Mobility impact in IEEE 802.11p infrastructureless vehicular networks

Vehicular ad hoc networks (VANETs) are an extreme case of mobile ad hoc networks (MANETs). High speed and frequent network topology changes are the main characteristics of vehicular networks. These characteristics lead to special issues and challenges in the network design, especially at the medium access control (MAC) layer. In this paper, we provide a comprehensive evaluation of mobility impact on the IEEE 802.11p MAC performance. The study evaluates basic performance metrics such as packet delivery ratio, throughput, and delay. An unfairness problem due to the relative speed is identified for both broadcast and unicast scenarios. We propose two dynamic contention window mechanisms to alleviate network performance degradation due to high mobility. The first scheme provides dynamic level of service priority via adaptation to the number of neighboring nodes, while the second scheme provides service priority based on node relative speed. Extensive simulation results demonstrate a significant impact of mobility on the IEEE 802.11p MAC performance, the unfairness problem in the vehicle-to-vehicle (V2V) communications, and the effectiveness of the proposed MAC schemes.     

Interference Analysis and Transmit Power Control in IEEE 802.11a/h Wireless LANs

Reducing the energy consumption by wireless communication devices is perhaps the most important issue in the widely deployed and dramatically growing IEEE 802.11 WLANs (wireless local area networks). TPC (transmit power control) has been recognized as one of the effective ways to achieve this goal. In this paper, we study the emerging 802.11a/h systems that provide a structured means to support intelligent TPC. Based on a rigorous analysis of the relationship among different radio ranges and TPC's effects on the interference, we present an optimal low-energy transmission strategy, called MiSer, which is deployed in the format of RTS-CTS(strong)-Data(MiSer)-Ack. The key idea of MiSer is to combine TPC with PHY (physical layer) rate adaptation and compute offline an optimal rate-power combination table, then at runtime, a wireless station determines the most energy-efficient transmission strategy for each data frame transmission by a simple table lookup. Simulation results show MiSer's clear superiority to other two-way or four-way frame exchange mechanisms in terms of energy conservation.     

Interference Evaluation of Bluetooth and IEEE 802.11b Systems

The emergence of several radio technologies, such as Bluetooth and IEEE 802.11, operating in the 2.4 GHz unlicensed ISM frequency band, may lead to signal interference and result in significant performance degradation when devices are colocated in the same environment. The main goal of this paper is to evaluate the effect of mutual interference on the performance of Bluetooth and IEEE 802.11b systems. We develop a simulation framework for modeling interference based on detailed MAC and PHY models. First, we use a simple simulation scenario to highlight the effects of parameters, such as transmission power, offered load, and traffic type. We then turn to more complex scenarios involving multiple Bluetooth piconets and WLAN devices.     

Throughput Analysis and Admission Control for IEEE 802.11a

We propose a new Markov model for the distributed coordination function (DCF) of IEEE 802.11. The model incorporates carrier sense, non-saturated traffic and SNR, for both basic and RTS/CTS access mechanisms. Analysis of the model shows that the throughput first increases, and then decreases with the number of active stations, suggesting the need for an admission control mechanism.We introduce such a mechanism, which tries to maximize the throughput while maintaining a fair allocation. The maximum achievable throughput is tracked by the mechanism as the number of active stations increases. An extensive performance analysis shows that the mechanism provides significant improvements.Mustafa Ergen received the B.S. degree in electrical engineering from Middle East Technical University (METU) and was the METU Valedictorian in 2000. He received the M.S. and Ph.D. degrees in electrical engineering in 2002 and 2004, the MOT certificate of HAAS Business School in 2003, and the M.A. degree in International and Area Studies in 2004 from the University of California, Berkeley.Dr. Ergen has been conducting research in wireless communication networks with an emphasis on sensor networks, wireless LAN and OFDM systems and is the author of many works in the field, including the book (with A.R.S. Bahai and B.R. Saltzberg) Multi-Carrier Digital Communications: Theory and Applications of OFDM (New York: Springer, 2004).He is National Semiconductor Post Doctoral Fellow and was awarded eight times Bulent Kerim Altay Award by department of electrical engineering in METU and received Best Student Paper Award in IEEE ISCC 2003 and has an invited paper in IEEE GLOBECOM CAMAD 200.Pravin Varaiya is Nortel Networks Distinguished Professor in the Department of Electrical Engineering and Computer Sciences at the University of California, Berkeley. From 1975 to 1992, he was also Professor of Economics at Berkeley. His research is concerned with communication networks, transportation, and hybrid systems. He has taught at MIT and the Federal University of Rio de Janeiro, Varaiya has held a Guggenheim Fellowship and a Miller Research Professorship. He received an Honorary Doctorate from L’Institut National Polytechnique de Toulouse, and the Field Medal of the IEEE Control Systems Society. He is a Fellow of IEEE and a member of the National Academy of Engineering. He is on the editorial board of several journals, including “Discrete Event Dynamical Systems” and “Transportation Research-C.” He has co-authored three books and more than 250 technical papers. The second edition of “High-Performance Communication Networks” (with Jean Walrand) was published by Morgan-Kaufmann in 2000. “Structure and interpretation of signals and systems” (with Edward Lee) was published in 2002 by Addison-Wesley.     

