A cross layer rate adaptation solution for IEEE 802.11 networks

Optimizing the performance of wireless networks for emerging network applications is a highly challenging issue. Cross layer solutions offer promising possibilities of joint optimization across the protocol suite. This paper proposes a cross layer framework for rate adaptation in 802.11× networks. Previous approaches for rate adaptation mainly focused on the factors related to the wireless medium. However, the applications’ demands and timing constraints of the underlying protocols were not considered. The solution proposed in this paper tackles these shortcomings with a novel approach by performing rate adaptation according to the changing channel conditions, application preferences and timing constraints of the underlying protocols. The proposed rate adaptation is based on a loss differentiation mechanism which identifies the real cause of channel variations and takes an appropriate action thereafter. The design provides flexibility of tuning various parameters according to application type and offers minimal complexity. The simulation results assert considerable improvements over the previous solutions in terms of the algorithm response to application requirements and dynamic adjustment with variation in underlying protocol performance.     

IEEE 802.11无线局域网标准研究*

介绍了IEEE 802.11全系列标准,研究了IEEE 802.11系列各标准的发展轨迹和相互关系,建立了该系列标准的层次模型。研究并分析了IEEE 802.11、IEEE 802.11a、IEEE 802.11b、IEEE 802.11g和IEEE 802.11n这几种常见标准,并对相应物理层和媒质访问控制层的关键技术作了重点分析。     

支持多速率自适应的IEEE 802.11网络性能分析

针对基于自动速率回退(ARF)算法的多速率IEEE 802.11网络,给出了ARF算法分析的一种离散时间Markov链模型.模型的建立,考虑了节点在发送过程中,通过Basic与RTS/CTS模式的选择,对是否因为信道传输误码导致的失败进行的区分.利用算法模型,分析了节点在以随机概率选择不同发送模式的方式下,其采用各个传输速率的稳态概率分布,并在此基础上结合协议退避机制的研究,给出了多速率网络系统吞吐量性能的计算方法.通过大量的仿真实验,验证了模型分析方法具有很好的准确性,并讨论了算法以及MAC协议参数设置对网络性能的影响.     

On the rate adaptation for IEEE 802.11 wireless networks

Rate adaptation (RA) is a mechanism to choose transmission rate based on the dynamic channel quality in wireless networks. This paper studied the adaptation algorithm run solely at the sender-side in IEEE 802.11 networks. The key insight is the inference discrepancy in inferring the relative order of transmission rates with respect to the expected performance, which indicates that one cannot always reach the correct order based solely on the channel state information collected by the sender itself. The consequence is wrong rate decision and significant performance loss. Therefore, we present a new RA structure to mitigate such effect by using a novel component, rate testing. Further, by employing the active measurement, a lightweight and effective testing mechanism, SFB, short frame burst, is proposed to detect and filter out the unsuitable transmission rate. Finally, an active measurement-based rate adaptation mechanism (AMRA) is designed and implemented. The experimental results show that AMRA outperforms many other well-known RA solutions in most scenarios.     

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.     

A practical framework for 802.11 MIMO rate adaptation

The emergence of MIMO antennas and channel bonding in 802.11n wireless networks has resulted in a huge leap in capacity compared with legacy 802.11 systems. This leap, however, adds complexity to optimizing transmission. Not only does the appropriate data rate need to be selected, but also the MIMO transmission technique (e.g., Spatial Diversity or Spatial Multiplexing), the number of streams, and the channel width. Incorporating these features into a rate adaptation (RA) solution requires a new set of rules to accurately evaluate channel conditions and select the appropriate transmission setting with minimal overhead. To address these challenges, our contributions in this work are two-fold. First, we propose a practical link metric that accurately captures channel conditions in MIMO 802.11n environments, and we call this metric diffSNR. Using diffSNR captured from real testbed environments, we build performance models that accuractely predict link quality in 95.5% of test cases. Practicality and deployability are guaranteed with diffSNR as it can be measured on all off-the-shelf MIMO WiFi chipsets. Second, we propose ARAMIS (Agile Rate Adaptation for MIMO Systems), a standard-compliant, closed-loop RA solution that jointly adapts rate and bandwidth, and we utilize the diffSNR-based 802.11n performance models within ARAMIS’s framework. ARAMIS adapts transmission rates on a per-packet basis; we believe it is the first closed-loop, 802.11 RA algorithm that simultaneously adapts rate and channel width. We have implemented ARAMIS with diffSNR on Atheros-based devices and deployed it on our 15-node testbed. Our experiments show that ARAMIS accurately adapts to a wide variety of channel conditions with negligible overhead. Furthermore, ARAMIS outperforms existing RA algorithms in 802.11n environments with up to a 10-fold increase in throughput.     

