lecture 12lecture 12 wireless network operations (2) mobility management + two issues in handoffs 2...
TRANSCRIPT
Lecture 12
WirelessNetworkOperations(2)MobilityManagement
+Two Issues in Handoffs
2
1. Handoff decision and initiation2. Restructuring the on-going connection
old
new(1)
(2)
anchor point
Theanchorpointissomenetworkentitythatis“fixed”andcanbeusedtotracktheMS
+Handoff Decision
n WhenshouldaMSswitchitsconnectionfromthecurrentBStoanewBS?n IftheMSpersistswiththecurrentBSfortoolong,itsconnectioncouldbebrokenn Importantforvoicecallsn Howaboutdata?
n IftheMSpersistswiththecurrentBSfortoolongandkeepsincreasingitstransmitpower,itcouldincreaseco-channelinterferenceorintra-cell/inter-cellinterference
n IftheMSswitchesitsconnectiontoosoon,itmaynotbetherightdecisionanditmayhavetochangeit– increasedsignalingload
n HandoffisthemechanismbywhichaMSmakesaswitchofitscurrentpointofaccess
n Ideally,theremustbeEXACTLYonehandoffatthecelledge
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+RSS based Handoff Decision
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8-140
-130
-120
-110
-100
-90
-80
-70
-60
distance between base stations in miles
rece
ived
sign
al s
treng
th +
unc
orre
late
d lo
gnor
mal
fadi
ng
n Thereceivedsignalstrength(RSS)ismostcommonlyusedinRRMalgorithms
n RSSbasedalgorithmsn Largestrelativesignal
strengthn Relativesignalstrength
withthresholdn Relativesignalstrength
withhysteresisn Relativesignalstrength
withhysteresisandthreshold
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BS1 BS2
+Traditional algorithms
n Received Signal Strength: The base station with the the largest strength isselected (choose BS Bnew if Pnew > Pold).
n Received Signal Strength plus Threshold: RSS of a new BS exceeds that of theold BS, RSS of old BS is below a threshold T (choose Bnew if Pnew > Pold andPold < T).
n Received Signal Strength plus Hysteresis: RSS of new BS is larger than that ofthe old BS by a hysteresis margin H (choose Bnew if Pnew > Pold + H).
n Received Signal Strength, Hysteresis and Threshold: RSS of a new BS exceedsthat of the old BS by a hysteresis margin H, and RSS of old BS is below athreshold T (choose Bnew if Pnew > Pold + H and Pold < T).
n Algorithm plus Dwell Timer: A timer is started at the instant when thecondition in the algorithm is true. If the condition continues to be true tillthe timer expires, a handoff is performed.
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+Handoff Initiation Criteria
6
100 1 2 3 4 5 6 7 8 9
-50
-110
-105
-100
-95
-90
-85
-80
-75
-70
-65
-60
-55
Distance from BSA (km)
Rec
eive
d S
igna
l Str
engt
h (d
Bm)
A
B
C
D
A: RSS OnlyB: RSS & ThresholdC: RSS & HysteresisD: RSS & Hysteresis & Threshold
Mobile is moving from left to right
BSA BSB
Hys
tere
sis
Threshold 1
Threshold 2
+The Ping-Pong Effect
nRepeatedhandoffsbackandforthbetweentwobasestationsnCausedbynotsettingsomeparametersofhandoffslikehysteresisandthresholdwisely
nResultsinlargesystemloads,multipleinterruptionsinvoicetransmissionsandinstability
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+Simple simulations of handoff
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+The Soft Handoff Concept in CDMAn TheMSsimultaneouslymaintainsconnectionwithtwoormorepointsofaccesstothefixednetworkn Eventually,oneofthemispickedasthestrongestconnectionandahandoffismadetothatcell
n WhySoftHandoff?n Positivefeedbackpowercontrolproblem
n Cellsusethesamefrequencyn MSmovesawayfromaBSandkeepsincreasingitstransmitpowern Theinterferenceintheneighboringcellincreases
n ThisforcesMSs intheneighboringcelltoincreasetheirTx power(Howdotheyknowtheyshouldincreasethepower?)
