class number – cs401

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Class Number – CS401Class Name – Mobile and Distributed Data Access

Instructor – Sanjay Madria

Lesson Title - Location Mgmt

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Personal Communication System (PCS)Personal Communication System (PCS)

A system where wired and wireless networks are integrated for

establishing communication.

PSTN: Public Switched Network.MSC: Mobile Switching Center. Also called MTSO

(Mobile Telephone Switching Office).BS: Base Station.MS: Mobile Station. Also called MU (Mobile Unit)

or Mobile Host (MH).HLR: Home Location Register.VLR: Visitor Location Register.EIR: Equipment Identify Register.AC: Access Chanel.

PSTN

BS

VLR

HLR

EIR

AC

MSC (MTSO)MSC (MTSO)

MSMS Wireless component

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Wireless Components

BS

MSC (MTSO)

MS Wirelesscomponent

MS

Cell

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Mobile cells

The entire coverage area is a group of a number of cells.

The size of cell depends upon the power of the base

stations.

PSTNMSC

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Problems with cellular structure

How to locate of a mobile unit in the entire coverage area?

Solution: Location management

How to maintain continuous communication between two parties in the presence of mobility?

Solution: Handoff How to maintain continuous communication

between two parties in the presence of mobility?

Solution: Roaming

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Handoff

A process, which allows users to remain in touch, even

while breaking the connection with one BS and

establishing connection with another BS.

Old BS New BS

MSC

Old BS New BS

MSC

MSC

Old BS New BS New BSOld BS

MSC

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HandoffTo keep the conversation going, the Handoff

procedure should be completed while the MS (the bus) is in

the overlap region.

G

Old BS New BS

Cell overlap region

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Roaming

Roaming is a facility, which allows a subscriber to

enjoy uninterrupted communication from anywhere in

the entire coverage space.

A mobile network coverage space may be managed

by a number of different service providers. They

must cooperate with each other to provide roaming

facility.

Roaming can be provided only if some administrative

and technical constraints are met.

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Roaming

Administrative constraints

Billing.

Subscription agreement.

Call transfer charges.

User profile and database sharing.

Any other policy constraints.

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RoamingTechnical constraints

Bandwidth mismatch. For example,

European 900MHz band may not be available

in other parts of the world. Integration of a new service provider into the network.

A roaming subscriber must be able to detect this new

provider.

Service providers must be able to

communicate with each other. Needs some

standard.

Mobile station constraints.

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Roaming

Two basic operations in roaming management are

Registration (Location update): The

process of informing the presence or

arrival of a MU to a cell. Location tracking: the process of locating

the desired MU.

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Roaming

Registration (Location update): There are six different

types of registration.

Power-down registration. Done by the MU when it

intends to switch itself off. Power-up registration. Opposite to power-down

registration. When an MU is switched on, it

registers. Deregistration. A MU decides to acquire control

channel service on a different type of network

(public, private, or residential).

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Roaming

Registration (Location update): There are six different

types of registration.

New system/Location area registration: when the

location area of the MU changes, it sends a registration

message. Periodic registration: A MU may be instructed to

periodically register with the network. Forced registration: A network may, under certain

circumstances, force all MUs to register.

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Personal Communication System (PCS)Personal Communication System (PCS)Registration

Two-Tier Scheme

HLR: Home Location Register

A HLR stores user profile and the geographical

location of each moving object at a pre-

specified location

VLR: Visitor Location Register

A VLR stores user profile and the current

location who is a visitor to a different cell than

its home cell.

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Registration

Two-Tier Scheme steps. MU1 moves to cell 2.

MU1

MU1

Cell 1 Cell 2

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Registration

Steps

1. MU1 moves to cell 2. The MSC of cell 2 launches a

registration query to its VLR 2.

2. VLR2 sends a registration message containing MU’s

identity (MIN), which can be translated to HLR address.

