Exploiting Physical Layer Power Control Mechanisms in IEEE 802.11b Network Interfaces ...

16 Pages·2012·0.13 MB·English

Exploiting Physical Layer Power Control Mechanisms
in IEEE 802.11b Network Interfaces
MarcoGruteser,AshishJain,JingDeng,FengZhao,andDirk Grunwald
CU-CS-924-01 December2001
University of Colorado at Boulder
TechnicalReportCU-CS-924-01
DepartmentofComputerScience
CampusBox430
UniversityofColorado
Boulder,Colorado80309
Copyright(cid:0)c 2001 by
MarcoGruteser,AshishJain,JingDeng,FengZhao,andDirkGrunwald
Abstract
Mobilecomputingreliesonlow-powercommunication. Sincead-hocnetworksareoftenidle,manyresearchers
have investigatedmechanismsto reduce idle power. This paper presents powermeasurements oncurrent IEEE
802.11bnetworkcardsanddiscussesphysicalandlinklayerpowercontrolmechanismsfortransmissionpower.
Specifically, we characterize the influence of bit-rate, transmission power, and packet size on total power con-
sumption. These measurements indicate a high per-packet overhead, indicating that packet aggregation might
beabletoreducethatoverhead. Packetaggregationcombinesmultiplesmallerpacketsintoalargerpacket. Our
experimentsshowthatourapplication-independentpacketaggregationmechanismyieldssubstantialtransmission
powersavingsforworkloadsthatfrequentlysendsmallpackets,suchasWebworkloads.
1 Introduction
Energyhasbecomeacentraldesignconstraintinmobilecomputing.Especially,astechnologyprogressestoever-
smaller computing devices, the significanceof energyconsumptionin the wireless network interface increases.
For example, the power consumption of an IEEE 802.11 network interface can make up 50% of a handheld
computingdevice[1]. Therefore, wirelessnetworkenergyconsumptionhasbeenthefocusofmanylow-power
computingresearchprojects.
Many applications use the network interface only sporadically. Since, idle power consumption on network
interfacesishigh,mostpreviousworkhasfocusedoncarddeactivationduringidlephases[2][1]. Withimprove-
mentsinwirelessnetworkcarddesign,idlepowerconsumptionisbecominglessofaproblem. Forexample,the
CiscoAironet350seriescard[3]hasaspecifiedsleeppowerconsumptionofonly50mW.Inaddition,theideaof
a“paging”mechanismhasthepotentialofvirtuallyeliminatingthisidlepowerconsumption.
Inthispaper,wefocusontheproblemofactivetransmissionpowerconsumption,includingbothtransmission
andreception. Wedevelopamodelofthetransmissionoverheadforvaryingpacketsizesandusedthistoimple-
mentanapplication-independentpacket-aggregationprotocolthatreducestransmissionandreceptionoverheads
whilemaintainingqualityofserviceconstraints.Thekeycontributionsofthispaperare:
Characterizingtheinfluenceofbitrateandtransmissionpoweronnetworkinterfacepowerconsumption
(cid:0)
Describingtheper-packetoverheadinmodern802.11bnetworkinterface
(cid:0)
Demonstratingthefeasibilityofapplication-independentpacketaggregation
(cid:0)
Packetaggregationcombinesmultiplesmallpacketsintoonebigpacketbeforetransmissionoverthewireless
link. Wewillshowthatthisresultsintransmissionenergysavingsofupto75%,whileimposingameandelayof
100msontopackets.
Therestofthispaperisstructuredasfollows:First,wewilldiscussrelatedworkinpower-awarenetworking.
WedescribetheimportantaspectsoftheIEEE802.11bstandardin2.1. Insection3wedescribeourexperimental
methodologyformeasuringnetworkcardenergyconsumption.Wemeasuredhowbitrateandtransmissionpower
affectpowerconsumptionandhowpacketsizesinfluencetransmissionenergyandpower.Wethenseehowpacket
aggrgeationcanexploittheseattributestoreduceactivepower.
2 Related Work
Energyefficiencyisnotonlyanissuefordifferentcomponentsin mobilecomputerssuchasnetwork,CPU[4],
and hard disks [5], it is also addressed at different levels. Network power management work can be catego-
rizedintomediaaccesscontrolmechanismsandoperatingsystemmechanismsfromasingle-nodeandanetwork
perspective.
