当前位置:文库下载 > 所有分类 > IEEE 802.11p的英文文档
免费下载此文档侵权投诉

IEEE 802.11p的英文文档

Runtime Optimization of IEEE802.11

Wireless LANs Performance

Luciano Bononi,Marco Conti,and Enrico Gregori

Abstract—IEEE802.11is the standard for Wireless Local Area Networks(WLANs)promoted by the Institute of Electrical and Electronics Engineers.Wireless technologies in the LAN environment are becoming increasingly important and the IEEE802.11is the most mature technology to date.Previous works have pointed out that the standard protocol can be very inefficient and that an appropriate tuning of its congestion control mechanism(i.e.,the backoff algorithm)can drive the IEEE802.11protocol close to its optimal behavior.To perform this tuning,a station must have exact knowledge of the network contention level;unfortunately,in a real case,a station cannot have exact knowledge of the network contention level(i.e.,number of active stations and length of the message transmitted on the channel),but it,at most,can estimate it.This paper presents and evaluates a distributed mechanism for contention control in IEEE802.11Wireless LANs.Our mechanism,named Asymptotically Optimal Backoff(AOB),dynamically adapts the backoff window size to the current network contention level and guarantees that an IEEE802.11WLAN asymptotically achieves its optimal channel utilization.The AOB mechanism measures the network contention level by using two simple estimates:the slot utilization and the average size of transmitted frames.These estimates are simple and can be obtained by exploiting information that is already available in the standard protocol.AOB can be used to extend the standard802.11access mechanism without requiring any additional hardware.The performance of the IEEE802.11protocol,with and without the AOB mechanism,is investigated in the paper through simulation.Simulation results indicate that our mechanism is very effective,robust,and has traffic differentiation potentialities.

Index Terms—Wireless LAN(WLAN),IEEE802.11,multiple access protocol(MAC),protocol capacity,performance analysis.

æ

1I NTRODUCTION

F OR decades,Ethernet has been the predominant network

technology for supporting distributed computing.In recent years,the proliferation of portable and laptop computers has led to the development of the wireless LAN(WLAN)technology([28],[43]).The success of WLANs is connected to the development of networking products that can provide wireless network access at a competitive price.A major factor in achieving this goal is the availability of appropriate networking standards.IEEE Standard802.11defines a Medium Access Control(MAC) and Physical Layer(PHY)specification for a wireless local area network to provide wireless connectivity for fixed, portable,and moving stations within a local area[42].

Two different approaches can be followed in the implementation of a WLAN:an infrastructure-based ap-proach or an ad hoc networking one([18],[25],[50]). Infrastructure-based802.11WLANs are currently widely used,while the use of IEEE802.11-based ad hoc networks is an 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.In WLANs,the medium access control(MAC)protocol is the main element that manages congestion situations that may occur inside the network.For this reason,in this paper,we focus on the efficiency of the IEEE802.11MAC protocol and we propose a solution for increasing both protocol efficiency and protocol’s ability to react to congestion conditions.

The IEEE802.11access scheme incorporates two access methods:Distributed Coordination Function(DCF)for asynchronous,contention-based,distributed access to the channel and Point Coordination Function(PCF)for centra-lized,contention-free access([42],[50]).PCF is intended to support real-time services(by using a centralized polling mechanism),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 Access protocol with Collision Avoidance,CSMA/CA,see,for example,([19],[38],[53]).The CSMA/CA protocol is typically adopted in a wireless environment due to its reliability, flexibility,and robustness.However,the performance of a WLAN based on the CSMA/CA protocol may be degraded by the presence of hidden terminals[54].A pair of stations is referred to as being hidden from each other if a station cannot hear the transmission from the other station.This event makes the carrier sensing unreliable as a station wrongly senses that the wireless medium has been idle while the other (hidden)station is transmitting.To avoid the hidden terminal problem,the CSMA/CA protocols are extended with a virtual carrier sensing mechanism,named Request To Send (RTS)/Clear To Send(CTS).This mechanism has been studied extensively;several variations and analyses of the RTS/CTS scheme can be found in the literature,see,for example,([4], [31],[29],[32]).IEEE802.11includes an optional RTS/CTS mechanism.In this work,we do not explicitly consider the RTS/CTS mechanism.The results presented hereafter always refer to the data transmission using the basic access only.A

66IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS,VOL.15,NO.1,JANUARY2004 .L.Bononi is with the Department of Computer Science,University of

Bologna,Mura Anteo Zamboni,7,40127Bologna,Italy.

E-mail:bononi@cs.unibo.it.

.M.Conti and E.Gregori are with the National Research Council(CNR),

IIT Institute,Via G.Moruzzi,1,56124Pisa,Italy.

E-mail:{marco.conti,enrico.gregori}@http://www.hjavira.netr.it.

Manuscript received14Mar.2001;revised5Aug.2002;accepted29May

2003.

For information on obtaining reprints of this article,please send e-mail to:

tpds@http://www.hjavira.net,and reference IEEECS Log Number113793.

1045-9219/04/$17.00ß2004IEEE Published by the IEEE Computer Society

第1页

免费下载Word文档免费下载:IEEE 802.11p的英文文档

(下载1-15页,共15页)

猜你喜欢

TOP相关主题

返回顶部