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Berkeley ELENG 228A - Dynamic Tuning of the IEEE 802.11 Protocol

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IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 8, NO. 6, DECEMBER 2000 785Dynamic Tuning of the IEEE 802.11 Protocol toAchieve a Theoretical Throughput LimitFrederico Calì, Marco Conti, Associate Member, IEEE, and Enrico Gregori, Associate Member, IEEEAbstract—In wireless LANs (WLANs), the medium access con-trol (MAC) protocol is the main element that determines the ef-ficiency in sharing the limited communication bandwidth of thewireless channel. In this paper we focus on the efficiency of theIEEE 802.11 standard for WLANs. Specifically, we analyticallyderive the average size of the contention window that maximizesthe throughput,hereafter theoretical throughput limit, and we showthat: 1) depending on the network configuration, the standard canoperate very far from the theoretical throughput limit; and 2) anappropriate tuning of the backoff algorithm can drive the IEEE802.11 protocol close to the theoretical throughput limit. Hence wepropose a distributed algorithm that enables each station to tuneits backoff algorithm at run-time. The performances of the IEEE802.11 protocol, enhanced with our algorithm, are extensively in-vestigated by simulation. Specifically, we investigate the sensitive-ness of our algorithm to some network configuration parameters(number of active stations, presence of hidden terminals). Our re-sults indicate that the capacity of the enhanced protocol is veryclose to the theoretical upper bound in all the configurations an-alyzed.Index Terms—Multiple access protocol (MAC), performanceanalysis, protocol capacity, wireless LAN (WLAN).I. INTRODUCTIONTHE DESIGN of wireless LANs (WLANs) needs toconcentrate more on bandwidth consumption than wirednetworks. This because wireless networks deliver much lowerbandwidth than wired networks, e.g., 1–2 Mb/s versus 10–150Mb/s [16]. In this paper we focus on the IEEE 802.11 WLAN([12], [16]). Since a WLAN relies on a common transmissionmedium, the transmissions of the network stations must becoordinated by the medium access control (MAC) protocol.The fraction of channel bandwidth used by successfullytransmitted messages gives a good indication of the overheadrequired by the MAC protocol to perform its coordination taskamong stations. This fraction is known as the utilization of thechannel, and the maximum value it can attain is known as thecapacity of the MAC protocol [14], [6].1MAC protocols for LANs can be roughly categorized into[10], [18]: random access (e.g., CSMA, CSMA/CD) and de-mand assignment (e.g., token ring). Due to the inherent flexi-Manuscript receivedSeptember17,1998,revisedOctober21,1999;approvedby IEEE/ACMTRANSACTIONSON NETWORKING Editor T. Todd. This paper waspresented in part at INFOCOM’98, San Francisco, CA, March, 1998.F. Calì is with Aleph, 56017 Pisa, Italy (e-mail: [email protected]).M. Conti and E. Gregori are with Consiglio Nazionale delle Ricerche, 56100Pisa, Italy (e-mail: [email protected]; [email protected]).Publisher Item Identifier S 1063-6692(00)06796-0.1Note that the protocol capacity univocally identifies the maximumthroughput and vice versa. Hence these two quantities will be used inter-changeably in the paper.bility of random access systems (e.g., random access allows un-constrained movement of mobile hosts) the IEEE 802.11 stan-dard committee decided to adopt a random access CSMA-basedscheme for WLANs. In this scheme there is no collision de-tection capability due to the WLANs inability to listen whilesending, since there is usually just one antenna for both sendingand receiving.The performances of CSMA protocols for radio channelswere investigated in depth in [13]. An analytical model of aCSMA/CD based LAN was presented in [15].Several works have investigated via simulation the IEEE802.11 protocol [1], [8], [20], and [21].By deriving an analytical model, in this paper we quantifythe maximum protocol capacity (hereafter referred to as theo-retical limit) that can be achieved by tuning the window sizeof the IEEE 802.11 backoff algorithm. To be more precise, wedevelop an analytical model to study the throughput of a-per-sistent IEEE 802.11 protocol. A-persistent IEEE 802.11 pro-tocol differs from the standard protocol only in the selection ofthe backoff interval. Instead of the binary exponential backoffused in the standard, the backoff interval of the-persistentIEEE 802.11 protocol is sampled from a geometric distribu-tion with parameter. In the paper we show that the -persis-tent IEEE 802.11 protocol closely approximates the standardprotocol (at least from the protocol capacity standpoint) if theaverage backoff interval is the same. Due to its memorylessbackoff algorithm, the-persistent IEEE 802.11 protocol is suit-able for analytical studies. By exploiting the similarity of thisprotocol with the standard one we used the analytical results toinfer the behavior of the standard protocol. These extrapolationsare validated via simulation. Specifically, we use the analyticalmodel to compute thevalue corresponding to the theoreticallimit, i.e., thevalue (optimal ) that maximizes the capacityof the-persistent IEEE 802.11 protocol. It is worth noting thatthe theoretical limit is the maximum throughput for the-per-sistent protocol. Due to the correspondence (from the capacitystandpoint) between the standard protocol and the-persistentone throughout this paper we use the theoretical limit as a ref-erence point for tuning the IEEE 802.11 protocol.In this paper we show that 1) depending on the network con-figuration, the standard protocol can operate very far from thetheoretical limit; and 2) the capacity of an IEEE 802.11 protocolwith a constant backoff window, tuned on the optimalvalue2is close to the theoretical limit. Hence, we propose to modifythe backoff algorithm of the IEEE 802.11 MAC Protocol, andwe name the resulting protocol as IEEE 802.11.2The average backoff interval must be equal to that of thep-persistent IEEE802.11 protocol.1063–6692/00$10.00 © 2000 IEEE786 IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 8, NO. 6, DECEMBER 2000TABLE IWLAN CONFIGURATIONThrough an extensive performance study we show that thecapacity of the IEEE 802.11is very close to the theoreticallimit for all the network and traffic configurations analyzed inthe paper.The paper is organized as follows. Section II presents the ana-lytical model used to estimate the protocol capacity. This modelis used in Section III to derive the upper bound of the protocolcapacity. Section IV presents, and


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Berkeley ELENG 228A - Dynamic Tuning of the IEEE 802.11 Protocol

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