兼容IEEE 802．11g标准的4口无线串口设备联网服务器

知名通讯产品制造商四零四科技的无线串行设备连网服务器NPort W2004／W2250／W2150，提供完整无线串行设备通讯连网解决方案：NPort W2004，为全球首创IEEE 802．11g／b标准的4埠无线串行设备连网服务器，2埠及1埠的W2250／2150则提供IEEE 802．11b无线网络。NPort W2004 IEEE 802．11g无线网络，最高数据传输速度可达54 Mbps；W2250／2150 IEEE 802．11b无线网络，最高数据传输速度可达11Mbps。标准的64／128-bit WEP编码加密功能，提供最佳的无线网络安全防护。     

无线宽带网络新技术Super G的研究

随着IEEE 802.11g无线网络标准的推出,WAPI标准在中国延缓实施。针对目前IEEE802.11g标准在实际应用中的带宽不足。一种基于IEEE802．11g增强型的无线网络技术及设备又出现在国人面前。本文主要讲述了Super G的技术概况、特点及性能。并对该技术的未来作了展望。     

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。     

Dr．Ben Q＆A热线

面对HTPC,IEEE 802.11g你行吗？HTPC的流行引起了玩家对无线网络产品的关注,目前市场占有量最多的仍是IEEE 802．11g产品。而IEEE 802．11n虽然来势汹汹,但用户似乎不买账。最近Dr.Ben和不少喜欢折腾HTPC的朋友聊起了相关话题。大家普遍认为受限于ADSL网络带宽、性能提升有限和价格偏高等因素,802．11n产品目前并不1直得购买,相反IEEE 802．11g仍然是市场主流,值得关注。但Dr．Ben却有不同的意见……     

IEEE802．11g为何成主流？——解析IEEE 802.11g

历史证明无止境的应用需求总能催生出让人振奋的新技术，在无线局域网领域，IEEE 802．119标准问世仅一年多就迅速普及．并博得广大用户的青睐。它凭借什么击败了IEEE 802．11a和IEEE 802．11b而登上主流无线标准的宝座？IEEE 802．11g在无线网络为人所诟病的安全性问题上有哪些改进？它高性能的秘诀是什么其自身又存在什么缺陷呢？这些值得我们深入地去探讨     

教你选购USB无线网卡的注意事项

随着IEEE802．11标准无线网络技术的成熟,产品性能的增强,以及产品价格的逐步下调。使得越来越多的企业、家庭用户开始部属基于IEEE802．11标准的无线网络。在用户终端方面,虽然英特尔迅驰笔记本电脑均已内置了无线网卡,并能和大多数主流的无线设备相兼容。但考虑到那些非迅驰平台的笔记本电脑以及台式机,选择一款合适的高性价比无线网卡。使终端产品能快速、     

延续IEEE802．11g的生命 240Mbps无线技术

现阶段，倘若你迫不及待地想体验高速无线网络带来的速度提升，那么240Mbps速率的IEEE802．11g增强型产品会是最好的选择，这项增强技术来自于Airgo的第三代True MIMO方案，它也是IEEE802．11n标准的一个有力竞逐者。[编者按]     

802．11b安全性评测与研究

构建无线网络系统的技术发展非常迅猛。目前存在着多种可用于创建无线网络的技术。其中,IEEE802．11b标准用途相当广泛,已经成为用于共享无线局域网（WLAN）技术的行业标准。本文集中研究IEEE802．11b标准的安全性问题及其解决措施。     

WLAN主要安全标准的演化与改进

无线网络因其连接方便又灵活已经非常普及了,但是无线网络的安全性一直是一个突出问题。IEEE和Wi—Fi先后出台了相应的标准和协议来保证无线网络的安全性,包括国际上通用的IEEE802．11标准,WPA,IEEE802．11i（WPA2）,并用密码技术在其演化过程中做了很多改进,提高了WLAN的安全性。     

无线网络中分布式协调功能的改进

文中提出了一种简单而有效的方法动态的估计无线网络中的活动节点数，并根据估计出的活动节点数调整MAC层的初始竞争窗口大小，从而降低冲突概率，提高吞吐率性能。而且，该方法没有增加任何控制开销。仿真结果表明，改进后的IEEE 802．11分布式协调功能机制的性能有明显改善。     

