最有希望改变未来Wi-Fi的几项技术未来802.11家族发展探秘

历时7年的IEEE802．11n无线Wi—Fi标准终于获得了通过。和以往相比,802．11n能够提供300Mbps的数据传输率和接近100～150Mbps的吞吐量,让更多人体验到了从未有过的高速。但802．11n标准认证的坎坷历程也给人一种假象：这是否会是Wi—Fi扩张的暂时停滞呢？显然不是。相反,它的出现意味着一个创新浪潮的开始。在未来的几年内。     

802.11n专题

现在的802．11n产品能够提供300Mbps的连接速率,而实际的数据传输速率在70-100Mbps左右,这使它已经可以在很多环境中替代传统的有线局域网。我们所测试的802．11n无线设备目前都是草案型,在本次测试中,我们一共收到了4款最新的草案型802．11n无线宽带路由器,以及一款MIMO型产品。     

极速！超越100Mbps有线网络 NETGEAR WNR854T

长期以来，也许你已经习惯性地认为，无线局域网的传输速率始终不如100MbPs有线网络。的确，即使是当前流行的Super国G（108Mbps）、Afterburner（125Mbps）等增强型802.11g产品，在实际无线传输速率上仍然不及100Mbps有线网络。不过802．11n无线路由器的最高理论无线传输速率将会达到600Mbps．而在《微型计算机》2006年10月下刊关于Draft-802．11n无线路由器的报道中．BUFFAL0G300N的实际表现也令人赞赏．其实际无线传输速率已经达到了100Mbps有线网络的水平。现在让我们来看看另一款Draft-802．11n无线路由器——NETGEAR WNR854T．它是否能给我们带来更大的惊喜呢？     

挑战802．11n 240Mbps无线路由器赏析

最快的802．11g无线传输速率是多少？是54Mbps、108Mbps，还是125Mbps？都不列，目前最快802．11g无线传输速率是240Mbps！那么，这种新的高速无线路由器和无线网卡究竟是什么样的，它们的性能与以前的802.11g有多大的进步，它们与300Mbps的802．11n草案标准1.0相比又如何？看完本文你就会明白。     

技术优势实现高吞吐

虽然802．11n标准尚未最终确定,但相比起802．11alb／g标准,802．11n标准具有明显的技术优势,能够提供高达600Mbps的传输速率,并能够减少噪音信号的干扰。作为下一代无线通迅标准,802．11n标准得到了业界众多重量级厂商的全力支持,下面将详细介绍802．11n标准和技术的优势及发展趋势。     

802.11n2．0草案标准无线路由器——性能与兼容性俱佳

众所周知,现在主流的无线网络标准是802．11g,其实际传输速率仅为22Mbps左右,而1080p高清电影的码率普遍在30-50Mbps．根本无法通过802．11g无线网络流畅地在线播放1080p高清电影。另外对于企业用户来说也越来越需要具有更高传输速率和更大覆盖范围的无线网络设备,2006年市场上就已经出现采用802．11n1．0草案标准的无线路由器,     

1920×1200震撼屏幕——惠普高性能移动工作站上市

日前，TP-Link在北京发布了全系列共11款IEEE 802．11n无线产品，无线传输速率最高可达300Mbps。其中9系列IEEE 802．11n无线产品配备了完整的3x3MIMO架构和先进的SST技术，通过发送冗余数据使错误帧大大减少，从而提高数据传输稳定性，减少掉线现象发生。     

腾达W311R无线路由器

在802．11n无线路由器中．150Mbps产品的优势在于价格便宜,相比802．11g的54Mbps传输速率来说,它的性能优势很明显。但是由于该产品大都采用单天线设计,因此信号覆盖范围没有多天线产品强。     

BUFFALO300M无线ADSL2＋路由器上市

近日．BUFFALO最新推出了无线宽带路由器WBMR—G300N,完全可以解决您的烦恼。这是国内市场第一款支持300M双天线的产品,它集无线路由器和ADSL2＋猫于一身,使用802．11n高速无线网络技术,最高可达270Mbps无线数据吞吐量,支持高速ADSL2＋连接,是一款高性价比无线宽带路由器。     

BUFFALO 300M无线ADSL2＋路由器上市

近日,BUFFALO最新推出了无线宽带路由器WBMR—G300N,完全可以解决您的烦恼。这是国内市场第一款支持300M双天线的产品,它集无线路由器和ADSL2＋猫于一身,使用802．11n高速无线网络技术,最高可达270Mbps无线数据吞吐量。支持高速ADSL2＋连接。是一款高性价比无线宽带路由器。     

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.     