n Theinterferenceincreasesfurther
n SofthandoffguaranteesthattheMSisalwaysconnectedtothestrongestBS
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+The Active Set
n ContainsthepilotchannelsassociatedwiththeforwardtrafficchannelsassignedtotheMSn Usuallythereisexactlyonepilotintheactivesetn Duringhandoff,theremaybemanypilotsinthisset
n Themaximumnumberofpilotsintheactivesetis3n ThisisrestrictedbythenumberofRAKEfingersn Ifpilotscanshareafinger,youcangoupto6pilots
n TheBSinformstheMSoftheactivesetusingtwomessagesn Channelassignmentmessagen Handoffdirectionmessage(HDM)
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+ 11
Handoffs in UMTS
n TheMSmaintainsalistofcellsthatitiscurrentlyusingormaylikelyuse
n ActiveSetn Allcellsthataresimultaneouslyinvolvedinacommunicationduringsofthandoff
n TheMScoherentlydemodulatesthereceivedsignalsfromthesecellsn ItcontainstwoormorecellsinFDDmode,butonlyonecellinTDDmode
n MonitoredSetn Thesearecellsnotintheactiveset,butintheneighborlistandmonitoredbytheMS
n DetectedSetn Cellsnotineithertheactiveorthemonitoredset,butdetectedbytheMSanyways
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Soft HandoffRegion
Basic idea in UMTS
n Thereisapermissiblenumberofcellsintheactiveset
n InitiallyCell1istheonlymemberoftheactivesetn Att1 thedifferenceinpowerbetween
thepilotfromcell1andcell2islessthanathresholdTh1
n Cell2isincludedintheactiveset
n TheMSisnowcommunicatingwithtwocellssimultaneouslyn Att2 thedifferenceinthepilotsbecomes
largerthananotherthresholdTh2n Thepilotfromcell1becomesunusable
anditisdroppedfromtheactiveset
Pilo
t ch
anne
l str
engt
h
time
t1 t2
Th1Th2
Pilot from cell 1
Pilot from cell 2
+Handoff decision algorithms in WiFi
nMostalgorithmsarebasedontheRSS
nMostalgorithmsareproprietaryn InIEEE802.11,mostMSs simplyuseeithertheRSSortheSNRtomakehandoffsnSomevendorsuseload
nTherearenodefinedthresholdsorhysteresismargins
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RRMMM
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+Mobility management
nForamessagetoreachaMSnThereshouldbeaknowledgeofwheretheMSiscurrentlynAroutemustbesetuptotheMSaccordinglyn Locationmanagement
nTheremustbeadjustmentsmadetotracktheMSasitmoveswhileconnectednTheroutemustberedefinedefficientlywhentheMSchangesitspointofaccesstothefixedinfrastructure
nHandoffManagement
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+
Radio NetworkControllers
UE
Mobile Switching Center
BS-A
BS-B
(1)
(2)
(3)
(4)(5)
(6)
HomeDatabase
Old VisitingDatabase
New VisitingDatabase
Generic Mobility Management16
+Steps in Mobility Management (1)
nMobiledecidesandinitiateshandoff(NCHO?)n RSSI,RSSIHysteresis,BLER,etc.