无线局域网在医疗中的应用

作为个人通信的一个重要组成部分，无线局域网已经掀起移动计算的新浪潮。无线技术在传榆速率、安全性方面不断改进，它不仅是有线网络的无线延伸，也可以作为LAN的无线替代。通过分析3种不同的IEEE802.11扩展标准的调制方式、工作频段等，突出说明了IEEE802.llg标准的优越性。根据医疗领域的特殊性，在基于IEEE802.11g基础上，初步构建了一个可扩展的无线医疗网络。     

Nonsaturation throughput enhancement of IEEE 802.11b distributed coordination function for heterogeneous traffic under noisy environment

In this paper, we propose a mechanism named modified backoff (MB) mechanism to decrease the channel idle time in IEEE 802.11 distributed coordination function (DCF). In the noisy channel, when signal-to-noise ratio (SNR) is low, applying this mechanism in DCF greatly improves the throughput and lowers the channel idle time. This paper presents an analytical model for the performance study of IEEE 802.11 MB-DCF for nonsaturated heterogeneous traffic in the presence of transmission errors. First, we introduce the MB-DCF and compare its performance to IEEE 802.11 DCF with binary exponential backoff (BEB). The IEEE 802.11 DCF with BEB mechanism suffers from more channel idle time under low SNR. The MB-DCF ensures high throughput and low packet delay by reducing the channel idle time under the low traffic in the network. However, to the best of the authors' knowledge, there are no previous works that enhance the performance of the DCF under imperfect wireless channel. We show through analysis that the proposed mechanism greatly outperforms the original IEEE 802.11 DCF in the imperfect channel condition. The effectiveness of physical and link layer parameters on throughput performance is explored. We also present a throughput investigation of the heterogeneous traffic for different radio conditions.     

一种IEEE 802.11接入机制的新退避算法

无线传感器网络采用类似开放式系统互联模型的协议栈,作为第一个完全针对无线传感网络设计的MAC协议,S-MAC采用了IEEE802.11DCF的接入机制。本文在分析了IEEE802.11协议的二进制指数退避算法(BEB),以及倍数增线性减退避算法(MILD)后,提出了一种基于网络性能指标的新退避算法。新算法将无线信道中的时隙利用率映射网络性能指标传输概率,对网络中连续两次发送的时隙利用率进行平均滤波处理,改变重传节点的发送优先级,来调整节点在竞争使用窗口的退避值,以达到有效减少网络的碰撞、提高无线信道的使用效率和网络吞吐率的目的。仿真结果表明,新退避算法能更准确地估计网络当前的竞争状态,有效地提高了网络吞吐率,获得较好的网络性能。     

IEEE802.11MAC层性能分析及仿真

无线网络通信作为新兴的通信技术已备受人们关注。无线局域网使用无线传输媒体,可以覆盖有线网络难于涉及的范围,成为传统有线网的必要补充。IEEE802.11b是当前无线局域网的主流标准,对其性能仿真和结果分析已做了大量工作,而对尚未普及的802.11a却没有太多的性能分析。论文利用文献眼1演算法得出802.11吞吐量理论极限值,并通过ns-2软件对802.11a进行仿真。     

Distributed channel assignment algorithms for 802.11n WLANs with heterogeneous clients