3. After registration, HLR sends an acknowledgment back

to VLR2.

4. HLR sends a deregistration message to VLR1 (of cell 1)

to delete the record of MU1 (obsolete). VLR1

acknowledges the cancellation.

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Personal Communication System Personal Communication System (PCS)(PCS)

Location tracking (MU2 wants to comm with MU1)

Steps

1. VLR of cell 2 is searched for MU1’s profile.

2. If it is not found, then HLR is searched.

3. Once the location of MU1 is found, then the

information is sent to the base station of cell

1.

4. Cell 1 establishes the communication.

Locating Objects in Mobile Computing

Location Management

Lookups and updates

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Introduction

• Our target is objects capable of changing their location

• We are interested in objects with identity

• We store user locations in multiple databases (DBs)

• Main questions: How do we update data when user moves? How do we locate user in DBs when it is

required?

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Locating Moving Objects• Example of moving objects

mobile devices (cars, cellular phones, palmtops, etc) mobile users (locate users independently of the device

they are currently using) mobile software (e.g., mobile agents)

• How to find their location - Two extremes Store their current location everywhere

search locally Cost of updates

Search everywhere No information is stored anywhere; search is

expensive No cost of updates

searching verses update cost

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Availability – either at all sites or at selective sites (frequently visited sites)

Currency – Stored location is always updated (it may not make sense if user moves very frequently

Precision – Exact location verses set of possible locations

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Locating Moving Objects

• What (precision), where (availability) and when (currency) to store

Av

aila

bili

ty

nowhere

at all sites

At selective sites (e.g., at frequent callers)

CurrencyNever update

Always update (at each movement)

Precision

Exact location

the whole network

Set of locations

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Overview

• Database schemas Two-tier Hierarchical

• Replication: Working set replication Replication in hierarchical schema

• Forwarding pointers: Two-tier schema Hierarchical schema

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Two Tier Scheme

• HLR – Home Location Register is associated with each mobile user, maintains current location of the user as part of the user’s profile

• HLR is located at a network location pre-specified for each user

• To locate X, X’s HLR is queried• When X is moved to new zone, X’s HLR is

contacted and updated

• Disadvantages : Global move is expensive

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Architectures of Location DBs

• Two-tier Schemes (similar to cellular phones) Home Location Register (HLR): store the

location of each moving object at a pre-specified location for the object

Visitor Location Register (VLR): also store the location of each moving object at a register at the current region

• Hierarchical Schemes Maintain multiple registries

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Two-tier Location DBs

• Search Check the VLR at your current location If object not in, contact the object’s HLR

• Update Update the old (delete) and new VLR (insert) Update the HLR

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VLRi-X|HLRi…

Two-tier Schema- Enhancement• User’s X profile is permanently stored in

Home Location Register (HLR)

• Each site maintains Visitor Location Register (VLR), stores info about users not at their home location

• During lookup at VLRi: VLRi is queried for X location HLR of X is queried upon failure

• During move from i to k: X’s HLR is updated X’s profile is deleted from VLRi

X’s profile is added to VLRk

VLR…|HLR-X

VLRk-X|HLRk…VLRi…|HLRi…

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Two-tier Schema Cont.

– Does not support locality search in nearby locations

impossible, always need to reg. with HA

possible distant HLR is always contacted upon move

– Home Location register is permanent resettlement is not supported Does not scale well; Home

location is always contacted

+ Relatively simplemax 2 operations for lookup3 operations for update

VLRi-X|HLRi…

VLR…|HLR-X


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Standards

• EIA/TIA (Electronic Indus Asso.) and GSM(Global System for mobile Comm.) – Use HLR and VLR

• Mobile-IP - Two IP addresses – Home address and Care-of –Address (Current point of attachment)

• Care-of-address is either the address of FA or IP address acquired by the node in the current network