nMobileregisterswith“new”visitingdatabasen Announcementofhandoffn Firstinformationtoanetworkentity
nNewVisitingDatabasecommunicateswithHomeDatabasetoobtainsubscriberprofileandforauthenticationn Firstinformationexchangebetweennetworkentitiesaboutthechangedlocationofthemobile
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LocationManagement
RadioResourceMgt
+Steps in Mobility Management (2)n HomeDatabaserespondstoNewVisitingDatabase
n Authenticationofmobilen Databaseupdateforredirectingmessagesn Newvisitingdatabaseincludesmobileinitslist
n HomeDatabaseasksOldVisitingDatabasetoflushpacketsintendedformobilen Packetsthatmayhavebeenroutedtotheoldvisitednetworkwhilethemobilewasmakingahandoffneedtobedroppedorredirected
n OldVisitingDatabaseflushesorredirectspacketstoNewVisitingDatabasen OldVisitingDatabaseremovesmobilefromitslist
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+IS-41 and GSM-MAP
n Twostandardstohandlemobility
n GSM-MAPn MAP=MobileApplicationpartn UsedbyGSM-likesystems
n IS-41n IS=InterimStandardn UsedbyNorthAmericanSystemsn IS-136andIS-95systemsuseIS-41forsignalingandmobilitymanagement
n MobileIPorvariationsn For3Gand4Gnetworks
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+Comparative Table of Network Elements involved in Mobility Management
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Generic GSM Mobile-IP
HomeDatabase HomeLocationRegister
HomeAgent
VisitingDatabase
VisitingLocationRegistor
ForeignAgent
Registration RegistrationProcedure Registration
HandshakebetweenVisitingandHome
Databases
PrepareHOSS-7
RegistrationactionbyForeignAgent
Flush ClearCommand Noneexists
+Location Management
n AssoonastheMSpowersupithasto“register”or“associate”itselfn Voicesystems
n MSs areNOTconnectedtillthereisacalln ButtheMSneedstobelocatedforincomingcallsifitis“on”whileitisonthemove
n Datasystemsn MSs areconnectedALLthetimen Todeliverpacketsefficiently,theMSmustbelocated
n IftheMSisnotpoweredup,orunreachablethenetworkshouldknowwhattodon Busytone,bufferingpackets,etc.
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+ Components of location management
n Locationupdatesn MessagessentbytheMSregardingthechangingpointsofaccesstothenetwork
n The“anchor”isupdatedwheneverthesemessagesaretransmittedn Theanchorisusuallythehomedatabase
n Pagingn Thelocationupdatesarenotcontinuousn ThereissomeuncertaintyastotheexactcellinwhichtheMSislocated
n TheMSis“paged”todetermineitsexactlocationifthereisanincomingcall
n Locationinformationdisseminationn ProcedurestostoreanddistributethelocationinformationofMSs
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+Issues in location management
n Tradeoffsbetweenn Costofthenumberandfrequencyoflocationupdates
n Costofpaging
n CaseI:n Numberoflocationupdatesistoofrequent
n Numberofincomingmessagesfew
n Networkloadistoocostlyn Signalingconsumesbandwidthn Processinganddatabaseupdates
arecostly
n CaseII:n Numberoflocationupdatesisfewandinfrequent
n AlargerareaneedstobepagedinordertolocatetheMS
n Resourcewasten PagingincellswheretheMSis
notn Delay
n Dependingonhowthepagingisperformed,theresponsefromtheMSmaybedelayed
n e.g.PagingisperformedinthecellinwhichtheMSislocatedafter pagingothercells
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+Location update algorithms (1)
nStaticAlgorithmsnNetworktopologydecidestheupdatesnMostcommonapproachisbasedonthe“locationarea”(LA)conceptnBSsaregroupedintoLAsnEachBSperiodicallybroadcastsitsLAoveracontrolchannel
nWhentheMSdiscoversthattheLAidentifierhaschanged,itwillinitiatealocationupdatewiththenewidentifier
nThedatabasesareupdated
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+ Example of Static Location Updates in GSM
nTheLAinGSMconsistsofallBTSsthatarecontrolledbyaBSC
nLocationupdatesareperformednWhentheMSpowersupanddetectsachangeinLAidentifier
nWhentheMScrossestheboundaryofaLAnPeriodicallyaftersometimeelapsesnThistimeisdeterminedbythenetwork
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+ Problems with static location updates
n IfaMSfrequentlycrossesaLA,itcanleadtotheping-pongeffect
