden nodes, only four constraints are needed to describe the interaction between stations and the AP with the consideration of both uplink and downlink traffic. Simulation evaluation shows that our model predicts network performance accurately over a wide range of network sizes and indicates the existence of a throughput starvation problem. To address this problem, based on our model, we formulate a bandwidth allocation problem to optimize the network throughput and fairness under some predefined requirements by systematically tuning the AP and stations contention windows. Simulation results show that the starvation problem is resolved with our approach, and the target throughput is met.     

Throughput and PER estimates harnessing link-layer measurements for indoor 802.11n WLAN

The research work reported in this paper investigates if a Markov chain can model the throughput and packet error rate (PER) performance of off-the-shelf IEEE 802.11n wireless LAN network interface cards (NICs). We draw together uplink -downlink information from the NIC with a Markov chain to examine the performance of 802.11n within an indoor environment. Site measurements and point-estimates are taken and compared with the model predictions. Errors of less than 4% were recorded for the Markov model estimates while point-estimates recorded average errors of 9% both compared to site-measured throughput.     

基于最小竞争窗口的IEEE 802.11区分服务机制及其性能分析

针对无线adhoc网络不能提供数据流优先级区分的问题,本文提出了一种基于802.11DCF退避算法的改进机制,该机制通过设置不同的MAC层最小竞争窗口来实现数据流的区分服务,使发送高优先级数据流(如实时数据流)的节点更容易连接信道,从而使高优先级数据流占有更多带宽资源。数学分析表明,该策略能有效的提高高优先级数据流的传输性能。     

基于Linux的802.11b无线局域网数据包捕获方法*

数据包捕获是进行网络监测、流量分析等网管活动中所使用的基本技术。从无线局域网（WLAN）和有线局域网的区别出发,研究了WLAN包捕获的原理及无线网卡两种不同的捕包方法。     

Energy-aware WLAN scanning in integrated IEEE 802.16e/802.11 networks

We consider an issue arising in a vertical handoff (VHO) between IEEE 802.16e and IEEE 802.11 networks, i.e., how efficiently the scanning operation can be controlled to find a target wireless local area network (WLAN) access point (AP). For this purpose, we propose that the 802.16e Base Stations (BSs) periodically broadcast the information about the density of 802.11 APs within their cell coverage. Based on this information, we develop a mathematical model which predicts the probability that any WLAN AP is found within a given scan time. The analytical model is validated via a comparison with the simulation results obtained for both random waypoint and city section mobility models. Based on the analytical model, we devise an energy-efficient scanning algorithm, which enables an mobile station (MSTA) to decide (1) whether to conduct a scan operation in the current 802.16e cell, and (2) if so, how to configure the inter-scan interval considering the energy consumption due to the scan operation. An intensive simulation study shows that our proposed scanning algorithm indeed works well as designed.