• Mobile node registers its care of address with its home address

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Hierarchical Schema• A hierarchy of location

databases is maintained• Internal node maintains

information about user registered in the set of zones in its subtree

• Leaf node contains actual location of objects in its coverage

• Intermediate node contains location information for all objects covered by its children in a form of:

Pointers to lower level DBs

or Actual location of each object

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1X

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X 12 Y 16

Y 16

Y 16

Y 16

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Hierarchical Scheme• LCA(I,j) – least common ancestor of nodes I and j• Parameter that affect the performance of the most location

management scheme Relative frequency of the move Call operations of each user

• Call to mobility ratio (CMRi) = Ci/Ui

Where Ci is the expected number of calls to user Pi over a period T and Ui the number of moves made by Pi over a period T

• LCMRij (local call to mobility ratio involves origin of calls)

= Cij/Ui

• For hierarchical scheme, LCMRij = LCMRik where k is a child of j; the call to mobility ratio for a user Pi and an internal node j is the ratio of the number of calls to Pi originated from any zone at J’s subtree to the number of moves made by Pi

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Lookup in Hierarchical Schema• Whenever lookup for object X at

ii is initiated at jj:• The tree is traversed from jj

upwards to LCA(i,j)

Then• Pointers are traversed

downwards from LCA(i,j) to ii • Location of X is found at ii

(pointer case)

OR• Location of X is found in

LCA(i,j)

(actual location case)• LCA(i,j) – Least Common

Ancestor of ii and jj• Example: (j,ij,i)) = {(8,12), (19,16)}

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1X

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X

X 12 Y 16

Y 16

Y 16

Y 16

LCA(8,12)LCA(19,16)

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Moving in Hierarchical Schema• Whenever object X moves from

jj to ii:• Pointers on the path jj,

…,LCA(j,i),…ii are altered

OR• All databases on paths root,…,jj

and root,…,ii are updated with new location of X

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Y 16

X Y

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14

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updates

• Pointer case - When user x moves from 15 to 18, the entries at 18,7, 6, 3, 15 are updated ; x is deleted from the database at 15 and 6, at 3 it is updated and are added at 18 and 7

• Actual location case – entries are updated at 0, 7,6,3,18,15

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Evaluation of Hierarchical Schema

+ Mobile object is not bound to HLR

+ Advantage of locality moves and lookups is taken

– Increased number of operations DB operations

Communication messages

– Increased load and storage requirements for DBsIntermediate DBs store location information for all objects covered by its children

Root DB stores location information for ALL objects

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Partitions

• Avoid maintaining locations at all levels, reduce search cost

• Partitions grouping zones for each user among which it moves frequently

• Partition exploits locality• Information whether a user is currently in the partition is

maintained at the LCA of all nodes in partition called representative of the partition

• Representative does not know exact location• Reduces overall search cost, increases update cost when

a user crosses partition, both representatives must be informed

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Partitions

P1 P2

P3

P4 P5

User x User x

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Overview




• Other topics• Relation to KDE3 project• Evaluation

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• Caching cache the callee’s location at the caller side

(large Call to Mobility Ratio)• Replication

replicate the location of a moving object at its frequent callers (large CMR)

• Forwarding Pointers do not update the VLR and the HLR, leave a forwarding

pointer from the old to the new VLR (small CMR) When and how forwarding pointers are purged?

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Caching – Two Tier Scheme

• Current location of the callee may be reused by subsequent calls originated from same region

• Every time a user x is called, its location is cached at the VLR in the caller’s zone so it can be reused

• Caching Is useful for those users who frequently receives calls relative to the rate at which they relocate

• To locate a user , the cache at the VLR of the caller’s zone is queries first, if found then query is launched to the indicated zone without contacting the user’s HLR else HLR is queried

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Caching – Hierarchical scheme• When a call is made from zone i to user x located at zone j, the search

traverse from i to LCA (i,j) and then down to j• Ack is returned back to i from j• Forward and Reverse bypass pointers

