Idle Sense: An Optimal Access Method for HighThroughput and Fairness in Rate Diverse Wireless LANsMartin Heusse, Franck Rousseau, Romaric Guillier, Andrzej DudaLSR-IMAG Laboratory∗Grenoble, France{Martin.Heusse, Franck.Rousseau, Romaric.Guillier, Andrzej.Duda}@imag.frABSTRACTWe consider wireless LANs such as IEEE 802.11 operatingin the unlicensed radio spectrum. While their nominal bitrates have increased considerably, the MAC layer remainspractically unchanged despite much research effort spent onimproving its performance. We observe that most propos-als for tuning the access method focus on a single aspectand disregard others. Our objective is to define an accessmethod optimized for throughput and fairness, able to dy-namically adapt to physical channel conditions, to operatenear optimum for a wide range of error rates, and to provideequal time shares when hosts use different bit rates.We propose a novel access method derived from 802.11DCF [2] (Distributed Coordination Function)inwhichallhosts use similar values of the contention window CW tobenefit from good short-term access fairness. We call ourmethod Idle Sense, because each host observes the meannumber of idle slots between transmission attempts to dy-namically control its contention window. Unlike other pro-posals, Idle Sense enables each host to estimate its frameerror rate, which can be used for switching to the right bitrate. We present simulations showing how the method leadsto high throughput, low collision overhead, and low delay.The method also features fast reactivity and time-fair chan-nel allocation.Categories and Subject DescriptorsC.2.5 [Computer-Communication Networks]: Local andWide-Area Networks—Access schemes∗LSR-IMAG is a joint research laboratory of CNRS (Cen-tre National de la Recherche Scientifique), INPG (InstitutNational Polytechnique de Grenoble), and UJF (Universit´eJoseph Fourier).Permission to make digital or hard copies of all or part of this work forpersonal or classroom use is granted without fee provided that copies arenot made or distributed for profit or commercial advantage and that copiesbear this notice and the full citation on the first page. To copy otherwise, torepublish, to post on servers or to redistribute to lists, requires prior specificpermission and/or a fee.SIGCOMM’05, August 22–26, 2005, Philadelphia, Pennsylvania, USA.Copyright 2005 ACM 1-59593-009-4/05/0008 ...$5.00.General TermsAlgorithms, PerformanceKeywordsWireless LANs, 802.11, Access Methods, Fairness1. INTRODUCTIONSince the advent of the first 802.11 wireless LANs, muchresearch effort has been spent on improving their perfor-mance. Successive variants have increased the nominal bitrate at the physical layer. However, the MAC layer remainspractically unchanged despite many proposals for tuning itsperformance. Most of this work focuses on a single aspectwhile disregarding others. For example, many researcherstry to improve throughput, but they neglect other perfor-mance aspects such as short-term fairness, adaptation tochannel conditions, or handling multiple bit rates. Unlikethese proposals, we follow a global approach to the designof an access method for wireless LANs by taking into ac-count all aspects and trying to find the best tradeoff betweenantagonistic objectives.We elaborate the principle of our method by considering amodification to the basic CSMA/CA (Carrier Sense Multi-ple Access/Collision Avoidance) access method: contendinghosts do not perform the exponential backoff algorithm af-ter collisions, but rather dynamically converge in a fully dis-tributed way to similar values of their contention windows.We optimize the throughput of the modified access methodto establish relations for estimating the current state of thenetwork and for dynamically controlling the contention win-dow CW. We call our access method Idle Sense, becauseeach host observes the mean number of idle slots betweentwo transmission attempts. By comparing the estimate witha theoretically derived value, hosts adjust their contentionwindows CW in a fully distributed way using an AIMD (Ad-ditive Increase Multiplicative Decrease) control algorithm. Itis the first access method to our knowledge that decouplesthe dynamic load control from collision perception—insteadof adjusting the contention window upon a collision, IdleSense controls transmission opportunities of hosts based onthe observed number of idle slots. Load control based on col-lisions results in lower performance and less fairness, becausesome collisions are just failed transmissions. Our methodapproaches the optimal tradeoff between high throughput,low collision overhead, and good short-term fairness, which121results in low delay. It is based on a surprisingly simple prin-ciple, yet it results in unprecedented overall performance.Our method also enables a host to estimate the collisionrate and distinguish it from the frame error rate due to poorchannel conditions. This feature solves other problems, usu-ally overlooked, that arise in wireless environments: adap-tation of the bit rate to channel conditions and handlinghosts with multiple bit rates. The information on the frameerror rate is essential for deciding (or not) on switching to adifferent bit rate. The method can handle hosts operatingat various bit rates by scaling their contention windows sothat they obtain equal channel time shares. In this way,the method provides a solution to performance anomaly inwhich the rate of a slower host limits the throughput ofa fast host [12]. We validate our method with simulationsshowing very good performance results in terms of efficiency,fairness, and reactivity.The rest of the paper is organized as follows. We first an-alyze the contention in wireless LANs from which we deriveour access method (Section 2). We then discuss the proper-ties of our access method (Section 3) and present simulationresults that show how the method meets the desired require-ments (Section 4). We compare our method with other pro-posals (Section 5) and present some conclusions (Section 6).2. IDLE SENSE ACCESS METHODOur objective is to define an access method optimized forthroughput and fairness, able to adapt dynamically to physi-cal channel conditions, to operate optimally for a wide rangeof error rates, and to provide equal time shares when hostsuse different bit rates. Usually, optimizing a CSMA/CAmethod is limited to only considering the best possible useof the radio channel,