The Jamming problem in IEEE 802.11‐based mobile ad hoc networks with hidden terminals: Performance analysis and enhancement

The hidden‐terminal problem significantly degrades the performance of IEEE 802.11 DCF. Many previous works have investigated its influence on the throughput of CSMA‐based medium access control (MAC) protocols, especially IEEE 802.11 DCF. In this paper, we introduce a new Jamming problem for IEEE 802.11‐based mobile ad hoc networks, which is caused by hidden terminals. An analytical model is established for this problem. Based on this model, an adaptive DCF (ADCF), is designed to solve the jamming problem through adaptively adjusting the minimum contention window of hidden terminals. Simulation results effectively demonstrate that the proposed A‐DCF can avoid the jamming and in turn greatly improve channel utilization and throughput. Copyright © 2009 John Wiley & Sons, Ltd.     

Adaptive backoff scheme for ad hoc networks based on IEEE 802.11

The performance of backoff scheme plays an important role in designing efficient Medium Access Protocols for ad hoc networks. In this paper, we propose an adaptive backoff scheme and evaluate the performance of the proposed scheme for ad hoc networks. The backoff mechanism devised by us grants a node access to the channel based on its probability of collision for a transmitted frame in comparison to the nodes in the two‐hop contention area. We use both an analytical model and simulation experiments to evaluate the performance of our adaptive backoff mechanism in an ad hoc network. The results show that our protocol exhibits a significant improvement in power saving, end‐to‐end goodput, packet delivery ratio, and hop‐put, compared with the existing IEEE 802.11 DCF. Copyright © 2010 John Wiley & Sons, Ltd.     

Saturation throughput analysis of multi‐rate IEEE 802.11 wireless networks

IEEE 802.11 protocol supports adaptive rate mechanism, which selects the transmission rate according to the condition of the wireless channel, to enhance the system performance. Thus, research of multi‐rate IEEE 802.11 medium access control (MAC) performance has become one of the hot research topics. In this paper, we study the performance of multi‐rate IEEE 802.11 MAC over a Gaussian channel. An accurate analytical model is presented to compute the system saturation throughput. We validate our model in both single‐rate and multi‐rate networks through various simulations. The results show that our model is accurate and channel error has a significant impact on system performance. In addition, our numerical results show that the performance of single‐rate IEEE 802.11 DCF with basic access method is better than that with RTS/CTS mechanism in a high‐rate and high‐load network and vice versa. In a multi‐rate network, the performance of IEEE 802.11 DCF with RTS/CTS mechanism is better than that with basic access method in a congested and error‐prone wireless environment. Copyright © 2008 John Wiley & Sons, Ltd.     

Analysis of a robust and energy efficient transmission scheduling protocol in single‐hop ad hoc networks

A fully connected one‐hop ad hoc network constitutes a basic unit for managing self‐organizing networks such as IEEE 802.11 and 802.15.3 networks. Since energy efficiency is a critical issue in ad hoc networks, we develop an energy‐saving framework that includes scheduling for node‐to‐node direct communication. The scheduling is performed by a coordinator that is selected by some simple rule. We enhance IEEE 802.11 protocol by using our proposed framework, and analyze its energy efficiency in transmitting and receiving data. Through mathematical analysis, we confirm that our enhanced protocol significantly saves energy compared to the IEEE 802.11 protocol. We also investigate the robustness of our algorithm by covering the cases of uncooperative users, system malfunctioning, and channel errors. The numerical results confirm that our protocol works well under these hostile environments and maintains its advantage over the conventional scheme. Copyright © 2009 John Wiley & Sons, Ltd.     

