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CU-Boulder TLEN 5520 - Runtime Optimization

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Runtime Optimization of IEEE 802.11Wireless LANs PerformanceLuciano Bononi, Marco Conti, and Enrico GregoriAbstract—IEEE 802.11 is the standard for Wireless Local Area Networks (WLANs) promoted by the Institute of Electrical andElectronics Engineers. Wireless technologies in the LAN environment are becoming increasingly important and the IEEE 802.11 is themost mature technology to date. Previous works have pointed out that the standard protocol can be very inefficient and that anappropriate tuning of its congestion control mechanism (i.e., the backoff algorithm) can drive the IEEE 802.11 protocol close to itsoptimal behavior. To perform this tuning, a station must have exact knowledge of the network contention level; unfortunately, in a realcase, a station cannot have exact knowledge of the network contention level (i.e., number of active stations and length of the messagetransmitted on the channel), but it, at most, can estimate it. This paper presents and evaluates a distributed mechanism for contentioncontrol in IEEE 802.11 Wireless LANs. Our mechanism, named Asymptotically Optimal Backoff (AOB), dynamically adapts the backoffwindow size to the current network contention level and guarantees that an IEEE 802.11 WLAN asymptotically achieves its optimalchannel utilization. The AOB mechanism measures the network contention level by using two simple estimates: the slot utilization andthe average size of transmitted frames. These estimates are simple and can be obtained by exploiting information that is alreadyavailable in the standard protocol. AOB can be used to extend the standard 802.11 access mechanism without requiring any additionalhardware. The performance of the IEEE 802.11 protocol, with and without the AOB mechanism, is investigated in the paper throughsimulation. Simulation results indicate that our mechanism is very effective, robust, and has traffic differentiation potentialities.Index Terms—Wireless LAN (WLAN), IEEE 802.11, multiple access protocol (MAC), protocol capacity, performance analysis.æ1INTRODUCTIONFOR decades, Ethernet has been the predominant networktechnology for supporting distributed computing. Inrecent years, the proliferation of portable and laptopcomputers has led to the development of the wirelessLAN (WLAN) technology ( [28], [43]). T he success ofWLANs is connected to the development of networkingproducts that can provide wireless network access at acompetitive price. A major factor in achieving this goal isthe availability of appropriate networking standards. IEEEStandard 802.11 defines a Medium Access Control (MAC)and Physical Layer (PHY) specification for a wireless localarea network to provide wireless connectivity for fixed,portable, and moving stations within a local area [42].Two different approaches can be followed in theimplementation of a WLAN: an infrastructure-based ap-proach or an ad hoc networking one ([18], [25], [50]).Infrastructure-based 802.11 WLANs are currently widelyused, while the use of IEEE 802.11-based ad hoc networks isan open research issue ([3], [21]).Since the wireless links will continue to have signifi-cantly lower capacity than wired links, the WLAN conges-tion is more problematic than in wired networks. InWLANs, the medium access control (MAC) protocol is themain element that manages congestion situations that mayoccur inside the network. For this reason, in this paper, wefocus on the efficiency of the IEEE 802.11 MAC protocol andwe propose a solution for increasing both protocolefficiency an d protocol’s abili ty to react to congestionconditions.The IEEE 802.11 access scheme incorporates two accessmethods: Distributed Coordination Function (DCF) forasynchronous, contention-based, distributed access to thechannel and Point Coordination Function (PCF) for centra-lized, contention-free access ([42], [50]). PCF is intended tosupport real-time services (by using a centralized pollingmechanism), but is not generally supported by current cards.Hereafter, we will concentrate our study on DCF only.The DCF is based on a Carrier Sensing Multiple Accessprotocol with Collision Avoidance, CSMA/CA, see, forexample, ([19], [38], [53]). The CSMA/CA protocol istypicallyadopted in a wireless environment due to its reliability,flexibility, and robustness. However, the performance of aWLAN based on the CSMA/CA protocol may be degradedby the presence of hidden terminals [54]. A pair of stations isreferred to as being hidden from each other if a station cannothear the transmission from the other station. This eventmakes the carrier sensing unreliable as a station wronglysenses that the wireless medium has been idle while the other(hidden) station is transmitting. To avoid the hidden terminalproblem, the CSMA/CA protocols are extended with avirtual carrier sensing mechanism, named Request To Send(RTS)/Clear To Send (CTS). This mechanism has been studiedextensively; several variations and analyses of the RTS/CTSscheme can be found in the literature, see, for example, ([4],[31], [29], [32]). IEEE 802.11 includes an optional RTS/CTSmechanism. In this work, we do not explicitly consider theRTS/CTS mechanism. The results presented hereafter alwaysrefer to the data transmission using the basic access only. A66 IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS, VOL. 15, NO. 1, JANUARY 2004. L. Bononi is with the Department of Computer Science, University ofBologna, Mura Anteo Zamboni, 7, 40127 Bologna, Italy.E-mail: [email protected] M. Conti and E. Gregori are with the National Research Council (CNR),IIT Institute, Via G. Moruzzi, 1, 56124 Pisa, Italy.E-mail: {marco.conti, enrico.gregori} received 14 Mar. 2001; revised 5 Aug. 2002; accepted 29 May2003.For information on obtaining reprints of this article, please send e-mail to:[email protected], and reference IEEECS Log Number 113793.1045-9219/04/$17.00 ß 2004 IEEE Published by the IEEE Computer Societymethodology for analyzing the optimal tuning of the backoffalgorithm when a portion of the traffic is transmitted usingthe RTS/CTS mechanism can be found, for example, in ([6],[13]). In addition, recent simulation and experimental resultsindicate that phenomena occurring at the physical layer makethe effectiveness of the RTS/CTS mechanism arguable sincethe hidden station phenomenon rarely occurs ([56], [11], [23]).The relevance of the IEEE 802.11 standard has generatedextensive literature on its MAC protocol. A completesurvey of the IEEE 802.11 literature

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