Themediaaccesscontrol(MAC)sublayergenerallyhasaroleofreducingcollisionsandschedulingaccess.
ThisinformationcaninfluencetheabilityoftheradiotocontrolenergyandQoS.Forinstance,asshownin[6,7],
theMAClayercanbeawareofnearbyradiostransmissionsscheduleswhenthechannelwillbebusyandsocan
1
simplyshutdownduringtheseintervalsinordertosavepower. Cellularsystemsalsousesegregatedcontroland
datachannelstominimizecontentioninefficiencies[8,9]. Similarly,Singh[6]proposesaseparatecontentionand
datachannel.
Transmission power control has first been investigated from the perspective of network capacity [10] [11].
Transmission with reduced power cause less interference for other nodes in the network. Thus, total network
capacityincreases. Fromanenergyconsumptionperspectivethisissueispresentedin[12].
Kravets and Krishnan [13] as well as Stemm and Katz [2] have used operating-system controlled admis-
sion scheduling to reduce idle power consumption. While Stemm’s results where simulation based, Kravets’
implementation used relatively simple beacon scheduling to coordinate system activity and the performance of
applicationswasreducedsignificantly.Thesystemwasdividedintoamaster/slaveorganizationwithacentralized
controlpoint, andnodiscussionwas givento distributedcontrol. Furthermore, goodperformancedependedon
theoperatingsystemknowinggoodcontrolparametersforeachapplication. Stemmalsomeasurestheeffectof
different transport protocols on a Metricom Ricochet Wireless Modem and a pre-802.11 AT&T Wavelan card.
Detailedmeasurementsfornetworkoperationson802.11and802.11bWavelancardsarepresentedbyFeeneyet
al. [14].
Recently RL has beendemonstrated on link layer adaptation in wireless networks[15]. The controller was
abletolearnapolicythatmeetsQoStargetswhilealsoreducingtheunnecessaryenergyuseby50%to80%.The
radiocontrollermonitorspacketssentandreceivedoverthewirelessinterfaceandlearnsapolicyforturningthe
radioonandoff.
Other studies have used transcoding or addressed systems that admit loss-allowing communication proto-
cols[16,17,18]byadaptingtheprotocoltochangesinthedatalinklayer.
Thegoalofthenetworklayeristomaintainconnectivitytoallparticipantsforthelongestperiodwhilemeeting
QoSguarantees.In[19],Singhanalyzeslinkmetricsthatleadtoincreasingthelifeofnodes.Appropriatemetrics
leadtoafactoroftworeductioninenergyusefortheworstcaseuser. Thisstudyconsideredarandomgraphand
notonebasedonphysicallocations.Morerealisticadhocnetworkstudiesincludeterminalsdistributedacrossthe
plane. Takagi[20] considerstheoptimaltransmissionrangetomaximizepacketprogress. Largerpowersmean
more connectivity, but more terminals for collision and vice versa. If transmitter power can vary from node to
node,connectiontoconnection,thentheminimumpowertoremainconnectedcanbecomputed[21]. In[22],the
authorsconsiderthegeneralproblemofdisseminatinginformationforlimitedenergyadhocnetworksofsensors.
Theyshowthattheroutingprotocolcanbeoptimizedforthisproblemtoyieldlowerenergyconsumption.Varying
transmitpower,thelinkmetricsusedfortheresultingconnectivity,andoptimizedprotocolscanbecombinedto
simultaneouslyguaranteeconnectivityandextendthebatterylifeoftheworstcaseparticipant.
Additionally, in [23] the concept of data fusion is proposed. Data from multiple nodes is combined before
itis forwardedto thenextroutinghop. Ourpacketaggregationapproachismore generallyapplicable, because
aggregationnodescanoperateatthenetworkpacketlevel. Theyarenotrequiredtoknowtheprotocolanddata
semanticsusedinthesetransmissions.
TheproblemofidlepowerconsumptionfromthenetworkperspectivehasbeenaddressedbyChenetal. [24].
Deactivationof networkinterfacesare coordinatedwithin thenetwork, so thatnetworksare notpartitionedand
bandwidthanddelayrequirementsareobserved.
