Mobile Networks and Applications 8, 223–235, 2003 2003 Kluwer Academic Publishers. Manufactured in The Netherlands.Quality of Service Schemes for IEEE 802.11 WirelessLANs – An EvaluationANDERS LINDGREN, ANDREAS ALMQUIST and OLOV SCHELÉNDivision of Computer Science and Networking, Department of Computer Science and Electrical Engineering, Luleå University of Technology,SE-971 87 Luleå, SwedenAbstract. This paper evaluates four mechanisms for providing service differentiation in IEEE 802.11 wireless LANs. The evaluated schemesare the Point Coordinator Function (PCF) of IEEE 802.11, the Enhanced Distributed Coordinator Function (EDCF) of the proposed IEEE802.11e extension to IEEE 802.11, Distributed Fair Scheduling (DFS), and Blackburst. The evaluation was done using the ns-2 simulator.Furthermore, the impact of some parameter settings on performance has also been investigated. The metrics used in the evaluation arethroughput, medium utilization, collision rate, average access delay, and delay distribution for a variable load of real time and backgroundtraffic. The simulations show that the best performance is achieved by Blackburst. PCF and EDCF are also able to provide pretty goodservice differentiation. DFS can give a relative differentiation and consequently avoids starvation of low priority traffic.Keywords: Quality of Service, wireless LANs, performance evaluation, MAC protocols1. IntroductionWireless networks are superior to wired networks with regardto aspects such as ease of installation and flexibility. They do,however, suffer from lower bandwidth, higher delays, higherbit-error rates, and higher costs than wired networks. Withthe advent of Wireless Local Area Networks (WLANs), band-width has increased and prices have decreased on wirelessnetworking solutions. These factors have made WLANs avery popular wireless networking solution. Given the cov-erage and low price, it is likely that demands for the abil-ity to run real-time applications such as voice over IP overthese networks will increase. If such applications shall beusable, considering the characteristics of wireless networks,some kind of service differentiation must be employed to letcertain types of traffic get better performance.The IEEE 802.11 standard [8] for WLANs is the mostwidely used WLAN standard today. Since it uses a sharedmedium, it has some inherent problems, such as low mediumutilization, risk of collisions and problem of providing differ-entiation between different types of traffic. There is a modeof operation in IEEE 802.11 that can be used to provide ser-vice differentiation, but it has been shown to perform poorlyand give poor link utilization [15], so several new servicedifferentiation schemes have been proposed. We study andevaluate four schemes for providing Quality of Service (QoS)over IEEE 802.11 wireless LANs: the PCF mode of the IEEE802.11 standard [8], Distributed Fair Scheduling [14], Black-burst [12], and Enhanced DCF [2].This paper summarizes work previously published as po-sition papers [9,10], and does a more thorough analysis thanthe previous papers. The rest of the paper is organized as fol-lows. Section 2 provides an overview of IEEE 802.11 andthe proposed schemes for service differentiation. Section 3describes our simulation scenarios and metrics. In section 4we present the results of our simulations, section 5 discussessome issues, and section 6 concludes.2. Overview of evaluated schemesIn this section we describe the QoS mechanisms we have eval-uated. For further details we refer to [2,8,12–14].2.1. IEEE 802.11IEEE 802.11 has two different access methods, the manda-tory Distributed Coordinator Function (DCF) and the optionalPoint Coordinator Function (PCF). The latter aims at support-ing real-time traffic.2.1.1. Distributed coordinator functionThe DCF is the basic access mechanism of IEEE 802.11. Ituses a Carrier Sense Multiple Access with Collision Avoid-ance (CSMA/CA) algorithm to mediate the access to theshared medium. Before a data frame is sent, the station sensesthe medium. If it is idle for at least a DCF interframe space1(DIFS) period of time, the frame is transmitted. Otherwise, abackoff time B (measured in time slots) is chosen randomlyin the interval [0, CW),whereCW is the so called ContentionWindow. After the medium has been detected idle for at leasta DIFS, the backoff timer is decremented by one for eachtime slot the medium remains idle. If the medium becomesbusy during the backoff process, the backoff timer is paused,and is restarted when the medium has been sensed idle for a1An interframe space, IFS, is the time a station waits when the mediumis idle before attempting to access it. IEEE 802.11 defines several IFSs,and by using shorter IFS, the medium is accessed prior to stations using alonger IFS. This is, e.g., used to ensure that an acknowledgment frame issent before any other station can send data.224 A. LINDGREN ET AL.Figure 1. The superframe of IEEE 802.11.DIFS again. When the backoff timer reaches zero, the frameis transmitted. Upon detection of a collision (which is de-tected by the absence of an acknowledgment frame to the dataframe), the contention window is doubled according toCWi= 2k+i−1− 1, (1)where i is the number of attempts (including the current one)to transmit the frame that has been done, and k is a constantdefining the minimum contention window, CWmin= 2k− 1.A new backoff time is then chosen and the backoff proce-dure starts over. The backoff mechanism is also used after asuccessful transmission before sending the next frame. Aftera successful transmission, the contention window is reset toCWmin.2.1.2. Point coordinator functionPCF is a centralized, polling-based access mechanism whichrequires the presence of a base station that acts as Point Coor-dinator (PC). If PCF is supported, both PCF and DCF coexistand in this case, time is divided into superframes as shownin figure 1. Each superframe consists of a contention periodwhere DCF is used, and a contention free period (CFP) wherePCF is used. The CFP is started by a special frame (a beacon)sent by the base station. Since the beacon is sent using ordi-nary DCF access method, the base station has to contend forthe medium, and therefore the CFP may be shortened.The PC keeps a list of mobile stations that have requestedto be polled to send data. During the CFP, it sends poll framesto the stations when they are clear to access the medium.Upon reception of a poll frame, the
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