Model-Based QoS Parameter Control for IEEE 802.11e EDCA

In this paper, we give an analytical justification how to control the QoS parameters using a throughput approximation model for IEEE 802.11e EDCA. Using the model, we propose a new parameter control algorithm which successfully achieves the dynamic parameter assignment under desired throughput ratio constraints.     

A Control Theoretic Approach for Throughput Optimization in IEEE 802.11e EDCA WLANs

The MAC layer of the 802.11 standard, based on the CSMA/CA mechanism, specifies a set of parameters to control the aggressiveness of stations when trying to access the channel. However, these parameters are statically set independently of the conditions of the WLAN (e.g. the number of contending stations), leading to poor performance for most scenarios. To overcome this limitation previous work proposes to adapt the value of one of those parameters, namely the CW, based on an estimation of the conditions of the WLAN. However, these approaches suffer from two major drawbacks: i) they require extending the capabilities of standard devices or ii) are based on heuristics. In this paper we propose a control theoretic approach to adapt the CW to the conditions of the WLAN, based on an analytical model of its operation, that is fully compliant with the 802.11e standard. We use a Proportional Integrator controller in order to drive the WLAN to its optimal point of operation and perform a theoretic analysis to determine its configuration. We show by means of an exhaustive performance evaluation that our algorithm maximizes the total throughput of the WLAN and substantially outperforms previous standard-compliant proposals.     

IEEE 802.11访问控制与MAC地址欺骗

对无线网络的访问控制机制进行了讨论，分析了目前所采用的IEEE 802．1lb无线网络设备在访问控制方面的不完善性，以及MAC地址访问控制的漏洞。采用序列号分析法对MAC地址欺骗进行检测，从而对无线网络的非授权访问进行监控。     

IEEE 802.11无线局域网标准发展历程及其发展方向

本详细介绍了IEEE802．11标准组成，标准发展历程和发展方向。     

Statistical characterization of the service time in saturated IEEE 802.11 networks

In this paper, we propose an analytical model for the service time in saturated IEEE 802.11 DCF networks. We derive a closed-form probability generating function for the packet service time of a cluster of IEEE 802.11 terminals, both for the RTC/CTS and the basic access mode. The probability generating function is inverted by numerical methods, providing the probability distribution function of the service time. Interestingly, it catches certain features of the DCF service time, which cannot be revealed with the second order analysis known in the literature. Finally, this analytical model is validated by means of extensive simulation outcomes.     

Coexistence Performance of IEEE 802.15.4 Wireless Sensor Networks Under IEEE 802.11b/g Interference

IEEE 802.15.4 Wireless Sensor Networks (WSNs) and IEEE 802.11b/g Wireless Local Area Networks (WLANs) are often collocated, causing a coexistence issue since these networks share the same 2.4GHz Industrial, Scientific, and Medical band. In our previous work, we built a coexistence model of IEEE 802.15.4 WSNs and IEEE 802.11b/g WLANs. By identifying three distinct coexistence regions, the model explained the coexistence behavior of IEEE 802.15.4 WSNs and IEEE 802.11b/g WLANs, and the model was experimentally validated. In this paper, we improve the model by introducing two important implementation factors: the transceiver’s Rx-to-Tx turnaround time and the Clear Channel Assessment partial detection effect. The enhanced model significantly improves the accuracy on explaining and predicting the coexistence performance of IEEE 802.15.4 WSNs in the real-life environment. Furthermore, under the guidance of the model, the coexistence performance of IEEE 802.15.4 WSNs is extensively investigated in various coexistence scenarios by analysis, simulation and experiments, respectively. The simulation and experimental results agree with our analysis. The coexistence model is believed to be helpful in resolving the coexistence issue.     

A simple and effective priority scheme for IEEE 802.11

In this letter, we propose an analytical model for a simple priority scheme for real-time applications in IEEE 802.11 by differentiating the initial window size, the window-increasing factor and the maximum backoff stage. Saturation throughputs and saturation delays of different priority classes are derived analytically.     