2.1 Background: IEEE802.11b
TheIEEE802.11bstandardprovidesastandardfortheISOphysicalandlinklayersofhigh-throughputwireless
networks.Itsupportsvariabletransmissionpowerandbitratesof1,2,5.5,and11Mbps.Twooperatingmodesare
defined: infrastructureandadhocmode. Ininfrastructuremode,allradiosassociatewithabasestation,whereas
in ad hoc mode they communicate directly. In the physical layer, spread-spectrum radio techniques are used
withinthe2.4GHzISMband.Thisbandisreservedforunlicensedoperationinmostcountries.Spread-spectrum
techniquesreduceinterferencewithuncooperativedevices. The11Mbpsthroughputwasachievedthroughaban-
donmentoffrequencyhoppingandimprovementsindataencoding.QuadraturePhaseShiftKeyingusestheradio
bandwidthmoreefficientlyandComplementaryCodeKeyingimprovesdatacoding.
ThelinklayerisbasedontheIEEEstandardsforwirednetworks. However,thewirelessenvironmentmade
modificationsnecessary. Sinceradiostationscannotsendandtransmitatthesametime,theCSMA/CAcollision
2
Desktop Network PCMCIA
receiver Airport Card 0.5 W Port
Preamp
Desktop
Voltage
(Measurement)
Meter
DAQ
Figure1:Measurementsetup
Resistor(mOhm) 100 500 1000
IdlePower(W) 1.2636 1.2732 1.2119
Table1:Variancewithdifferentresistors
avoidanceprotocolisusedinsteadofCSMA/CD.The802.11specificationalsosupportsa“requesttosend/clear
tosend”mechanismtoavoidthehiddenterminalproblem,butthismechanismistypicallydisabledinmostim-
plementations1. Tocompensateforhighererrorrates,checksumsandexplicitacknowledgementswereintroduced
intotheMAClayer,whichcausesignificantoverheadcomparedtowirednetworks.
3 Equipments & Methodology
Inthissection,wedescribeourgeneralmeasurementsetupasillustratedinfigure1. Detailsforspecificexperi-
mentsaregivenintheexperimentsections.
Theenergyconsumedatthenetworkcardistheintegralintimeoverpower. Poweristheproductofvoltage
andcurrent. ThevoltageatthePCMCIAcardisknowntobeeitheraconstant5Vor3.3V.However,weneedto
measurethechangingcurrent. Sinceourmeasurementstationonlytakesvoltagesamples,weintroduceaknown
resistor of 0.5 ohm in series with network card and measure the voltage drop across this resistor. We can then
calculatethecurrentusingOhm’slaw,resultingin(cid:0)(cid:2)(cid:1)(cid:4)(cid:3)(cid:6)(cid:5)(cid:8)(cid:7)(cid:10)(cid:9)(cid:11)(cid:9)(cid:13)(cid:12)(cid:14)(cid:7)(cid:15)(cid:5)(cid:17)(cid:16)(cid:19)(cid:18)(cid:19)(cid:18)(cid:20)(cid:7)(cid:21)(cid:5)(cid:17)(cid:16)(cid:19)(cid:18) .
(cid:22) (cid:23)(cid:25)(cid:24)
WemeasurenetworkcardpowerconsumptiononthePCMCIAportofaDellInspiron3800laptop. AKaitek
EngineeringCardbusExtendercardisinsertedbetweenthePCMCIAportandthenetworkcardtogiveusaccess
to the powersupply of the card. Our measurement equipment providesa varying sample rate; we found that a
20Khzsamplingrateeliminatedmostmeasurementerrors. ThemeasurementstationiscomprisedofaNational
InstrumentsDAQBNC-2090andaLinuxmachinerunningtheComediinterfacev0.7.60[25]andKtimetrace[26]
utility. TheKtimetraceutilitycreatesalogofthevoltagesamplesinasimpleASCIIfileformat. Energy,average
current,averagepower,andpeakpowerconsumptioniscalculatedfromthislogwithaPerlscript.
To ensure that we chose a suitable resistor, we compared the variance from measurements with different
resistors. Iftheresistorischosentoosmall,thevoltagedropcannotbeaccuratelymeasuredwithourequipment.
However,ifitistoolarge,itmightaffectthepowerconsumptionoreventheoperationofthenetworkcard. We
experimentedwithresistorsintherangeof0.1to1ohm. Theresults,averagedfromthree10smeasurementsare
presentedintable1. Thevarianceislessthan5%.
