DemystifyUndesiredHandoffinCellularNetworks

Chunyi PengTheOhioStateUniversity

Yuanjie LiUniversityofCalifornia,LosAngeles

MobileEra:“AlwaysConnected”

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In-building Outdoor Walking

Driving On the subway On the high-speed train

“AlwaysConnected”viaCellularNetworks

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cellular networks

cellular networks

“AlwaysConnected”viaMobilitySupport(Handoff)

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DesiredHandoffs:“AlwaysWellConnected”

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cellular networks

2G

3G

4G-2

4G-1

§ Multiplechoices¨ RAT:4G,3G,2G¨ Freq bands:700MHz,

1900MHz,etc.

§ Desiredhandoff:¨ BetterRAT(4G>3G)¨ Strongersignal

coverage¨ …

UndesiredHandoffs:“NotWellConnected”§ Notaniceorwisehandoffchoice

¨ Ex1:in2Gwhen4G/3Gisavailable¨ Ex2:evenoutofservicewhen4G/3Gisavailable

§ Questionsinthiswork¨ Q1:Dotheyhappeninreality?(Yes)¨ Q2:Howtodetectthem?¨ Q3:Whydotheyoccur?¨ Q4:Whatlessonslearnt?

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Fromtheorytopractice

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Q2: HowtoDetect?

Background:HandoffProcedure§ 3steps:trigger-decision-execution

§ Configurable¨ Versatilehandoffgoals

- Idle-state- active-stateE.g.seamlessconnectivity,loadbalancingviaoffloading

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Cell 1

2

decisionrulesparas

meas

Cell 2

3

1

Handoff decision@C1

Background:HandoffProcedure§ 3steps:trigger-decision-execution

§ Configurable¨ Versatilehandoffgoals

§ Distributed¨ Localdecision&configurations

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Cell 1

2

decisionrulesparas

meas

Cell 2

3

1

Handoff decision@C1

……5

decisionrules’paras’

meas’

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Handoff decision@C2

OurMethodology:Formulation§ Step1:Buildahandoffmodel

¨ Eachiteration:oneatomichandoffdecision:

- s, t:serving/target cell- C:setofcandidatecells- Ω:decisionlogic- G:configuration(tunableparameters)- O:measurement(runtimeobservation)

¨ Handoffsequences:sàc1à…à ci à [ci+1 =Ωci (ci)]à …10

s→ t =Ωs (Gs,Os ), t ∈Cs

OurMethodology:Analysis§ Step2:Undesiredreachabilityanalysis

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S

C1

Ci

t

……

Possible path

Handoff path

OurMethodology:Twocases§ Step2:Undesiredreachabilityanalysis

¨ ClassI:convergencesplit

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Possible path

Handoff path

S

C1

Ci

t

……Cj

topt…

OurMethodology:Twocases§ Step2:Undesiredreachabilityanalysis

¨ ClassI:convergencesplit¨ ClassII:prematureconvergence

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Possible path

Handoff path

S

C1

Ci

t

Cjtopt………

FromTheorytoPractice§ Step3:Buildanin-devicedetectorviaMMDIAG++

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ConfigurationCollector111

ConvergenceAnalyzer2

Scenario Emulator

ReachabilityAnalyzer

Counterexamples

Empirical Validation

MM Automata

1MobileInsight (Mobicom’16): http://metro.cs.ucla.edu/mobile_insight

Static

Dynamic

2MMDIAG (SIGMetrics’16)

Reality-checkandroot-causeanalysis

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Q1:Dotheyexistinreality?Q3:Andwhy?

RealityCheck§ Experimentsettings

¨ TwoUScarriers¨ Intwocities(LosAngeles,CAandColumbus,OH)¨ 50 outdoor locations,63 indoor locations,¨ Macrocells plusfemtocells (self-deployed)¨ Collectedhandoffprofiles(logic,config.,meas.)

§ Fourinstancesidentified¨ Inbothcategories¨ Threecauses

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CategoryI.A:Unaccessible intermedite cells(1/5)

§ Instance#1:Failtoreach4Gfrom2G

§ Cause:missingconfigurationfor2Gà 4G¨ Likelynoupdatein2G

§ Morerealcasesinthepaper17

Prefs,c > Prefs,sradioc>-108dBm

s (2G) c (3G) t (4G)

Prefc,t > Prefc,cradiot>-108dBmx

CategoryI.A:Unaccessible intermedite cells(2/5)

§ Realitycheck:¨ US-I:missing2Gà4Gconfigurationduringidleandactive¨ US-II:missing2Gà4Gconfigurationduringactive¨ 5out63locations:3Gisnotaccessible(<-105dBm)

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Prefs,c > Prefs,sradioc>-108dBm

s (2G) c (3G) t (4G)

Prefc,t > Prefc,cradiot>-108dBmx

CategoryI.A:Unaccessible intermedite cells(3/5)