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.     

多速率IEEE 802.11b网络控制包速率调整

针对IEEE802．11b无线自组网中控制帧使用的速率较低、降低网络性能的问题,提出1种速率调整策略,其中心思想是使用AARF（Adaptive Auto Rate Fallback）算法自动调整RTS（Request to Send）和ACK（ACKnowledge）与DATA包的速率,使其速率一致,但不调整CTS（Clear to Send）以减少控制帧占用信道的时间．仿真结果表明改进后的速率调整策略能使IEEE802．1lb无线自组网的吞吐量显著提高．     

802.11无线局域网标准与安全

介绍了无线局域网标准IEEE 802．11安全机制,描述了数据加密和用户身份认证机制与过程,以及802．11标准的安全问题。讨论了旨在提高802．11安全性的802．1x协议以及动态密钥管理方法,提出了使用无线局域网应采取的行之有效的安全措施。     

一种通用的无线网络AAA机制的性能评价模型

目前针对AAA机制的性能分析评价主要有两种手段,即定量的数学分析和仿真对比.提出了一种基于安全关联(security association,SA)路径长度的分析评价模型,并且基于该模型给出了AAA机制性能改进的方向.分析结果表明,该模型能够更简单有效地评价对比各种AAA机制的性能优劣.     

一种基于IEEE 802.11e的多跳无线网络拥塞控制协议

多跳无线网络中网络拥塞的出现将严重降低网络的性能。基于802.11e提供的区分业务类型的信道接入优先级机制,提出了一种对尽力而为(best-effort)类型业务动态调整其优先级进行网络拥塞控制的协议。该算法的主要思想是对发生网络拥塞的节点提升其业务流传输的优先级使其获得更多的传输机会以缓解拥塞状况,并对严重的网络拥塞状况采取反向施压的策略降低上游节点业务流的转发速率。仿真结果表明,该算法有效地提高了网络重负载情况下的吞吐量。     

Modelling and performance evaluation of the IEEE 802.11 DCF for real-time control

Popular wireless networks, such as IEEE 802.11/15/16, are not designed for real-time applications. Thus, supporting real-time quality of service (QoS) in wireless real-time control is challenging. This paper adopts the widely used IEEE 802.11, with the focus on its distributed coordination function (DCF), for soft-real-time control systems. The concept of the critical real-time traffic condition is introduced to characterize the marginal satisfaction of real-time requirements. Then, mathematical models are developed to describe the dynamics of DCF based real-time control networks with periodic traffic, a unique feature of control systems. Performance indices such as throughput and packet delay are evaluated using the developed models, particularly under the critical real-time traffic condition. Finally, the proposed modelling is applied to traffic rate control for cross-layer networked control system design.     

On the feasibility of IEEE 802.11 multi-channel multi-hop mesh networks

Several scientific works have considered the possibility to build Wireless Mesh Networks (WMN) using multi-channel IEEE 802.11 architectures. At the basis of these works is the notion of “non-overlapping” channels, i.e. with a frequency separation equal or greater than 25 MHz. It is now a common assumption that multiple independent transmissions over these channels can coexist without mutual interference even in physical proximity. In this work we demonstrate that this assumption does not hold in general. Through an extensive set of experiments we illustrate the presence of cross-channel interference between “non-overlapping” channels at relay nodes due to the “near-far” effect. We analyze in what manner the MAC layer reacts to such an interference and how this problem extends to higher layers, with detrimental effects on the global throughput. The central problem is that cross-channel interference is not handled adequately by the MAC layer, and in some cases single-channel multi-hop settings perform better than multi-channel. Our results highlight a serious mismatch between some routing and channel assignment schemes proposed recently by the research community, assuming full separation between non-overlapping channels, and what is achievable in practice. More generally, as the presence of cross-channel interference can not be neglected at relay nodes, our findings point to a fundamental problem in building Multi-channel Multi-hop WMN based on IEEE 802.11b/g technology.     