TocompletelyverifyourmeasurementsetupwecomparedmeasurementsofanAironet350seriescardwith
manufacturerspecifications[3],whicharepresentedintable2. Ourmeasurementsdeviatelessthan5%.
1Inparticular, theRTS/CTSmechanismisonlyusedforpacketsaboveaspecificthreshold,andthedefaultthresholdislargethanthe
defaultMTUfor802networks
3
2
1.9
1.8
1.7
W)
on ( 1.6 1
pti 2
m
u 1.5 5.5
s
on 11
c
er 1.4 idle
w
o
P
1.3
1.2
1.1
1
0 4 8 12 16 20
Transmission power (dBm)
Figure2:Influenceofbitrateandtransmissionpoweronpowerconsumption
4 Effect of Bitrate and Transmission Power
Weconfiguredanetworkcardintoeverypossiblecombinationoftransmitpowerandbitrateandrecordedpower
measurementswhile itwasactivelysending. We usedNetperf[27], a standard TCPbenchmarkingtool, to keep
the card transmitting during our measurement. The average power is obtained from 10s measurements. Every
experimentwasrepeatedthreetimesandwecalculated95%confidenceintervalsfromthisdata. Althoughthisis
veryfewsamplesandwouldnormallyresultinverylargeerrorbars,thevariancewasexceptionallysmallbetween
samplesandtherewaslittlereasontocollectmoresamples.
WeusedaCisco350seriesPCMCIA802.11bforthisexperiment.Toourknowledge,theCisco350serieshas
themost fine-grainedcontroloftransmitpower. Ithas sixdifferenttransmitpowerlevels: 1mW,5mW, 20mW,
30mW,50mW,and100mW.ItalsosupportsthestandardIEEE802.11bbitrates: 1Mbps,2Mbps,5.5Mbps,and
11Mbps.
Wewereunabletoforcetheaccesspointand802.11bcardstouseaspecificbitrateininfrastructuremode;
when the cards were forced to used a specific bit rate, they lost connectivity with the access point (an Apple
AirportV1.0). Thus,forthismeasurementonly,weconductedtheexperimentsinadhocmode,whichallowedus
tovarythebitratesusingthedevicedrivers.WeconductedtheexperimentinadhocmodewithaLinuxdriver.We
experimented with the Microsoft Windows98driverbecauseof better manufacturersupport; however,we were
unabletoselectdifferentbitratesusingthisdriver.
WeplacedtwolaptopswithCisco350seriescardsapproximately10feetapart. Wesetupthemeasurement
hardwareandaNetperfclientononemachineandaNetperfserverontheotherlaptop. Inthefollowingdescrip-
tion, we refer to thosemachinesasclient laptop andserverlaptop, respectively. We configured thebit-rateand
transmitpowerontheclientlaptopwiththeiwconfigutilityfromLinuxwirelessextensions.
Theresultsofourmeasurementsareshowninfigure 2. Thestraightlineatthebottomshowsthe idlebase-
line. Eachoftheotherlinesrepresentsthemeasurementsforonebitratesetting. Asexpected,thelowestpower
consumptions were encounteredat1mW (=0dBm) transmissionpower. At thislevel, theyare veryclose to the
idle baseline. Fromthere, theygraduallyincreasebyabout 50% tothe highestpowerconsumptionsat 100mW
(=20dBm).
Mode Transmit Receive Sleep
Power(mW) 1750 1250 50
Table2:CiscoAironet350seriesmanufacturerspecifications
4
Theeffectofbitrateonpowerconsumptionismorecomplex. Surprisingly,atthehightransmitpowerlevels
faster bitrates havelower averagepower consumption thenslower bitrates. The lines cross at medium transmit
power levels and at lower levels, the faster bitrates have higher power consumption. We conjecture that the
lowerbitratesconsumemorepowerthanhigherbitratesathightransmitpowerbecauseofdifferencesinbitrate
utilization. WhileNetperfmaximize transport-levelthroughput for allmeasurements, thethroughputdata show
thattherearedifferencesinlink-levelutilization. Itismuchclosertothetheoreticmaximumthroughputatlower
bitrates.Therefore,thecardshouldbeactivelysendingforahigherpercentageoftimeatlowerbitratescompared
tohigherbitrates. Ifthecardisnotsending,thenthepowerconsumptionwillbeattheidlelevel. Ergo,athigher
bitrate the card spends more time idling and thus the average power consumption is lower. Why does this not
holdfor lowertransmissionpowerlevels? We speculatethatthisisdueto differentamountsofpacketloss. As
welowerthetransmitpower,packetlossprobablybecomesmoresignificant. Inaddition,weexpecttheamount
ofpacketlosstobedependentonthebitrate. Higherpacketlossathigherbitrateswouldcausemoreresendsand
thereforehigherpowerconsumption.