As the latest IEEE 802.11 standard, 802.11n applies several new technologies, such as multiple input multiple output (MIMO), channel bonding, and frame aggregation to greatly improve the rate, range and reliability of wireless local area networks (WLANs). In 802.11n WLANs, access points (APs) are often densely deployed to provide satisfactory coverage. Thus nearby APs should operate at non-overlapping channels to avoid mutual interference. It is challenging to assign channels in legacy 802.11a/b/g WLANs due to the limited number of channels. Channel assignment becomes more complex in 802.11n WLANs, as the channel bonding in 802.11n allows WLAN stations (APs and clients) to combine two adjacent, non-overlapping 20MHz channels together for transmission. On the other hand, IEEE 802.11n is backward compatible, such that 802.11n clients will coexist with legacy clients in 802.11n WLANs. Legacy clients may affect the performance of nearby 802.11n clients, and reduce the effectiveness of channel bonding. Based on these observations, in this paper, we study channel assignment in 802.11n WLANs with heterogeneous clients. We first present the network model, interference model, and throughput estimation model to estimate the throughput of each client. We then formulate the channel assignment problem into an optimization problem, with the objective of maximizing overall network throughput. Since the problem is NP-hard, we give a distributed channel assignment algorithm based on the throughput estimation model. We then present another channel assignment algorithm with lower complexity, and aim at minimizing interference experienced by high-rate, 802.11n clients. We have carried out extensive simulations to evaluate the proposed algorithms. Simulation results show that our algorithms can significantly improve the network throughput of 802.11n WLANs, compared with other channel assignment algorithms.     

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.     

Channel measurement-based access point selection in IEEE 802.11 WLANs

With the wide deployment of IEEE 802.11 Wireless Local Area Networks, it has become common for mobile nodes (MNs) to have multiple access points (APs) to associate with. With the Received Signal Strength Indicator (RSSI)-based AP selection algorithm, which is implemented in most commercial IEEE 802.11 clients, the AP with the best signal strength is selected regardless of the candidate AP’s available throughput, resulting in unbalanced distribution of clients among the APs in the network. Several studies have shown performance improvement in not just the new MN (nMN), but also the network as a whole when the selection process considers the current load status of candidate APs. However, the proposed algorithms in these studies assume that there are no hidden terminal problems that severely affect the performance of the network. Hidden terminal problems frequently occur in wireless networks with unlicensed frequencies, like IEEE 802.11 in the 2.4 GHz band. Moreover, none of the previous studies have considered frame aggregation, a major improvement in transmission efficiency introduced and widely deployed with the IEEE 802.11n standard. In this paper, we propose a new AP selection algorithm based on the estimation of available throughput calculated with a model based on the IEEE 802.11 distributed coordination function in consideration of hidden terminal problems and frame aggregation. The proposed algorithm is evaluated through extensive simulation, and the results show that the nMN with the proposed AP selection algorithm can achieve up to 55.84% and 22.31% higher throughput compared to the traditional RSSI-based approach and the selection algorithm solely based on the network load, respectively.     

IEEE802．11n聚合机制的研究与性能分析

分析了IEEE802．11n无线局域网提高网络吞吐量的瓶颈,介绍帧聚合机制的原理,其中着重对A-MSDU和A-MPDU两种聚会机制的性能进行仿真,仿真结果表明采用聚合机制降低了网络额外开销,与未采聚会机制相比在网络利用率上有较大地提高,同时降低了网络时延,有效地提高了网络性能。     

Mixed-mode simulation for IEEE 802.11-operated WLANs

In this paper, we address the issue of integrating packet-level simulation with fluid-model-based simulation for IEEE 802.11-operated wireless LANs (WLANs), so as to combine the performance gain of the latter with the accuracy and packet-level detail afforded by the former. In mixed-mode simulation, foreground flows operate in the packet mode, while the other background flows are approximated into a collection of fluid chunks and simulated in the fluid mode. As these two types of flows influence each other at the point of interaction, e.g. the wireless channel in a WLAN, we derive the model of interaction at the wireless medium. We then implement mixed-mode simulation in ns-2 as well as MATLAB, and conduct a comprehensive simulation study to evaluate its performance with respect to the capability of keeping track of network dynamics (in terms of the TCP behavior), accuracy (in terms of the error discrepancy in throughput), and efficiency (in terms of the speed-up in carrying out simulation).Simulation results indicate that, for IEEE 802.11-operated WLANs, mixed-mode simulation significantly expedites the simulation, and yet provides the same level of packet details and accuracy as packet-level simulation does for flows of interest. Specifically, the error discrepancy incurred in mixed-mode simulation is within 2% of the maximum channel bandwidth, and reduces the execution time, in the best case, by two orders of magnitude.