Increasing fairness and efficiency using the MadMac protocol in ad hoc networks

The IEEE 802.11 MAC layer is known for its unfairness behavior in ad hoc networks. Introducing fairness in the 802.11 MAC protocol may lead to a global throughput decrease. It is still a real challenge to design a fair MAC protocol for ad hoc networks that is distributed, topology independent, that relies on no explicit information exchanges and that is efficient, i.e. that achieves a good aggregate throughput. The MadMac protocol deals with fairness and throughput by maximizing aggregate throughput when unfairness is solved. Fairness provided by MadMac is only based on information provided by the 802.11 MAC layer. MadMac has been tested in many configurations that are known to be unfair and compared with three protocols (IEEE 802.11 and two fair MAC protocols). In these configurations, MadMac provides a good aggregate throughput while solving the fairness issues.     

Ad Hoc中一种改进后退机制的基于多包接收的MAC算法

物理层多包接收技术的发展给利用物理层多包接收能力的媒体接入控制（MAC）协议的设计带来了挑战。IEEE802.11 DCF是目前WIAN最成熟的分布式MAC协议之一，对其在多包接收模型下进行性能改善将有很大的应用价值。在物理层具有多包接收能力的基础上，提出了一种改进的802.11 DCF协议，并将该协议应用于现有的基于802.11 DCF的多包接收MAC算法（MDCF），理论分析和NS-2仿真实验结果表明，该算法与IEEE 802.11 DCF和MDCF相比，在网络吞吐量和时延性能方面有很大的改善。     

E‐MAC: an elastic MAC layer for IEEE 802.11 networks

We present a system for real‐time traffic support in infrastructure and ad hoc IEEE 802.11 networks. The proposed elastic MAC (E‐MAC) protocol provides a distributed transmission schedule for stations with real‐time traffic requirements, while allowing a seamless coexistence with standard IEEE 802.11 clients, protecting best‐effort 802.11 traffic from starvation by means of admission control policies. Our scheduling decisions are based on an ‘elastic’ transmission opportunity (TXOP) assignment which allows for efficient wireless resource usage: whenever a real‐time station does not use the assigned TXOP, the other real‐time stations can take over the unused access opportunity, thus preventing the well‐known inefficiencies of static time division multiple access (TDMA) schemes. Unlike other TDMA‐based solutions for 802.11, E‐MAC does not require a tight synchronization among the participating clients, thus allowing its implementation on commodity WLAN hardware via minor software changes at the client side, and no changes at the access points (APs). We studied the performance of our mechanism via ns‐2 simulations and a mathematical model, showing that it outperforms IEEE 802.11e in terms of throughput, delay, and jitter. We finally provide a proof of concept through the results obtained in a real testbed where we implemented the E‐MAC protocol. Copyright © 2011 John Wiley & Sons, Ltd.     

MARS: a multiple access scheme with sender driven and reception first for smart antenna in ad hoc networks

The deployment of directional antennas offers many advantages, such as transmission range extension, co‐channel interference reducing, the increasing of spatial reuse degree, throughput improving of networks, and transmission power saving. Hence, many ad hoc MAC protocols with directional antennas have been proposed. However, these protocols do not provide significant improvement of network performance due to the lack of supporting multiple transmissions and receptions simultaneously. With the adaptive beam‐forming system (Smart Antenna), a newly designed MAC protocol (MARS) that enables nodes with multiple transmissions and receptions is proposed. Simulation results show that our MARS do exploit the advantage of space division multiple access. In terms of total number of data forward and complete sessions in bottleneck nodes, MARS achieves three times better than Novel and nine times better than IEEE 802.11. In addition, the end‐to‐end delay keeps very short. Copyright © 2008 John Wiley & Sons, Ltd.     

Performance Analysis of IEEE 802.11 DCF in Imperfect Channels

IEEE 802.11 is the most important standard for wireless local area networks (WLANs). In IEEE 802.11, the fundamental medium access control (MAC) scheme is the distributed coordination function (DCF). To understand the performance of WLANs, it is important to analyze IEEE 802.11 DCF. Recently, several analytical models have been proposed to evaluate the performance of DCF under different incoming traffic conditions. However, to the best of the authors' knowledge, there is no accurate model that takes into account both the incoming traffic loads and the effect of imperfect wireless channels, in which unsuccessful packet delivery may occur due to bit transmission errors. In this paper, the authors address this issue and provide an analytical model to evaluate the performance of DCF in imperfect wireless channels. The authors consider the impact of different factors together, including the binary exponential backoff mechanism in DCF, various incoming traffic loads, distribution of incoming packet size, queueing system at the MAC layer, and the imperfect wireless channels, which has never been done before. Extensive simulation and analysis results show that the proposed analytical model can accurately predict the delay and throughput performance of IEEE 802.11 DCF under different channel and traffic conditions.     

