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

3

+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

4

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

8

+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

12

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

13

RRMMM

14

+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

20

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

25

+ Problems with static location updates

n IfaMSfrequentlycrossesaLA,itcanleadtotheping-pongeffect

nCommonsolutionn Useadwelltimern Useabiasingfactortodefinethecellboundary

26

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

32

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

34

SimpleCaseoftwoHandoffForwards– resultsin thecallpathshownaboveafterhandoffforwardtoSystemC.CurrentSolutionisHANDOFFtoaTHIRDcommand

MSC-B MSC-A

PSTN

MSC-C

+Handoff to a Third

35

•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)

36

+Handoff in 802.11

37

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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