IEEE802.11无线网络中一种新的切换技术

作为当前无线局域网的主流标准,IEEE802.11网络提供了种类繁多的服务.然而,由于无线网络的移动性本质,切换技术成为IEEES02.11网络的关键技术.对IEEE802.11无线网络中现有的切换技术进行了研究,分析了可行的改进方向,并实现了一种改进的切换方案.     

A Cross-Layer Multi-Hop Simulator for IEEE 802.11e

In this work we simulate the ad hoc mode of IEEE 802.11e for routing optimisation. We simulate the behaviour of routing algorithms at the network layer by using a custom-made cross-layer network simulator developed by our team, which simultaneously considers the physical and Medium Access Control (MAC) layers. Although the simulator also supports the infrastructure mode, in this paper we focus on the ad hoc feature which was introduced by the authors. We opted for the simulator approach over the theoretical analysis, but we also present a mathematical model for IEEE 802.11 ad hoc networks. Some initial tests were performed by using a simple routing algorithm (to evaluate the behaviour of the system in terms of selection of the path between a source and a destination, and the correctness of the calculated metrics, which include end-to-end delay, packets lost, packets delivered), but more advanced cross-layer design solutions were also tested. When information from the physical and MAC layers is used as an input to the routing algorithm, improvements are achieved in the performance of the network. Several functions were compared and the algorithm that privileges shorter links accounting with the metric “collision rate” achieves the best results. When compared with a standard routing solution, this cross-layer approach allows to increase the number of packets delivered, while not significantly affecting the end-to-end delay of the packets.     

A delay model for IEEE 802.11e EDCA

In this paper, we present a model to analyze the delay behavior of the enhanced distributed channel access (EDCA) mechanism of IEEE 802.11e. Simulation results validate the accuracy of the analysis.     

A Multichain Backoff Mechanism for IEEE 802.11 WLANs

The distributed coordination function (DCF) of IEEE 802.11 standard adopts the binary exponential backoff (BEB) for collision avoidance. In DCF, the contention window is reset to an initial value, i.e., CWmin, after each successful transmission. Much research has shown that this dramatic change of window size may degrade the network performance. Therefore, backoff algorithms, such as gentle DCF (GDCF), multiplicative increase–linear decrease (MILD), exponential increase–exponential decrease (EIED), etc., have been proposed that try to keep the memory of congestion level by not resetting the contention window after each successful transmission. This paper proposes a multichain backoff (MCB) algorithm, which allows stations to adapt to different congestion levels by using more than one backoff chain together with collision events caused by stations themselves as well as other stations as indications for choosing the next backoff chain. The performance of MCB is analyzed and compared with those of 802.11 DCF, GDCF, MILD, and EIED backoff algorithms. Simulation results show that, with multiple backoff chains and collision events as reference for chain transition, MCB can offer a higher throughput while still maintaining fair channel access than the existing backoff algorithms.     

一种基于IEEE 802.11的多速率自适应MAC协议

提出了一种新颖的基于连续ACK帧统计信息的IEEE 802.11多速率自适应MAC协议EARF(EnhancedARF),其主要思想是:每一个速率有各自的成功阈值——速率升高的门限值,并且该值根据信道状况(用延时因子量化)动态地变化。协议不需对现有的IEEE 802.11标准做任何修改,因此易于通过编写驱动程序实现。仿真表明在大多数信道条件下,该协议性能较现有的基于ACK帧统计的速率自适应协议如ARF,ARF3-10都有较大的提高。     

Distributed Contention Window Control for Selfish Users in IEEE 802.11 Wireless LANs

In this paper, we study non-cooperative user behavior in random-access wireless networks in which users have freedom to choose their back-off contention window size according to network's congestion status. We formulate a non- cooperative game and show the existence and uniqueness of its equilibrium point. We also propose an iterative method leading to the equilibrium point of the game. A discussion of alternative game formulations in the same problem context is also given.     

A Game Theoretic Framework of Distributed Power and Rate Control in IEEE 802.11 WLANs

We present a game-theoretic study on the power and rate control problem in IEEE 802.11 WLANs where network participants choose appropriate transmission power and data rate to achieve maximum throughput with minimum energy consumption. In such game-theoretic study, the central issues are the existence, uniqueness of the Nash equilibrium (NE), the convergence to the NE and the system performance at the NE. We conduct our study for three specific games: the fixed-rate power control game G/sub NPC/, the fixed-power rate control game G/sub NRC/and the joint power rate control game G/sub NJPRC/. As main contributions, we establish the existence, uniqueness and convergence of the NE for the three games. In G/sub NRC/where the NE is inefficient, we provide pricing scheme to improve the efficiency. Based on our analysis on G/sub NJPRC/, we propose the joint power and rate control procedure to approach the NE which is proven to be social optimal. The procedure is distributed and simple to incorporate into the existing IEEE 802.11 MAC protocol.