Forward bypass is an entry at an ancestor of i, say s, that points to ancestor of j, say, t

Reverse is from t to s• During the next call from zone i to user x, search will travel until s then

follow t via LCA (i,j) or via a shorter path• In case of simple caching, s and t can be at the leaf while in level

caching, s and t belongs to any level and possibly to a different one.• Placing a bypass at higher level node s makes this entry available to

all calls made in s’s subtree, but will travel long path to reach s• Placing high node t will increase lookup cost, cache entry remains

valid as long as the user move’s inside t’s subtree

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Cache Invalidation• Eager caching – Every time a user is moved new location,

all cache entries for this user’s location are updated • Location of the cache entries must be centrally known,

failure of central location can cause problem• Lazy caching – a move operation does not mean updating

cache• When look up, either the user is still in the indicated

location or it has moved out (cache miss) Cache miss- HLR is contacted and then cache is updated Cache update only on cache miss Overhead – cache location must be visited first

• Saving of lazy caching over eager caching if hit ratio threshold for a user in a zone must exceed the (cost of lookup when there is a hit) / (the cost of lookup in non-caching scheme). Depends on querying HLR and VLR

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Replication

• Location (profile) of selected users is replicated at selected sites:

Enhances lookup response time Reduces network load during lookup Creates overhead during updates

• Replication is judicious if following holds:

number of lookups for object i from location j during T number of updates for i during T - savings per lookup; - cost per update

• Additional parameters: service capacity of DBs, etc.• Replication is possible at both caller and receiver locations

(1)jiC ,

iU

iji UC ,

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Working Set Replication• Applicable to two-tier schema• Replicas are kept at frequent callers of X

– working set of X• Equation holds for every member j of the

set:

• Every time a call to X is made: From a member of a set –

no updates required

From nonmember k of a set –

if (1) holds for k, k is added to the set

• Every time X moves: (1) is evaluated for each member of X

working set If (1) does not hold for member k, it is

removed from the set

(1)xjx UC ,

1

3

4

5

2

8

7

9

X1

5

6(1)? 66

X

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Replication in Hierarchical Schema• Takes advantage of locality in

movement• Local Call to Mobility Ratio is

used to determine feasibility

• and are thresholds for determining replication of nodes

• If , j is assigned a replica

• If , j is not used for replication

• If , database depends on topology of network

k U

CLCMR

i

ki

ji

,

,

maxS minS

max, SLCMR ji

min, SLCMR ji

maxmin , SLCMRS ji 18 19 20

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12 13 14

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8 9 10

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2 3

1

…

+ +

+

+

Ci 8, Ci 14,i

U

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Overview




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VLRi-X|HLRi…

VLR…|HLR-X


VLRi-X |HLRi…

VLR…|HLR-X

VLRk-X |HLRk…

Forwarding Pointers (two-tier schema)• Pointers could be used to reduce

communication overhead and query load at the HLR

• When X moves from ii to jj a pointer from VLR at ii to VLR at jj is added

• During lookup if no information on X is found at current VLR, HLR of X is queried and pointers are followed

• Chain of pointers is managed not to exceed length K

• Useful for users receiving calls infrequently and reallocate often

• It has been showed that cost savings for K<5, are 20%-60%

5.0, iki UCVLRn-X|HLRn…

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Forwarding Pointers (hierarchical schema)• Updating and lookup in

hierarchical schema lack efficiency when LCA(i,j) is at higher levels

• Consider schema with entries as pointers to lower level DBs

• In simple forwarding a forwarding pointer connects two leaf nodes

Allows cheaper updates

• In level forwarding a forwarding pointer connects two intermediate nodes

Allows cheaper lookups18 19 20

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LCA(12,15)

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Simple forwarding

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Level forwarding

class number – cs401

Documents

continuous communication

uninterrupted communication

registration message

periodic registration

forced registration

location managementhow

location tracking

mu changes