nCommonsolutionn Useadwelltimern Useabiasingfactortodefinethecellboundary
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LA- 1
LA- 2
Location updates
+Location update algorithms (2)
nDynamicAlgorithmsn State-based
n MSdecidestomakeanupdatebasedonitscurrentstaten Stateinformationincludes
n Timeelapsedsincethelastupdaten Distancetraveledn Numberofcallsreceivedn NumberofLA’s crossed
n User-profilebasedn MaintainsasequentiallistofLAs wheretheMSisusuallylocated(e.g.Oakland,Wilkinsburg,Monroeville)
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+Paging
n BroadcastingamessagetotheMSrequiringittorespondinagroupofcellswhereitisexpectedtobelocated
n Blanketpagingn PageallcellsinaLAsimultaneouslyn Responseisreceivedinthefirstpagingcycle– delaysarereducedn UsedinGSM
n Closestcellsfirstn ThecellinwhichtheMSwaslastseenispagedfirstfollowedbythefirstringofcellsandsoon
n Sometimesseveralringsofcellsarepagedsimultaneouslylikeblanketpaging
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+Location information dissemination
n Atthebareminimum,theanchorinthenetworkneedstobeupdatedaboutthelocationoftheMSn AsingleanchorforallMSscanleadtobottlenecksandfailure
n The“homedatabase”maintainsallinformationassociatedwithaMSn TheMSidentificationn Authenticationkeysn Subscriberprofilen Accountingn Locationasdeterminedbythe“visitingdatabase”
n The“visitingdatabase”keepstrackofMSsinitsservingareaandwhathappenstothemn AsingleVDBmayserveseveralLAs
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+Handoff Management
n Twocategoriesofhandoffn Intrasystem handoff(3cases)
n Intra-cellhandoff(differentsectorofsamecell)n Standardhandoff(cellsattachedtosameBSC)n InterBSChandoff(sameMSC)
n Intersystemhandoffn CellsattachedtotwodifferentMSCsn Requirespecializedsignalingn Threecases
n A.HandoffForwardn B.HandoffBackn C.HandofftoaThird
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+Types of Handoff
31
MSC MSC
BSC BSCBSC
BTS BTS BTSBTS
MS MS MS MS
Intracell StandardInter-BSC Intersystemhandoff
+Intersystem Handoff – Handoff Forward
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The situation after a handoff forward from System A (anchor system) toSystem B (serving system).
MSC-B MSC-A
PSTN
+Handoff Back
nAfteraHandoffForwardnFromMSC-AtoMSC-B
nUsermaymovebacktoacellattachedtoanchorMSC-AnUseHANDOFFBACKcommandtopreventcallgoingfromMSC-AtoMSC-BbacktoMSC-Ainwirednetwork
nCalledthe“shoelace”effect
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+Trombone Effect
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SimpleCaseoftwoHandoffForwards– resultsin thecallpathshownaboveafterhandoffforwardtoSystemC.CurrentSolutionisHANDOFFtoaTHIRDcommand
MSC-B MSC-A
PSTN
MSC-C
+Handoff to a Third
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•IftherearecircuitsconnectingMSC-AandMSC-C,thesystemcanperformhandofftothirdwiththisresult.•Yieldsbetterroutesinwirednetwork
MSC-B MSC-A
PSTN
MSC-C
+IEEE 802.11 Mobility Types
nNoTransitionnMSisstaticormovingwithinaBSA
nBSSTransitionn TheMSmovesfromoneBSStoanotherwithinthesameESS
nESSTransitionn TheMSmovesfromoneBSStoanotherBSSthatispartofanewESS
n Upperlayerconnectionsmaybreak(needsMobileIP)
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+Handoff in 802.11
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Beaconperiodically
1.Strongsignal
2.Weaksignal;startscanningfor
handoff
3.ProbeRequest
5.ChooseAPwithstrongestresponse
8.IAPPindicatesreassociationtooldAP
AP2AP1 AP3
IAPP:InterAccessPointProtocol
+Inter-AP Protocol (802.11f)
n APsregisterwitha“RegistrationService”inthedistributionsystemn TheyusetheIAPP-INITIATEandIAPP-TERMINATEtoregisterand
deregister
n AnMSin802.11canbeassociatedwithonlyoneAP
n WhentheMSsendsareassociationrequestandobtainsanassociationframe,thenewAPsendsanIAPP-MOVE-notifypackettotheoldAPn TheoldAPaddressisobtainedfromtheregistrationservicen Iftheregistrationservicecannotbelocated,theAPwillissuean
IAPP-ADD-notifypackettothebroadcastMACaddressontheLAN
n TheoldAPsendsanIAPP-MOVE-responsepacketwithanycontextinformationithadfortheMS
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