1. Location Management for Mobile Computing
Sandeep Gupta
Arizona State University
Department of Computer Science
The Hong Kong University of Science & Technology
November 9, 1998

2. Tutorial Outline 1. Introduction - Technical Issues & Technologies
- New (database) Applications
2. Location Information Services
3. Location Management
4. Location Based Queries
5. Summary
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3. Introduction

4. Motivation Why do we care about mobile database management?
Information is the core of business success.
The Internet has changed the way information is accessed.
Wireless networks are in place.
Mobile devices are available in the market.
Users like to go mobile and get ubiquitous access of information.
New applications requiring data management
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5. Mobile Applications
Expected to create an entire new class of Applications
new massive markets in conjunction with the Web
Mobile Information Appliances - combining personal computing and consumer electronics
Applications:
Vertical: vehicle dispatching, tracking, point of sale, information service (yellow pages), Law enforcement
Horizontal: mail enabled applications, filtered information provision, collaborative computing…
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6. Medical Example 911 Call Ambulance arrives/departs Closest hospital
Access patient records
Send vital signs
Update patient records
Page hospital personnel
Order medical supplies
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7. Party on Friday Update Smart Phone’s calendar with guests names.
Make a note to order food from Dinner-on-Wheels.
Update shopping list based on the guests drinking preferences.
Don’t forget to swipe that last can of beer’s UPS label.
The shopping list is always up-to-date.
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8. Party on Friday
AutoPC detects a near Supermarket that advertises sales.
It accesses the shopping list and your calendar on the Smart Phone.
It informs you the soda and beer are on sale, and reminds you.
that your next appointment is in 1 hour.
There is enough time based on the latest traffic report.
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9. Party on Friday
TGIF…
Smart Phone reminds you that you need to order food by noon.
It downloads the Dinner-on-Wheels menu from the Web on your PC with the guests’ preferences marked.
It sends the shopping list to your
CO-OP’s PC.
Everything will be delivered by the time
you get home in the evening.
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10. Wireless Networks Cellular - GSM (Europe+), TDMA & CDMA (US)
FM: Kbps; Digital: Kbps (ISDN-like services)
Cellular Subscribers in the United States:
90,000 in 1984; 4.4 million in 1990; 13 million in 1994; 120 million in 2000; million by 2004 (Cahner In-State Group Report).
Handheld computer market will grow to $1.77 billion by 2002
Public Packet Radio - Proprietary
19.2 Kbps (raw), 9.6 Kbps (effective)
Private and Share Mobile Radio
Paging Networks – typically one-way communication
low receiving power consumption
Satellites – wide-area coverage (GEOS, MEOS, LEOS)
LEOS: 2.4 Kbps (uplink), 4.8Kbps (downlink)
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11. Wireless Networks (Cont.)
Wireless Local Area Networks
IEEE Wireless LAN Standard based systems, e.g., Lucent WaveLan.
Radio or Infrared frequencies: 1.2 Kbps-15 Mbps
Packet Data Networks
ARDIS
RAM
Cellular Digital Packet Data (CDPD)
Private Networks
Public safety, UPS.
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12. Wireless Local Area Network
Data services: IP packets
Coverage Area: Offices, buildings, campuses
Roaming: Within deployed systems
Internet access: via LAN.
Type of services: Data at near LAN speed.
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13. Wireless Data Data can be everywhere: Air! Computers LAN Internet
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14. Recent History of Wireless Data
1970s: First use of mobile data systems in police cars.
1980s: IBM/Motorola build ARDIS data network.
1990s: RAM Mobile Data Network.
1990s: CDPD packet network.
1990s: GSM cellular systems support circuit-switched data access.
1998: CDMA PCS systems support 14.4 circuit-switched data access.
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15. If the history repeats itself
Wireless data will be booming pretty soon!
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16. Mobile and Wireless Computing
Goal: Access Information Anywhere, Anytime, and in Any Way.
Aliases: Mobile, Nomadic, Wireless, Pervasive, Invisible, Ubiquitous Computing.
Distinction:
Fixed wired network: Traditional distributed computing.
Fixed wireless network: Wireless computing.
Wireless network: Mobile Computing.
Key Issues: Wireless communication, Mobility, Portability.