These results indicate that significant overall power savings can be achieved by lowering the transmission
powerandthatvaryingthebitratehaslessimpactontheoverallpowersavings.
Whetherthesepowersavingscouldbetranslatedintoenergysavingsisunclear;ifanapplicationmusttransfer
aspecificamountofdata,ourdataindicatesthatisismoreusefultotransmitthedataatthehighestbitratepossible,
particularlyiftheidlemodepowercanbereduced.Iftheapplicationtransfersdataatafixedrate(suchasamedia
application),itmaybethatreducingthebitratecanreducetheoverallenergy,particularlyifthetransmissionrate
preventsthe effectiveuse of idle energyreduction. Likewise, the transmission powerlevel must be adjustedto
reducethebiterrorrateleastpacketretransmissionseliminateanyenergysavings.
Reducingthepowerleveltotheminimumrequiredtoreachthereceiveralsocauseslessinterference. Since
thesignalisweaker,nodesthatarefurtherawaycanoperateundisturbed.Toourknowledge,theCisco350cards
are already capableof automaticallyadjusting transmissionpower. However,the resultssuggestanother power
managementstrategy.Theeffectiverangeofthewirelesssignalcanbeincreasedbyeitherincreasingthetransmit
powerorloweringthebitrate[3]. Thedatashowthatthelatterismorepower-efficient.Hence,adaptivelyvarying
thebitratebasedonthesignalstrengthandthebandwidthrequiredbyapplicationsmayresultinadditionalpower
savings.
5 Effect of Packet Size
We explored the effect of packet size on the power overhead. For older network cards; this relationship has
beenexploredbyFeeney[14]. We verifythattheresultsstillholdontwomorerecentcardswiththefollowing
experiment. Wealsogenerateatransmissionoverheadenergymodelforourfollowingexperiment.
Wesentafixednumberofsame-sizedpacketsduringa30sintervalandmeasurethetotalenergyconsumed.
Onepacketissentevery40ms.Thisexperimentisrepeatedwithpacketsizesof50,100,250,500,1000,and1450
bytes.Tocompareagainstthebaselinepowerconsumption,wealsotakemeasurementswhilethecardisidle.We
implementedapacketgeneratorintheClickModularRouterFramework[28]. TheClickFrameworksimplifies
theconstructionofroutersthroughamodularapproach.WeusedClickbecauseitsimplifiestheimplementationof
packetmanipulationcodeandwelaterusedthesameinfrastructureforpacketaggregation.Ourpacketgenerator
sends Ethernetframesof specifiedsizeandat a specifiedrate. We conducted thisexperimentonthe Cisco350
withthelinuxdriverandanSMC2632Wusingthelinux-wlan-ngdriverfortheIntersilPrisimchipset. Again,we
repeatthemeasurementforeveryconfigurationthreetimesandshow95%confidenceintervals.
Figure3showstheaveragepowermeasuredfordifferentpacketsizes. Wealsocalculatedtheenergyperbyte
dependentonpacketsize.Wearrivedatthesenumbersbyfirstsubtractingtheidleenergyfromourmeasurements.
Therefore,theresultsshowninfigure4onlyrepresenttheactualtransmissionoverheads.
Comparingtheresultsinfigure3,weseethattheCisco350cardconsumesabout10%lessidleenergythan
theSMC2632W.Furthermore,theper-bytetransmissionoverhead(i.e.,theadditionalpowerneededtotransmit
eachadditionalbyteasthepacketsizeisincreased)oftheCiscocardisanorderofmagnitudelowerthanthatfor
theSMCcard.