A hybrid multi-channel MAC protocol for wireless ad hoc networks

In a regular wireless ad hoc network, the Medium Access Control (MAC) protocol coordinates channel access among nodes, and the throughput of the network is limited by the bandwidth of a single channel. The multi-channel MAC protocols can exploit multiple channels to achieve high network throughput by enabling more concurrent transmissions. In this paper, we propose a hybrid and adaptive protocol, called H-MMAC, which utilizes multi-channel resources more efficiently than other multi-channel MAC protocols. The main idea is to adopt the IEEE 802.11 Power Saving Mechanism and to allow nodes to transmit data packets while other nodes try to negotiate the data channel during the Ad hoc Traffic Indication Message window based on the network traffic load. The analytical and simulation results show that the proposed H-MMAC protocol improves the network performance significantly in terms of the aggregate throughput, average delay, fairness and energy efficiency.     

Design and performance of an enhanced IEEE 802.11 MAC protocol for multihop coverage extension

Ad hoc communication is gaining popularity, not only for pure ad hoc communication networks but also as a viable solution for coverage extension in wireless networks. Especially for upcoming WLAN hotspots, this is an interesting option to decrease installation costs. In this article we introduce a new MAC protocol that needs only marginal changes to the standard and enables efficient multihop networking. We advocate the use of multiple IEEE 802.11 channels, where one channel is reserved as a common signalling channel for the task of assigning the others (data channels) among wireless terminals. The proposed MAC protocols are based on a four-way handshake over the common signalling channel, while data transmission occurs on a dedicated channel. We propose a further optimization applying multiple wireless network interface cards. This improvement in performance comes at the price of a slightly more complex hardware. Two different simulation models are implemented to investigate our approach. The first model investigates the MAC protocol and its improvements, while the second model analyzes the multihop performance in terms of delivery ratio and transmission delay. BY means of numerous simulations we present the performance of our MAC approach in comparison with two standard approaches in terms of bandwidth, packet delivery, and transmission delay. For our performance evaluation we apply the IEEE 802.11a technology, but we note that our approach can also be used for IEEE 802.11b.     

IEEE 802.11在无线自组网中的应用研究

深入分析了IEEE 802.11 DCF机制应用于无线自组网存在的固有缺陷及潜在原因,并阐明了数据流竞争、物理层机制对MAC协议性能的影响。IEEE 802.11 DCF是针对全连通adhoc网络结构设计的,分析表明,要在实际的多跳无线自组网中应用还存在很多问题需要解决。在此基础上提出了将IEEE 802.11有效应用于多跳无线自组网的进一步研究方向。     

A Distributed Mechanism for Power Saving in IEEE 802.11 Wireless LANs

The finite battery power of mobile computers represents one of the greatest limitations to the utility of portable computers. Furthermore, portable computers often need to perform power consuming activities, such as transmitting and receiving data by means of a random-access, wireless channel. The amount of power consumed to transfer the data on the wireless channel is negatively affected by the channel congestion level, and significantly depends on the MAC protocol adopted. This paper illustrates the design and the performance evaluation of a new mechanism that, by controlling the accesses to the shared transmission channel of a wireless LAN, leads each station to an optimal Power Consumption level. Specifically, we considered the Standard IEEE 802.11 Distributed Coordination Function (DCF) access scheme for WLANs. For this protocol we analytically derived the optimal average Power Consumption levels required for a frame transmission. By exploiting these analytical results, we define a Power Save, Distributed Contention Control (PS-DCC) mechanism that can be adopted to enhance the performance of the Standard IEEE 802.11 DCF protocol from a power saving standpoint. The performance of an IEEE 802.11 network enhanced with the PS-DCC mechanism has been investigated by simulation. Results show that the enhanced protocol closely approximates the optimal power consumption level, and provides a channel utilization close to the theoretical upper bound for the IEEE 802.11 protocol capacity. In addition, even in low load situations, the enhanced protocol does not introduce additional overheads with respect to the standard protocol.     

