locn-mgmt 12

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Location management inMobile Networks

By

C.Manoj KumarT. Aswin KumarB. Anuradha

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Location managementInvolves two basic operations

PagingSearch by system to track the mobileMSC broadcasts message on the FCCTarget replies in the reverse channel

UpdateUpper bound on the location uncertaintyMobile sends update message on the reversechannel

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IssuesCost of Communicating with mobile user isaugmented by ― Cost of Searching for the Currentlocation of the user .

Paging costNumber of calls arrived & Number of cells paged

Update costNumber of times mobile updates

Trade-off : More the paging, less the updateRequirement for an optimal algorithm

Call routed with allowable time constraintLess information exchange

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Update SchemasThe vicinity of the last update information gives themost probable location of the mobile terminalWhen to update?Static

Partition of cells in LAsNon-overlapping grouping of cellsMobile updates when it crosses an LA boundaryDrawback: traffic generated only on theboundary cells, reducing BW availability forother calls

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Update SchemasSelection of designated reporting cells

Mobile must update in some designated cellsOptimal set of reporting cells is NP-Complete

Dynamic – based only on user‘s activity Distance based

Updates when Euclidean distance crosses a threshold DDistance can be specified in terms of cells covered

Movement basedUpdates when number of cell boundaries crossedreaches a threshold M

Time basedMobile sends periodic updates

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a b c

hg

f e

d

Dynamic Update Schemes

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Location DatabasesDistributed DBs used to store thelocation of mobile usersTypes of Architectures

Two-tierHierarchicalRegional Directories

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Two-Tier ArchitectureUsed in GSM

A home DB called ― Home Location Register (HLR) isassociated with each mobile user

―Visitor Location Register (VLR) is maintained ateach zoneHLR

Located at a pre-specified zone for each userMaintains the current location of the user as partof the ―User Profile To locate user ‗x‘, x‘s HLR is identified and queried When ‗x‘ moves, contacts HLR and updates it to

―New Current Location

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VLRstores profiles of users not at home location andcurrently located inside it‘s area. When call is placed from zone ‗i‘ to user ‗x‘

Query for ‗x‘ in i‘ s VLR If ( not found )

contact x‘s HLR When ‗x‘ moves from zone ‗i‘ to zone ‗j‘

Delete entry ‗x‘ from i‘s VLR Add new entry ‗x‘ to j‘s VLR

Update x‘s HLR

Two-Tier Architecture (contd..)

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Design Choice in GSMcompromise solutionused in GSM is to groupcells into ―Location

areas

Updating messages arerequired when moving

between location areas,and mobile stations arepaged in the cells oftheir current locationarea.

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Mobility Management (contd..)Location updating

When mobile powered ‘ON’ Performs update indicating it‘s IMSI( I nternational Mobile Subscription I d )

The above procedure called ― IMSI Attach Procedure ― When moves to new Location Area or a Different PLMN

Update message sent to new MSC/VLRIf MS authorised in the new MSC/VLRthen

subscriber‘s HLR updates the cuurent locationsends a message to the old MSC/VLR to cancel it‘s VLR entry

Periodic Location UpdatingIf after the updating time period, the mobile station has not registered, it is thenderegistered

When a mobile station is powered offit performs an “ IMSI detach procedure ” in order to tell the network that it is no longerconnected.

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MSC 1

To otherMSCs

HLR V

LR

MSC 2

HLR V

LR

MSC 3

HLR V

LR

USER INFO CL

A MSC2

USER INFO

A

USER INFO

A

VLR

VLR

HLR

Fixed Network

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Drawbacks of HLR/VLR Assignment of HLR to a mobile ispermanent

users permanently shifted to differentregion still contact the same HLRScalability

Not scalable to highly distributed systemsLocality of mobility is not takenadvantage of

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Static LMTLocation mgt involves

Updates

SearchesSearch-updates A LMS is a combination of all the abovestrategies

Cost of LMS includes number, size and thedistance message needs to travel An efficient LMS should attempt to minimizethe above cost

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System ModelMobile networks comprise

. Static backbone network

. A wireless networkTwo distinct set of entities exist.

. Mobile hosts

. Fixed hostsNotion of Mobile Support Station(MSS) is there whichstands as a gate way between wireless and staticnetwork.

A Mobile host can communicate with only one MSS ata time.

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Logical Network Architecture.Each LMS assumes following logical architecture for the system -

• MSS at the leaves

• Location Server at the intermediate nodes• Every Mobile host(MH) will have an unique identifier called Home Address(HM). But the Physical addresses change

The aim of the location management strategy reduces toefficiently maintaining HM-PA mappings

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Data Structures .Every node (MSS/LS) maintains the information ofmobile hosts as three tuples-

(h-id, fp-dest, fp-time)

. h- id : Mobile host identifier (Home Address). fp-dest : Forwarding pointer indicating dest.

. fp-time : Last time where forwarding pointerupdate took place.

