Hongqiang Zhai

Bio: Hongqiang Zhai is an academic researcher from University of Florida. The author has contributed to research in topics: Wireless ad hoc network & Throughput. The author has an hindex of 23, co-authored 34 publications receiving 2095 citations. Previous affiliations of Hongqiang Zhai include Tsinghua University.

Performance analysis of IEEE 802.11 MAC protocols in wireless LANs

Abstract: Summary IEEE 802.11 MAC protocol is the de facto standard for wireless local area networks (LANs), and has also been implemented in many network simulation packages for wireless multi-hop ad hoc networks. However, it is well known that, as the number of active stations increases, the performance of IEEE 802.11 MAC in terms of delay and throughput degrades dramatically, especially when each station’s load approaches its saturation state. To explore the inherent problems in this protocol, it is important to characterize the probability distribution of the packet service time at the MAC layer. In this paper, by modeling the exponential backoff process as a Markov chain, we can use the signal transfer function of the generalized state transition diagram to derive an approximate probability distribution of the MAC layer service time. We then present the discrete probability distribution for MAC layer packet service time, which is shown to accurately match the simulation data from network simulations. Based on the probability model for the MAC layer service time, we can analyze a few performance metrics of the wireless LAN and give better explanation to the performance degradation in delay and throughput at various traffic loads. Furthermore, we demonstrate that the exponential distribution is a good approximation model for the MAC layer service time for the queueing analysis, and the presented queueing models can accurately match the simulation data obtained from ns-2 when the arrival process at MAC layer is Poissonian. Copyright # 2004 John Wiley & Sons, Ltd.

How well can the IEEE 802.11 wireless LAN support quality of service

Abstract: This paper studies an important problem in the IEEE 802.11 distributed coordination function (DCF)-based wireless local area network (WLAN): how well can the network support quality of service (QoS). Specifically, this paper analyzes the network's performance in terms of maximum protocol capacity or throughput, delay, and packet loss rate. Although the performance of the 802.11 protocol, such as throughput or delay, has been extensively studied in the saturated case, it is demonstrated that maximum protocol capacity can only be achieved in the nonsaturated case and is almost independent of the number of active nodes. By analyzing packet delay, consisting of medium access control (MAC) service time and waiting time, accurate estimates were derived for delay and delay variation when the throughput increases from zero to the maximum value. Packet loss rate is also given for the nonsaturated case. Furthermore, it is shown that the channel busyness ratio provides precise and robust information about the current network status, which can be utilized to facilitate QoS provisioning. The authors have conducted a comprehensive simulation study to verify their analytical results and to tune the 802.11 to work at the optimal point with maximum throughput and low delay and packet loss rate. The simulation results show that by controlling the total traffic rate, the original 802.11 protocol can support strict QoS requirements, such as those required by voice over Internet protocol (VoIP) or streaming video, and at the same time achieve high channel utilization.

Supporting QoS in IEEE 802.11e wireless LANs

Abstract: In the emerging IEEE 802.11e MAC protocol, the enhanced distributed channel access (EDCA) is proposed to support prioritized QoS; however, it cannot guarantee strict QoS required by real-time services such as voice and video without proper network control mechanisms. To overcome this deficiency, we first build an analytical model to derive an average delay estimate for the traffic of different priorities in the unsaturated 802.11e WLAN, showing that the QoS requirements of the real-time traffic can be satisfied if the input traffic is properly regulated. Then, we propose two effective call admission control schemes and a rate control scheme that relies on the average delay estimates and the channel busyness ratio, an index that can accurately represent the network status. The key idea is, when accepting a new real-time flow, the admission control algorithm considers its effect on the channel utilization and the delay experienced by existing real-time flows, ensuring that the channel is not overloaded and the delay requirements are not violated. At the same time, the rate control algorithm allows the best effort traffic to fully use the residual bandwidth left by the real-time traffic, thereby achieving high channel utilization

A call admission and rate control scheme for multimedia support over IEEE 802.11 wireless LANs

Abstract: Quality of service (QoS) support for multimedia services in the IEEE 802.11 wireless LAN is an important issue for such WLANs to become a viable wireless access to the Internet. In this paper, we endeavor to propose a practical scheme to achieve this goal without changing the channel access mechanism. To this end, a novel call admission and rate control (CARC) scheme is proposed. The key idea of this scheme is to regulate the arriving traffic of the WLAN such that the network can work at an optimal point. We first s how that the channel busyness ratio is a good indicator of the network status in the sense that it is easy to obtain and can accurately and timely represent channel utilization. Then we propose two algorithms based on the channel busyness ratio. The call admission control algorithm is used to regulate the admission of real-time or streaming traffic and the rate control algorithm to control the transmission rate of best effort traffic. As a result, the real-time or streaming traffic is supported with statistical QoS guarantees and the best effort traffic can fully utilize the residual channel capacity left by the real-time and streaming traffic. In addition, the rate control algorithm itself provides a solution that could be used above the media access mechanism to approach the maximal theoretical channel utilization. A comprehensive simulation study in ns-2 has verified the performance of our proposed CARC scheme, showing that the original 802.11 DCF protocol can statically support strict QoS requirements, such as those required by voice over IP or streaming video, and at the same time, achieve a high channel utilization.