IEEE 802.11移动自组织网络节点竞争窗口长度的概率分布

 在基于IEEE 802.11的移动自组织网络中,MAC(Medium Access Control)层提供了DCF(Distributed Coordinate Function)以控制节点对无线信道的争用.DCF包括了BEB (Binary Exponential Backoff)算法.该文对BEB的重要参数——竞争窗口CW(Contention Window)进行研究,通过随机建模,导出了竞争窗口长度的概率分布,并进行数值分析.研究结果可应用于IEEE 802.11移动自组织网络.     

Modeling of Collision Avoidance Protocols in Single-Channel Multihop Wireless Networks

Although there has been considerable work on the performance evaluation of collision avoidance schemes, most analytical work is confined to single-hop ad hoc networks or networks with very few hidden terminals. We present the first analytical model to derive the saturation throughput of collision avoidance protocols in multi-hop ad hoc networks with nodes randomly placed according to a two-dimensional Poisson distribution. We show that the sender-initiated collision-avoidance scheme achieves much higher throughput than the ideal carrier sense multiple access scheme with a separate channel for acknowledgments. More importantly, we show that the collision-avoidance scheme can accommodate much fewer competing nodes within a region in a network infested with hidden terminals than in a fully-connected network, if reasonable throughput is to be maintained. Simulations of the IEEE 802.11 MAC protocol and one of its variants validate the predictions made in the analysis. It is also shown that the IEEE 802.11 MAC protocol cannot ensure collision-free transmission of data packets and thus throughput can degrade well below what is predicted by the analysis of a correct collision avoidance protocol. Based on these results, a number of improvements are proposed for the IEEE 802.11 MAC protocol.     

A novel self‐adaptive transmission scheme over an IEEE 802.11 WLAN for supporting multi‐service

The IEEE 802.11 wireless local area network (WLAN) media access control (MAC) specification is a hybrid protocol of random access and polling when both distributed coordination function (DCF) and point coordination function (PCF) are used. Data traffic is transmitted with the DCF, while voice transmission is carried out with the PCF. Based on the performance analysis of the MAC protocol for integrated data and voice transmission by simulation, this paper puts forward a self‐adaptive transmission scheme to support multi‐service over the IEEE 802.11 WLAN. The simulation results show that, on the premise of satisfying the maximum allowable delay of packet voice, the self‐adaptive transmission scheme can improve the data traffic performance and increase the WLAN capacity through dynamic and appropriate adjustment of the protocol parameters. Especially, voice traffic is sensitive to delay jitter, and the self‐adaptive scheme can effectively decrease it. Finally, it is worth noting that the adaptive scheme is easy to be realized, whereas no change in the MAC protocol is needed. Copyright © 2006 John Wiley & Sons, Ltd.     

A Scalable CSMA and Self-Organizing TDMA MAC for IEEE 802.11 p/1609.x in VANETs

vehicular ad hoc networks (VANETs) have been a key topic for research community and industry alike. The wireless access in vehicular environment standard employs the IEEE 802.11p/1609.4 for the Medium Access Control (MAC) layer implementation for VANETs. However, the carrier sense multiple access (CSMA) based mechanism cannot provide reliable broadcast services, and the multi-channel operation defined in IEEE 1609.4 divides the available access time into fixed alternating control channel intervals (CCH) and service channel (SCH) intervals, which may lead to the low utilization of the scarce resources. In this paper, a novel multichannel MAC protocol called CS-TDMA considering the channel access scheduling and channel switching concurrently is proposed. The protocol combines CSMA with the time division multiple access (TDMA) to improve the broadcast performance in VANETs. Meanwhile, the dwelling ratio between CCH and SCH changes dynamically according to the traffic density, resulting in the improvement of resource utilization efficiency. Simulation results are presented to verify the effectiveness of our mechanism and comparisons are made with three existing MAC protocols, IEEE MAC, SOFT MAC and VeMAC. The simulation results demonstrate the superiority of CS-TDMA in the reduction of transmission delay and packet collision rate and improvement of network throughput.     