At the MSS, the fp-time value of a host residing in itscell is NULL.Initial conditions: Every LS should have correctinformation regarding the MHS residing in itssubtree.(Root contains information about all MHS).

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Update ProtocolsStrategies for updating the location information at MSSand LS when MH moves

LAZY-UPDATE(LU)

- Simple, updates take place only at the source and dest.- S-{h, D, t} , D-{h, dest, NULL)- Cost of update is ZERO.

FULL UPDATE(FU)- Location updates take place in all the LS‘s located on the

path from Source and destination cells to the root.- Cost is 2(h-1) where h is the height of the tree.LIMITED UPDATE(LMU)

- Update take place in the LS‘s whose height is less thanm(m<H).

- Cost is 2(m-1).

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Full Update algorithm (FU)Source cell

At the MSS: for host h set fp_dest = dest andfp_time =tThe MSS of src sends update messages to all theLocation Servers upon which an entry (h,dest,t1)is added (t1 – local time of the location server)

Destination cell At the MSS: An entry (h,dest,NULL) is added forhost hMSS of the dest sends update messages to all thelocation servers upon which an entry (h,dest,t1) is

added (t1 – local time of the location server)

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Search ProtocolInitially, the search_cell is the source cell C STEP 1 :

If the MSS of the search_cell has an entry for h1If fp_dest= search_cell, search completed.else search_cell = fp_dest. Repeat step1

Else Forward the query to the next higher level LS.

STEP 2 :If the location server has an entry (h1, fp-dest, fp-time) for h1

search_cell = fp_dest. Go to step 1Else If the location server is the root,

Root broadcasts to find out the location of h1.Else Forward the query to the next higher level LS along the root andgo to STEP1.

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Search Update ProtocolsMake sense to perform updates at certain locationservers after a successful search

Example

There are three search-update protocols .

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Search-update protocols cont.NO-UPDATE(NU)

. As name indicates, no update-action will be taken aftersuccessful search.

. But, the fp-time field value updated in the search path.JUMP UPDATE(JU)

. Location update takes place only at the callers MSS.

. Update cost is 1.

PATH COMPRESSION UPDATE(PCU). Location update takes place at all the nodes in the searchpath.

. Cost is the length of the search path.

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Purging of the forwarding

pointers.There is a need to purge the forwarding pointers for tworeasons.

- Save storage

- Avoid storing stale informationSo, the forwarding pointers are purged periodically.

THE PROBLEM ! As LU and LMU do not update forward pointers at higher levels,they will be purged as they become stale. But the problem isthat root LS need to do frequent broadcasts.The solution is that, along with purging, forwarding pointers atthe LS on the path to the root from the current MSS must beupdated periodically.

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SimulationsLocation Management Strategy can be simulatedon two type of environments.

Environment where the moves of mobile and also the calls

for it are arbitrary.Environment where short moves and a set of callers for amobile is assumed.(A realistic assumption).

PARAMETERS FOR THE TWO TYPES.Time b/n the moves of a host and calls for it are assumed

to be an exponential distribution with mean C and Mrespectively.Height of the tree is chosen based on some probabilitydistribution function

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ObservationsType I

FU - LMU : high cost of updates upon each moveLU – NU : high search cost

LU – JU : update on successful search does not reduce search costType II

LU – JU outperforms all the other strategies as employing JU atthe caller‘s end reduces the search cost

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Finding mobility and call

frequenciesFINDING MOBILITY.

A system parameter MTMI(Maximum Threshold Move Interval)is defined. Let Delta be the average time interval b/n successivemoves.

Then if Delta<MTMI, the host is fast moving otherwiseslow moving.FINDING CALL-FREQUENCY.

Again here, MTCI(Maximum Threshold Call Interval) is defined.Let Delta1 be average time interval b/n successive calls for thehost.

Then if Delta1<MTCI, then the host is frequent callerotherwise in-frequent caller.

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Adaptive algorithmIf(host makes a lot of long moves)

Employ LU-PCelse if(Frequent caller and low mobility)

Employ LU-JUelse if(Frequent caller and high mobility)

Employ LU-PCelse if( Not frequently called and high mobility)

Employ LU-JU.else Employ LU-PC

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Pros and Cons of Hierarchical Approach

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ConclusionsThe Location DBs must support very highupdate rates.

Various enhancements to these approachesinclude caching, replication etc..,Some approaches also try to predict theprobability of a user to be in a given cell

based on it‘s mobility pattern. Support of advanced queries involving thelocation of moving objects is an ongoingresearch topic.

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ReferencesStatic and Adaptive Location Management in MobileWireless Networks, P. Krishna, Nitin H. Vaidya andDhiraj K. Pradhan,

LeZi-Update: An Information-Theoretic Approach toTrack Mobile Users in PCS Networks, MOBICOM'99,SeattleLocating Objects in Mobile Computing, EvaggeliaPitoura and George Samaras,Efficient and Flexible Location ManagementTechniques for Wireless Communication Systems, JanJannik, Derek Lam, N Shivakumar, Jennifer Widom ,Donald C. Cox

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