Evaluating transport protocol performance over a wireless mesh backbone

IEEE 802.11n wireless physical layer technology increases the deployment of high throughput wireless indoor mesh backbones for ubiquitous Internet connectivity at the urban and metropolitan areas. Most of the network traffic flows in today’s Internet use ‘Transmission Control Protocol’ (TCP) as the transport layer protocol. There has been extensive works that deal with TCP issues over wireless mesh networks as well as noisy wireless channels. Further, IEEE 802.11n is well known for its susceptibility to increased channel losses during high data rate communication. This paper investigates the dynamics of an end-to-end transport layer protocol like TCP in the presence of burst and correlated losses during IEEE 802.11n high data rate communication, while maintaining fairness among all the end-to-end flows. For this purpose, we evaluate four TCP variants-Loss Tolerant TCP (LT-TCP), Network Coded TCP (TCP/NC), TCP-Horizon and Wireless Control Protocol (WCP), where the first two protocols are known to perform very well in extreme lossy networks, and the last two are specifically designed for mesh networks. Our evaluation shows that WCP performs better in a IEEE 802.11n supported mesh networks compared to other three variants. However, WCP also results in negative impact at high data rates, where end-to-end goodput drops with the increase in physical data rate. The analysis of the results reveals that explicit loss notifications and flow balancing are not sufficient to improve transport protocol performance in an IEEE 802.11n supported mesh backbone, rather a specific mechanism is required to synchronize the transport queue management with lower layer scheduling that depends on IEEE 802.11n features, like channel bonding and frame aggregation. The findings of this paper give the direction to design a new transport protocol that can utilize the full capacity of IEEE 802.11n mesh backbone.     

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.     

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.     

基于竞争窗口递减因子自适应调整退避算法

包括IEEE802.11、802.15.4标准在内的许多无线网络协议都采用二进制指数退避机制管理数据的重发。在动态分布式的网络环境中,二进制指数退避算法固定的竞争窗口递减方式难以适应动态变化的网络规模。针对这一问题,提出了一种改进的回退机制,该机制通过引入竞争窗口递减因子,自适应地调整无线节点的等待时间,以实现网络吞吐量的最大化。同时,在算法实现上提出一种启发式算法以跟踪网络中竞争节点数量的改变。在IEEE 802.11DCF协议中以相同的物理层参数进行仿真,结果表明改进算法提高了网络吞吐量,降低了分组平均接入时延。     

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.     

SRA-MSDU: Enhanced A-MSDU frame aggregation with selective retransmission in 802.11n wireless networks

The main goal of the IEEE 802.11n standard is to achieve more than 100 Mbps of throughput at the MAC service access point. This high throughput has been achieved via many enhancements in both the physical and MAC layers. One of the MAC enhancements is the frame aggregation in which multiple frames are concatenated into a single large frame before being transmitted. The 802.11n MAC layer defines two types of aggregation, aggregate MAC service data unit (A-MSDU) and aggregate MAC protocol data unit (A-MPDU). The A-MPDU outperforms A-MSDU due to its large aggregation size and the subframes retransmission in erroneous channels. However, in error free channels and under the same aggregation size the A-MSDU performs better than the A-MPDU due to its smaller headers. Thus, adding a selective retransmission capability to the A-MSDU would improve the system performance. In this paper, we have proposed an MSDU frame aggregation scheme that enables selective retransmission at the MSDU level without altering the original MAC header. In this proposed scheme an implicit sequence control mechanism has been introduced in order to keep the frames in sequence and preserve their correct order at the receiver side. The results show that the proposed scheme improves the system performance in terms of throughput and delay even under highly erroneous channels.     

The status and future of 802.11-based WLANs

The main attraction of WLANs is their flexibility. They can extend access to local area networks, such as corporate intranets, as well as support broadband access to the Internet - particularly at "hot spots," public venues where people tend to gather. WLANs can provide quick, easy wireless connectivity to computers, machinery, or systems in a local environment where a fixed communications infrastructure does not exist or where such access is not permitted. These hosts can be stationary, handheld, or even mounted on a moving vehicle. Bandwidth considerations have thus far been secondary in WLAN design and implementation: the original 802.11 standard allowed a maximum channel bit rate of only 2 megabits per second, while the current 802.11 b standard supports an 11 Mbps maximum rate. However, the widespread deployment of 802.11a and 802.11g standards, which allow a bit rate of up to 54 Mbps, will pave the way for new types of mobile applications, including m-commerce transactions and location-based services.     

IEEE 802.11n中速率、模式及信道的联合自适应算法

针对IEEE 802.11n无线网络中的速率、MIMO模式与信道宽度的联合自适应问题,提出了一种基于非静态Multi-Armed Bandit学习方法的联合自适应算法,并设计了一种新颖的报酬函数.为解决该算法收敛时间较慢的问题,基于分类回归树设计了MCS、MIMO模式以及信道宽度预测算法,其能够有效利用无线网卡驱动程序采集的相关统计数据预测不同MCS、MIMO模式以及信道宽度组合的报酬函数,大幅度缩小联合自适应算法的搜索空间.该算法具有易实现、近似最优及计算复杂度低的特点.真实实验结果表明:在无干扰和不同干扰环境下,联合自适应算法都能够有效地提高UDP吞吐量.     

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。