5
1.45
1.4
SMC Card
(send
1.35 data)
Cisco
W) 1.3 Card
er ( (Send
w data)
Po 1.25 SMC idle
1.2
1.15
1.1
0 200 400 600 800 1000 1200 1400 1600
Packet Size (Unit: byte)
Figure3:Influenceofpacketsizeonpowerconsumption
0.0016
0.0014
SMC
0.0012 Card
(send
W) 0.001 data)
m
r ( 0.0008
e
w Cisco
Po 0.0006 Card
(Send
data)
0.0004
0.0002
0
0 200 400 600 800 1000 1200 1400 1600
Packet Size (Unit: byte)
Figure4:Energy/byteatdifferentpacketsizes
6
Asexpected,transmissionoverheadincreasesforlargerpackets.However,evensmallpacketsexhibitasignif-
icantoverhead.Fromfigure4weconcludethatthetransmissionenergyoverheadperbytedecreasessignificantly
forlargerpackets.
FortheCisco350card,thepackettransmissionenergy(inJoules)canbeapproximatedbythefollowinglinear
model (cid:0)(cid:1)(cid:0)(cid:3)(cid:2)(cid:5)(cid:4)(cid:7)(cid:6)(cid:5)(cid:8) (cid:1)(cid:10)(cid:9)(cid:12)(cid:11)(cid:13)(cid:15)(cid:14)(cid:17)(cid:16)(cid:19)(cid:18)(cid:21)(cid:20) (cid:12)(cid:23)(cid:22)(cid:25)(cid:24) (cid:16)(cid:27)(cid:26)(cid:29)(cid:28)(cid:30)(cid:18)(cid:15)(cid:11)(cid:31)(cid:15)(cid:31)(cid:17)(cid:16)(cid:19)(cid:18)(cid:21)(cid:20) (cid:12)! "(cid:24) . We assume a fixedenergyoverheadfor everypacketand
(cid:16) (cid:26)
additionalenergydependentonthepacketsize .
Weattributethedifferenceintransmissionoverheadtotransmissionpowercontrolfeatures. EithertheCisco
cardorthedriverautomaticallylowersthetransmissionpowertotheminimumrequiredtoreachtheaccesspoint.
Our SMC configuration in comparison does not seem to implement this and therefore has higher transmission
overhead.
Thehigherenergy/byteratioforsmallerpacketscanbeexplainedwithafixedprocessingoverheadforevery
packetintheDSPandMACprotocoloverhead.Everypacketrequiresthatthecardlistensforacarriersignalwith
possibleback-offsbeforetransmission.Inaddition,itneedstoreceiveanacknowledgementafterthetransmission.
Our per-packet calculation we do not consider the additional overhead of Ethernet, IP, and other headers.
Thus,theper-packetoverheadwouldbeevenhigherifmeasuredonapplicationdata.
6 Packet Aggregation
We believe that idle power in wireless data networks will eventually be reduced by including some sort of a
“paging”mechanism, wherein a simple verylow-powerradio listensfor a “wakeup”messagethat thenenables
the standard transceiver.2. Likewise, our experiments with the Cisco 350 card appear to indicate that modern
802.11bcardsareincludingmechanismstoautomaticallyadjusttransmissionpowerininfrastructuremodebased
onthesignalstrengthorretransmissionrates.
It’s clear that these physical layer attributes are best handled by interface firmware; however, our measure-
ments indicate that some energyoptimizations can be performed at the link layer. Combining multiple smaller
packets and transmitting them over the wireless link as a single larger packet reduces the number of packets
transmitted;ourmeasurementsshowthismaydecreasetheenergyneededfornetworkingbyreducingper-packet
overheadandthustotalpowerconsumption.
However, it isn’t clear howmuch packet aggregationcan be performed in practice; simply counting packet
distributionsisn’tsufficientbecauseitwouldbeunacceptabletosignificantlyincreasepacketdelay,andprotocol
dependencesmaybeexacerbatedbythisincreaseddelay.
WeimplementedaQoS-sensitivepacketaggregationmechanismusingtheClickModularRouter. Inorderto
increasethechancesofhavingpacketstoaggregateweneedtodelayoutgoingpacketsandbufferthem. Packets
canbenolargerthantheMTUofthewirelesscard.3 Whenpacketsaresentbythenetworkstack,weplacethem
into our transmission buffer. Whenever, the buffer cannot fit the next packet, it is flushed and transmitted as a
singlepacket. Weassignatimeouttothefirstpacketinthebufferandtherebyplaceanupperboundonthedelay
foreachpacket. Thebufferisflushedwheneveritisfullorthetimeoutoccurs.