Incomplete cooperation‐based service differentiation in WLANs

In the IEEE 802.11 wireless LAN (WLAN), the fundamental medium access control (MAC) mechanism—distributed coordination function (DCF), only supports best‐effort service, and is unaware of the quality‐of‐service (QoS). IEEE 802.11e enhanced distributed channel access (EDCA) supports service differentiation by differentiating contention parameters. This may introduce the problem of non‐cooperative service differentiation. Hence, an incompletely cooperative EDCA (IC‐EDCA) is proposed in this paper to solve the problem. In IC‐EDCA, each node that is cooperative a priori adjusts its contention parameters (e.g., the contention window (CW)) adaptively to the estimated system state (e.g., the number of competing nodes of each service priority). To implement IC‐EDCA in current WLAN nodes, a frame‐analytic estimation algorithm is presented. Moreover, an analytical model is proposed to analyze the performance of IC‐EDCA under saturation cases. Extensive simulations are also carried out to compare the performances of DCF, EDCA, incompletely cooperative game, and IC‐EDCA, and to evaluate the accuracy of the proposed performance model. The simulation results show that IC‐EDCA performs better than DCF, EDCA, and incompletely cooperative game in terms of system throughput or QoS, and that the proposed analytical model is valid. Copyright © 2010 John Wiley & Sons, Ltd.     

Improved channel‐aware MAC protocols for wireless local area networks

This paper studies the fading properties of the communication channel assumed in wireless local area networks (WLANs) and devises efficient channel‐aware protocols for the distributed coordination function (DCF) and the point coordination function (PCF), the two modes of communication defined in the IEEE standard for WLAN. Our simulations show that the proposed PCF protocol improves the channel capacity usage up to 14% and the proposed DCF protocol improves the channel capacity up to 90%, when compared with standard IEEE 802.11 implementations, depending on the loss rate and temporal characteristics of the wireless channel. The proposed protocols introduce minimum computational overhead. We also show that, compared with standard DCF protocol defined in IEEE 802.11, the proposed DCF protocol can lower the SNR requirements for a given packet error rate thus potentially extending the battery life of portable devices that use WLAN.. Copyright © 2004 John Wiley & Sons, Ltd.     

Performance of a MANET directional MAC protocol with angle‐of‐arrival estimation

The use of directional antennas in mobile ad hoc networks (MANETs) has shown to offer large throughput gains relative to omnidirectional antennas. When used in ad hoc networks, directional medium‐access‐control (DMAC) protocols usually require all nodes, or part of nodes, to be aware of their exact locations. This location information is typically provided using a global positioning system (GPS). Although GPS systems are designed to be as nearly accurate as possible, there are still estimation errors that can cause a relatively large deviation from the actual GPS receiver position. In this paper, we investigate the effect of inaccurate node position estimation on the throughput of these protocols. Our results clearly indicate that the advantages of DMAC protocols diminish if the available position information is not accurate enough. As an alternative, we propose an efficient DMAC protocol that utilizes signal parameter estimation via the rotational invariance technique (ESPRIT) for direction‐of‐arrival (DOA) estimation; alleviating the need for GPS and, hence, avoiding the degrading associated with typical GPS position estimation errors. Moreover, unlike GPS‐based protocols, our protocol is suitable for both outdoor and indoor applications. Under different operating conditions and channel models, our simulation results show the throughput improvement achieved using the proposed protocol relative to the IEEE 802.11. Copyright © 2007 John Wiley & Sons, Ltd.