§ Performanceimpact:muchsmaller¨ Webbrowsing(cnn.com):every1min

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2G

4G

0 600 1200 1800 2400 3000 36Time (s)

US-IUS-II

0

30

60

90

0 10 20 30 40 50 6CD

F (%

)Page loading time (s)

US-IUS-II

CategoryI.A:Unaccessible intermedite cells(4/5)

§ Instance#2:OutofservicefromFemto to4G

§ Cause:missingconfigurationfromFemtoà 4G¨ Improperconfigurationsin3GFemtocells

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Prefs,c = Prefs,sradioc> radios

s (Femto) c (3G)(No/weak relay)

t (4G)

Prefc,t > Prefc,cradiot>-108dBm

CategoryI.A:Unaccessible intermedite cells(5/5)

§ Realitycheck¨ US-I(only):allfemtocells (Nofemtocells inUS-II)

¨ 5outof63locations:4Gbutno/weak3G

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CategoryI.A:Unaccessible intermedite cells(5/5)

§ Realitycheck§ Performanceimpact:

¨ Exp:out-of-servicedurationw/wo 3G

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W 3G

W/o 3G

0 30 60 90 120 150Out-of-service Duration (s)

0

30

60

90

0 20 40 60 80 100CD

F (%

)Out-of-service duration (s)

W/o 3GW 3G

CategoryII.B:BlockedDecision(1/2)§ Instance#3:3Gblockedby2G

¨ Scenario:both3Gand2Gavailablewhenleaving4G;¨ Duringactive

§ Causes:¨ Device:2Gmeas.comesfirst¨ Servingcell:first-come-first-serve¨ improperdevice-networkcoordination

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CategoryII.B:BlockedDecision(2/2)§ Realitycheckonmeasurementcriteriasatisfied

¨ OP-I:60outof63locations(95.2%)¨ OP-II:100%locations

§ Realitycheckonactualhandoffresults¨ OP-I:100%to2G(allservingcellsuseFCFS)¨ OP-II:5.7%to2G(notallservingcellsuseFCFS)

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CategoryII.B:BlockedDecision(2/2)§ Realitycheckonmeasurementcriteriasatisfied§ Realitycheckonactualhandoffresults§ Performanceimpact:handofflatency

¨ 10.8%calldropinUS-II(when2G+3Gavailable)

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0 20 40 60 80

100

0 5 10 15 20 25 30

CDF

(%)

Handoff latency (s)

2G+3G3G

0 20 40 60 80

100

0 5 10 15 20 25 30CD

F (%

)

Handover latency (s)

2G+3G3G

US-I US-II

CategoryII.C:ProblematicDevice-Networkcoordination

§ Instance#4:Outofservicewhen3Gbandunsupported¨ Scenario:movinginto3GareaafterleavingFemtocells

§ Cause:¨ Servingcell:measuringall3Gbands¨ Device’s3Gcapabilitynotcompatible¨ Devicerejection(halt)

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CategoryII.C:ProblematicDevice-Networkcoordination

§ Reality-check¨ Whenmovingoutof3Gfemtocells (US-Ionly)¨ Alltestphones:100%failure

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Q4: LessonsLearnt

BlameWhom?§ Operator:Practicalchallenges

¨ Manyreasonsfortoday’schoices

§ Device(chipsetvendors):Freedom¨ Nothingwrongfromitsperspective

§ Lessons:¨ Verificationrequired(@operator,@device-operator)¨ Easyfixshouldbemadepossible(NFV,easyconfigurationupdates)

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BlameWhom?§ Operator:Practicalchallenges

¨ 2Gcells:expensiveupgrade(Nodirect2Gà4Gpath)¨ Full3Gdeploymentnotguaranteed¨ Seeminglyreasonablestrategies

- FCFS(handoffuponfirstmeas):reducelatencybutmissbetterchoices- Measureall3Gbands:don’tmisshandoffwithoutpriorknowledge

§ Device(chipsetvendors):Freedom¨ Freedom:conductmeas inanyorder;rejectoncefailure

§ Lessons:¨ Verificationrequired(@operator,@device-operator)¨ Easyfixshouldbepossible(NFV,configurationupdatesmadeeasy)

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PossibleFixes§ Device-side:Beamoreproactivelocalcontroller

¨ Self-checkandcorrectifimproperhandoffs¨ Lesson:devicesshouldnotsimplyfollow

§ Network-side:acentralizedcontroller¨ Self-checkandcoordinatehandoffconfigurationsamongcells¨ Makeconfigurationupdateseasy(NFV,ongoing5G)

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Summary§ Firstworktostudyundesiredhandoffreachability

¨ Overlookedinthepast¨ Real-worldcasesreported¨ Rootcausesinmobilitymanagementmisconfigurationsexplored

¨ Makeacallforattentionfromresearchandindustrycommunity

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