Wewantedtodetermineifenoughpacketscanbeaggregatedtoreachsubstantialpowersavingsandhowthis
wouldincreasedelay. Weanalyzedthesequestionswithrespecttowebbrowsingworkloads,becauseweexpect
webaccesstobeoneofthebasicapplicationsofhandheldswithwirelessnetworks.
Ourimplementationisindependentofspecificapplications;thepacketsareencapsulatedanddeencapsulated
atthelinklayer.Thissolutionrequiresthatbothcommunicationendpointsbeabletounderstandtheencapsulation
protocol. TheaggregationtestbediscomprisedaDellInspiron3800laptopwitha Cisco350wirelesscard, an
Apple Airport, and a workstation with an Ethernet connection to the airport and the Internet gateway. We use
aninfrastructurenetworktodirectalltrafficthroughaspecificaccesspoint,insuringthatourtestsystemswould
capturealltraffic.AlthoughnotshownintheClickspecificationsbelow,wehaveimplementedanICMPextension
thatallowsustoqueryremotenodestodetermineiftheysupportaggregation.
2ThisideahasbeenproposedbyBroderson&RabeayoftheBerkeleyWirelessResearchCenter,amongothers
3TheMTUof802.11islargerthanthatforstandardEthernet,butweusedthestandard1500byteMTUtoavoidproblemswiththenetwork
drivers.
7
Application
Forward De-aggregator 14 1
11 6
IP
7
10 Stack
12
eth0
2
13
5
8
9 eth0 tap0
3
Cs-gw-wireless 4
Aggregator
De-aggregator
Aggregator
Figure5:AggregationSetup
We implemented an aggregationtest bed to obtain realistic aggregationand delay data. We can then apply
ourpowermodelsfromthepreviousexperimenttocalculatetheenergysavings. Theexperimentisrepeatedfor
differentbuffertimeoutintervals.Specifically,weusedthefollowingtimeouts: 50,100,150,200,250,300,350,
400,450,and500ms.
We implemented an aggregator and de-aggregator module in the Click Modular Router Framework [28].
These are installed on the laptop and workstation, respectively. The aggregator module performs the packet
buffering and aggregation function described in the previous section. The de-aggregator extracts the original
packetsfromanaggregatedpacketandforwardsthemtotheInternet. Theexactsetupisdescribedinmoredetail
inthefollowingsubsections.
Inordertogetrepeatablewebbrowsingbehavior,wecreatedascriptthatdownloadsHTMLfilesandpictures
from severalpopularwebsites with the Linuxwget program. It simulates”user thinking”duringweb browsing
through delays between accesses of different web pages. In addition, it mimics web browser communication
behaviorbydownloadingpictureswithmultipleTCPconnectionssimultaneously.
6.1 AggregationSetup
Thesoftwareconfigurationofouraggregatorandde-aggregatormachineisillustratedinfigure5.Ontheaggrega-
tionmachine,theaggregatormoduleneedstointerceptallpacketssentbyapplicationssuchasourwebbrowser.
Therefore,weinstallanethertapvirtualnetworkinterface(tap0)andchangethedefaultrouteintheIP-Stack’s
routing table to this interface. Then, we configured the Click router to receive packetsfrom the tap0 interface.
OurClickmodulethensendstheaggregatedpacketsdirectlythroughthewirelessnetworkinterface(eth0)tothe
de-aggregatormachine;ourimplementationcanalsobeuseddirectlyinthekernel.
ThecorrespondingClickconfigurationfortheaggregatormachineisshowninthefollowingcodefragment.
Thefirstpartroutespacketsfromthetap0interfacethroughtheaggregatormoduletoeth0.Thesecondpartroutes
receivedpacketsfrometh0throughthetap0interfacebacktotheapplication.
tun :: KernelTap(192.168.0.99/25);
tun -> MarkIPHeader(14)
-> IPPrint
-> Aggregator(SIZE 1400, // maximum payload size
TIMEOUT 100, // delay threshold
ETHSRC 00:40:96:38:C8:52, // client
ETHDST 00:06:5B:4A:50:E0, // deaggregator
DST 192.168.0.1) // special IP address to indicate
8