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17. Mobile Network Architecture
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18. Wireless characteristics
Variant Connectivity
Low bandwidth and reliability
Frequent disconnections
predictable or sudden
Asymmetric Communication
Broadcast medium
Monetarily expensive
Charges per connection or per message/packet
Connectivity is weak, intermittent and expensive
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19. Portable Information Devices
PDAs, Personal Communicators
Light, small and durable to be easily carried around
dumb terminals [InfoPad, ParcTab projects],
palmtops, wristwatch PC/Phone, walkstations
will run on AA+ /Ni-Cd/Li-Ion batteries
may be diskless
I/O devices: Mouse is out, Pen is in
wireless connection to information networks
either infrared or cellular phone
specialized HW (for compression/encryption)
Li-Ion (Lithium-Ion)
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20. Portability Characteristics
Battery power restrictions
transmit/receive, disk spinning, display, CPUs, memory consume power
Battery lifetime will see very small increase
need energy efficient hardware (CPUs, memory) and system software
planned disconnections - doze mode
Power consumption vs. resource utilization
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21. Portability Characteristics
Resource constraints
Mobile computers are resource poor
Reduce program size – interpret script languages (Mobile Java?)
Computation and communication load cannot be distributed equally
Small screen sizes
Asymmetry between static and mobile computers
Query By Icons (QBI): Iconic visual Language [Massari&Chrysanthis95]
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22. Mobility Characteristics
Location changes
location management - cost to locate is added to communication
Heterogeneity in services
bandwidth restrictions and variability
Dynamic replication of data
data and services follow users
Querying data - location-based responses
Security and authentication
System configuration is no longer static
bursty network activity during connections
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23. What Needs to be Reexamined?
Operating systems
File systems
Data-based systems
Communication architecture and protocols
Hardware and architecture
Real-Time, multimedia, QoS
Security
Application requirements and design
PDA design: Interfaces, Languages
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24. Query/Transaction Processing
Concern shifts from CPU time and network delays to battery power, communication costs, and tariffs
Updates may take the form of long-running transactions
nodes may continue in disconnected mode
need new transaction models [Chrysanthis 93, Satya 94]
Context (location) based query responses
Consistency, autonomy, recovery
Providing uniform access in a heterogeneous environment
Design of human-computer interfaces (pen-based computing)
Dynamic system info: Location information, user profiles
prioritization of actions upon reconnection
PDAs are unreliable
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25. Recurrent Themes Handling disconnections (planned failures?)
caching strategies
managing inconsistencies
Delayed write-back and prefetch: use network idle times
increases memory requirements
Buffering/batching: allows bulk transfers
Partitioning and replication
triggered by relocation
Compression: increase effective bandwidth increases battery power requirements
Receiving needs less power than sending
Caching: what, when and how
charging the laptop with "local" info
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26. Everybody is ready!
End users: they want to access to data ubiquitously.
Wireless networks: there are many choices of digital wireless data networks.
Mobile devices: they are wireless-ready today.
Applications: Internet standards are in progress to facilitate developments of wireless applications.
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27. Location Information Services

28. Location Information Services:Types
Location-Transparent
Location-Tolerant
Location-Aware
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29. Location-Transparent Services
Hides the effect of mobility to applications and users.
Network services and resources can be transparently accessed by means of a resource and service broker functions which map the application’s service type requests on adequate service provider instances.
Application defined quality-of-service (QoS) for the underlying network connections is sustained through a QoS manager.
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30. Location-Tolerant Services
Allows applications and users to tolerate the effects of mobility that can not be hidden by the platform.
A trader function allows the application to perform a service and service type re-negotiation to achieve a graceful degradation instead of dumb service termination.
A profile handler function allows to retrieve user and terminal characteristics to perform application adaptation according to terminal type currently being used.
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31. Location-Aware Services
Allows applications and users to be aware of their mobility and the absolute and real physical positions of real-world objects.
Applications can exploit this information for customizing their functionality and users can benefit from this information for navigation purposes.
This abstraction level is realized by the location information server (LIS) function that allows to query location information and to be notified about the occurrences of predefined location-related events.