Dynamic adaptation of CSMA/CA MAC protocol for wide area wireless mesh networks

Though the popular IEEE 802.11 DCF is designed primarily for Wireless LAN (WLAN) environments, today it is being widely used for wide area wireless mesh networking. The protocol parameters of IEEE 802.11 such as timeout values, interframe spaces, and slot durations, sufficient for a general WLAN environment, need to be modified in order to efficiently operate in wide area wireless mesh networks. The current wide area wireless mesh network deployments use manual configuration of these parameters to the upper limit which essentially makes the networks operate at lower system efficiency. In this paper, we propose d802.11 (dynamic 802.11) which dynamically adapts the protocol parameters in order to operate at varying link distances. In fact, in 802.11, a transmitter can face ACK/CTS timeout even when it started receiving ACK/CTS packet before the timeout value. We present three strategies, (i) multiplicative timer backoff (MTB), (ii) additive timer backoff (ATB), and (iii) link RTT memoization (LRM), to adapt the ACK_TIMEOUT in d802.11 in order to provide better adaptation for varying link dimensions. Through extensive simulation experiments we observed significant performance improvement for the proposed strategies. We also theoretically modeled the maximum link throughput as a function of the link dimension for the proposed system. Our results show that the LRM technique provides the best adaptation compared to all other schemes.     

Rate Adaptive Transmission Scheme for IEEE 802.11 WLANs

In this paper, a novel rate adaptive transmission scheme named ＂EACK＂ for IEEE 802.11 WL,ANs is presented. which can react to channel variations quickly and has less overhead. Simulation shows EACK outperforms the existing schemes under various channel conditions.     

Throughput analysis of IEEE 802.11 multirate WLANs with collision aware rate adaptation algorithm

This paper presents a mathematical model that analyzes the throughput of the IEEE 802.11b distributed coordination function (DCF) with the collision aware rate adaptation (CARA) algorithm. IEEE 802.11 WLANs provide multiple transmission rates to improve system throughput by adapting the transmission rate to the current channel conditions. The system throughput is determined by some stations using low transmission rates due to bad channel conditions. CARA algorithm does not disturb the existing IEEE 802.11b formats and it can be easily incorporated into the commercial wireless local area networks (WLAN) devices. Finally, we verify our findings with simulation.     

支持区分服务的无线传感器网络802.11 MAC协议

针对无线传感器网络不能提供数据流优先级区分的问题,提出了一种基于IEEE802.11MAC协议的改进机制。该机制通过引入新的控制分组,使目的节点可以拒绝接收来自低优先级节点的数据,从而确保了高优先级数据业务的服务质量。数学分析表明,该机制能够使高优先级数据流占用更多的带宽,并且降低了高优先级数据流的分组丢弃概率。能耗分析表明,高优先级数据业务能耗低于低优先级数据业务,随着节点密度的增加,该机制节能效果逐渐明显,适合于节点密度高的无线传感器网络。     

Route Maintenance in IEEE 802.11 wireless mesh networks

In this paper, we propose a novel Route Maintenance scheme for IEEE 802.11 wireless mesh networks. Despite lack of mobility and energy constraints, reactive routing protocols such as AODV and DSR suffer from frequent route breakages in 802.11 based infrastructure wireless mesh networks. In these networks, if any intermediate node fails to successfully transmit a packet to the next hop node after a certain number of retransmissions, the link layer reports a transmission problem to the network layer. Reactive routing protocols systematically consider this as a link breakage (and therefore a route breakage). Transmission failures can be caused by a number of factors e.g. interference or noise and can be transient in nature. Frequent route breakages result in significant performance degradation. The proposed mechanism considers multiple factors to differentiate between links with transient transmission problems from those links which have permanent transmission problems and takes a coherent decision on link breakage. The proposed mechanism is implemented in AODV for single-radio single-channel mesh network and an extension is incorporated in multi-radio multi-channel scenarios. Simulation results show substantial performance improvement compared to classical AODV and local route repair schemes.