IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 3, MARCH 2000 535Performance Analysis of the IEEE 802.11 DistributedCoordination FunctionGiuseppe BianchiAbstract—Recently, the IEEE has standardized the 802.11 pro-tocol for Wireless Local Area Networks. The primary medium ac-cess control (MAC) technique of 802.11 is called distributed coor-dination function (DCF). DCF is a carrier sense multiple accesswith collision avoidance (CSMA/CA) scheme with binary slottedexponentialbackoff.Thispaperprovides a simple, butneverthelessextremely accurate, analytical model to compute the 802.11 DCFthroughput, in the assumption of finite number of terminals andideal channel conditions. The proposed analysis applies to both thepacket transmission schemes employed by DCF, namely, the basicaccessandthe RTS/CTS access mechanisms. In addition, it also ap-plies to a combination of the two schemes, in which packets longerthan a given threshold are transmitted according to the RTS/CTSmechanism. By means of the proposed model, inthis paper we pro-vide an extensive throughput performance evaluation of both ac-cess mechanisms of the 802.11 protocol.Index Terms—802.11, collision avoidance, CSMA, performanceevaluation.I. INTRODUCTIONIN recent years, much interest has been involved in thedesign of wireless networks for local area communication[1], [2]. Study group 802.11 was formed under IEEE Project802 to recommend an international standard for Wireless LocalArea Networks (WLAN’s). The final version of the standardhas recently appeared [3], and provides detailed medium accesscontrol (MAC) and physical layer (PHY) specification forWLAN’s.In the 802.11 protocol, the fundamental mechanism to accessthe medium is called distributed coordination function (DCF).This is a random access scheme, based on the carrier sense mul-tiple access with collision avoidance (CSMA/CA) protocol. Re-transmission of collided packets is managed according to bi-nary exponential backoff rules. The standard also defines an op-tional point coordination function (PCF), which is a centralizedMAC protocol able to support collision free and time boundedservices. In this paper we limit our investigation to the DCFscheme.DCF describes two techniquesto employ for packet transmis-sion. The default scheme is a two-way handshaking techniquecalled basic access mechanism. This mechanism is character-ized by the immediate transmission of a positive acknowledge-ment (ACK) by the destination station, upon successful recep-tion of a packet transmitted by the sender station. Explicit trans-Manuscript received November 1998; revised July 25, 1999. this work wassupported in part by CNR and MURST, Italy.The author is with the Universitá di Palermo,Dipartimento di Ingegneria Elet-trica, Viale delle Scienza, 90128 Palermo, Italy (e-mail: [email protected]).Publisher Item Identifier S 0733-8716(00)01290-7.mission of an ACK is required since, in the wireless medium, atransmitter cannot determine if a packet is successfully receivedby listening to its own transmission.In addition to the basic access, an optional four way hand-shaking technique, known as request-to-send/clear-to-send(RTS/CTS) mechanism has been standardized. Before transmit-ting a packet, a station operating in RTS/CTS mode “reserves”the channel by sending a special Request-To-Send short frame.The destination station acknowledges the receipt of an RTSframe by sending back a Clear-To-Send frame, after whichnormal packet transmission and ACK response occurs. Sincecollision may occur only on the RTS frame, and it is detectedby the lack of CTS response, the RTS/CTS mechanism allowsto increase the system performance by reducing the durationof a collision when long messages are transmitted. As animportant side effect, the RTS/CTS scheme designed in the802.11 protocol is suited to combat the so-called problem ofHidden Terminals [4], which occurs when pairs of mobilestations result to be unable to hear each other. This problemhas been specifically considered in [5] and in [6], which, inaddition, studies the phenomenon of packet capture.In this paper, we concentrate on the performance evaluationof the DCF scheme, in the assumption of ideal channel con-ditions and finite number of terminals. In the literature, perfor-mance evaluation of802.11 has been carried out either by meansof simulation [7], [8] or by means of analytical models with sim-plified backoff rule assumptions. In particular, constant or geo-metrically distributed backoff window has been used in [5], [9],[10] while [11] has considered an exponential backoff limited totwo stages (maximum window size equal to twice the minimumsize) by employing a two dimensional Markov chain analysis.In this paper, which revises and substantially extends [12], wesucceed in providing an extremely simple model that accountsfor all the exponential backoff protocol details, and allows tocompute the saturation (asymptotic) throughput performance ofDCF for both standardized access mechanisms (and also for anycombination of the two methods). The key approximation thatenables our model is the assumption of constant and indepen-dent collision probability of a packet transmitted byeach station,regardlessof the numberof retransmissionsalready suffered. Asproven by comparison with simulation, this assumption leads toextremely accurate (practically exact) results, especially whenthe number of stations in the wireless LAN is fairly large (saygreater than ten).The paper is outlined as follows. In Section II we briefly re-view both basic access and RTS/CTS mechanisms of the DCF.In Section III we define the concept of Saturation Throughput,and in Section IV we provide an analytical technique to com-0733–8716/00$10.00 © 2000 IEEE536 IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, VOL. 18, NO. 3, MARCH 2000pute this performance figure. Section V validates the accuracyof the model by comparing the analytical results with that ob-tained by means of simulation. Additional considerations on themaximum throughput theoretically achievable are carried out inSection VI. Finally, the performance evaluation of both DCF ac-cess schemes is carried out in Section VII. Concluding remarksare given in Section VIII.II. 802.11 DISTRIBUTED COORDINATION FUNCTIONThis section briefly summarizes the DCF as standardized bythe 802.11 protocol. For a more complete and detailed presen-tation, refer to the 802.11 standard [3].A station with a new packet to transmit