DUCHA: A New Dual-Channel MAC Protocol for Multihop Ad Hoc Networks

Abstract: IEEE 802.11 MAC protocol has been the standard for wireless LANs and is also implemented in many simulation software for mobile ad hoc networks. However, IEEE 802.11 MAC has been shown to be quite inefficient in the multihop mobile environments. Besides the well-known hidden terminal problem and the exposed terminal problem, there also exists the receiver blocking problem, which may result in link/routing failures and unfairness among multiple flows. Moreover, the contention and interference from the upstream and downstream nodes seriously decrease the packet delivery ratio of mulitihop flows. All these problems could lead to the "explosion" of control packets and poor throughput performance. In this paper, we first analyze these anomaly phenomena in multihop mobile ad hoc networks. Then, we present a novel effective random medium access control (MAC) protocol based on IEEE 802.11 MAC protocol. The new MAC protocol uses an out-of-band busy tone and two communication channels, one for control frames and the other for data frames, and can give a comprehensive solution to all the aforementioned problems. Extended simulations demonstrate that our protocol provides a much more stable link layer, greatly improves the spatial reuse, and works effectively in reducing the packet collisions. It improves the throughput by up to 20% for one-hop flows and by up to 5 times for multihop flows under heavy traffic comparing to the IEEE 802.11 MAC

Fundamentals of Queueing Theory

Abstract: Praise for the Third Edition: "This is one of the best books available. Its excellent organizational structure allows quick reference to specific models and its clear presentation . . . solidifies the understanding of the concepts being presented."IIE Transactions on Operations EngineeringThoroughly revised and expanded to reflect the latest developments in the field, Fundamentals of Queueing Theory, Fourth Edition continues to present the basic statistical principles that are necessary to analyze the probabilistic nature of queues. Rather than presenting a narrow focus on the subject, this update illustrates the wide-reaching, fundamental concepts in queueing theory and its applications to diverse areas such as computer science, engineering, business, and operations research.This update takes a numerical approach to understanding and making probable estimations relating to queues, with a comprehensive outline of simple and more advanced queueing models. Newly featured topics of the Fourth Edition include:Retrial queuesApproximations for queueing networksNumerical inversion of transformsDetermining the appropriate number of servers to balance quality and cost of serviceEach chapter provides a self-contained presentation of key concepts and formulae, allowing readers to work with each section independently, while a summary table at the end of the book outlines the types of queues that have been discussed and their results. In addition, two new appendices have been added, discussing transforms and generating functions as well as the fundamentals of differential and difference equations. New examples are now included along with problems that incorporate QtsPlus software, which is freely available via the book's related Web site.With its accessible style and wealth of real-world examples, Fundamentals of Queueing Theory, Fourth Edition is an ideal book for courses on queueing theory at the upper-undergraduate and graduate levels. It is also a valuable resource for researchers and practitioners who analyze congestion in the fields of telecommunications, transportation, aviation, and management science.

Vehicular Networking: A Survey and Tutorial on Requirements, Architectures, Challenges, Standards and Solutions

Abstract: Vehicular networking has significant potential to enable diverse applications associated with traffic safety, traffic efficiency and infotainment. In this survey and tutorial paper we introduce the basic characteristics of vehicular networks, provide an overview of applications and associated requirements, along with challenges and their proposed solutions. In addition, we provide an overview of the current and past major ITS programs and projects in the USA, Japan and Europe. Moreover, vehicular networking architectures and protocol suites employed in such programs and projects in USA, Japan and Europe are discussed.

Flying Ad-Hoc Networks (FANETs)

Abstract: One of the most important design problems for multi-UAV (Unmanned Air Vehicle) systems is the communication which is crucial for cooperation and collaboration between the UAVs. If all UAVs are directly connected to an infrastructure, such as a ground base or a satellite, the communication between UAVs can be realized through the in-frastructure. However, this infrastructure based communication architecture restricts the capabilities of the multi-UAV systems. Ad-hoc networking between UAVs can solve the problems arising from a fully infrastructure based UAV networks. In this paper, Flying Ad-Hoc Networks (FANETs) are surveyed which is an ad hoc network connecting the UAVs. The differences between FANETs, MANETs (Mobile Ad-hoc Networks) and VANETs (Vehicle Ad-Hoc Networks) are clarified first, and then the main FANET design challenges are introduced. Along with the existing FANET protocols, open research issues are also discussed.

Interference-aware topology control and QoS routing in multi-channel wireless mesh networks

Abstract: The throughput of wireless networks can be significantly improved by multi-channel communications compared with single-channel communications since the use of multiple channels can reduce interference influence. In this paper, we study interference-aware topology control and QoS routing in IEEE 802.11-based multi-channel wireless mesh networks with dynamic traffic. Channel assignment and routing are two basic issues in such networks. Different channel assignments can lead to different network topologies. We present a novel definition of co-channel interference. Based on this concept, we formally define and present an effective heuristic for the minimum INterference Survivable Topology Control (INSTC) problem which seeks a channel assignment for the given network such that the induced network topology is interference-minimum among all K-connected topologies. We then formulate the Bandwidth-Aware Routing (BAR) problem for a given network topology, which seeks routes for QoS connection requests with bandwidth requirements. We present a polynomial time optimal algorithm to solve the BAR problem under the assumption that traffic demands are splittable. For the non-splittable case, we present a maximum bottleneck capacity path routing heuristic. Simulation results show that compared with the simple common channel assignment and shortest path routing approach, our scheme improves the system performance by 57% on average in terms of connection blocking ratio.