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32. Platform for Mobile and Location-Aware Applications
APIs
mobility
manager
mobile application platform
resource
& service
broker
QoS
location
manger
server
profile
handler
wired & wireless
networks
locating
infrastructure
mobile and location-aware
multimedia applications
mobility-management services
platform access protocols
mobility
management
signalling
protocol
generic support functions
Infrastructure and network
interfaces
Mobility Management Domain
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33. Location Information Server
profile
handler
directory
database
Location information server (LIS)
event
location-aware
application
LIS API
other
midlleware
components
directory access protocol
other APIs
to remote platforms
locater
Infrastructure to locate physical objects
Acquires information about the – absolute or relative – physical location of real world objects in which an application is interested.
Hides from applications which locating technology is being used.
Has map and relationship knowledge to translate the low-level position information from the locating infrastructure into location information having meaningful abstraction level for application.
Application can query the LIS about current location of objects (LIS directory database) or can request to be notified when certain location-related conditions between objects locations are fulfilled (LIS event handler).
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34. A Generic Locating Model
LIS locates objects (identified by object-ID) representing either persons or resources inside areas (identified by area-ID).
Objects have a tag (identified by tag-ID) attached to them that identifies and localizes them.
Resources can serve as tags e.g. badges, cordless telephones, PDAs, or laptops.
A LIS:
maintains relations between objects and tags.
provides graphical map in human-readable format and calculates shortest paths between areas.
finds objects that are nearest to a specified area and fulfill certain conditions.
supports both querying and notification services.
A locator provides the interface between locating infrastructure and LIS.
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35. location information server
LIS Data Model
database/
event handler
application
maps
relations
position  area
sensor  area
object  tag
object  area
tag  sensor
tag  position
tag
sensor
locator
positioning system
location information server
Locating rules:
A tag is located within an area by means of sensors installed in that area. This information is collected by the locator and published to LIS, which uses a map to translate sensor-ID into area-ID.
A tag may determine its absolute geographical position (e.g. using GPS) and publish this through the locator to LIS, which uses map to translate geographical location into area-ID.
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36. LIS Service Types Location retrieval service
Location-dependent object and area selection service
Event notification service
Map retrieval service
Identifier mapping service
Relationship service
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37. Location Retrieval Service
Provides physical location of static/mobile object with/without tags, e.g.
In which area is person A (and when was her last sighting)?
How many (or which persons) are present in area B?
Where is equipment C (e.g. printer) in area B?
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38. Location-dependent Object and Area Selection Service
Provides type or class oriented location information, e.g.
Which object of type D is present in area B?
Which one is the nearest object of type D (relative to my own location) and where it is located?
Which (nearest) area fulfils certain condition (e.g. which one is the nearest unoccupied meeting room)?
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39. Event Notification Service
Avoids repeated polling the LIS by an application. This functionality is, e.g., needed to start a function on a mobile device as soon as it enters a certain area. Examples of events are
Inform me when person A enters (or leaves) area B!
Inform me when area B is empty!
Inform me next time person A meets person B anywhere!
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40. Map Retrieval Service
To help users (of navigation based applications) in finding others persons, resources, or areas by providing maps and navigation instructions, e.g.
Provide me with with a (human-readable) map of area A!
Provide me with the shortest-path between area A and area B!
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41. Identifier Mapping Service
Provides mappings between identifiers of areas, sensors, and positions
to support internal LIS operations,
for other support functions of mobile application platform, and
for location-aware applications operating at tag-level.
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42. Relationship Service
Provides relationship information between objects and tags for applications operating at tag level instead of object level, or for applications that want to communicate with objects through its tag, e.g.
Which object is associated with tag A?
Which (if any) tag does object B posses ?
Associate tag A with objet B!
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43. LIS: Implementation Issues
DBMS-based or Directory-based (such as X.500 [Maas MONET 98])
LIS do not require full functionality of DBMS
Should support distributed implementation on multiple platforms
Communication with external locator
Dramatically increases search time
Minimize the interaction with external locator using
Precise search filters
Location query optimization techniques
Event Monitoring
Active Databases
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44. Location Management Schemes

45. Location Management: Context
Mobility Management: Enables users to support mobile users, allowing them to move, while simultaneously offering them incoming calls, data packets, and other services.
Types of mobility:
Terminal mobility: ability of terminal to retain connectivity with the network so that all on-going communication services remain active despite terminal’s migration.
Personal mobility: disassociates user from the terminal (e.g. in GSM a mobile station = mobile terminal + smart card with subscriber identification module (SIM)).
Service mobility: provides continuous service to mobile clients across multiple administrative domains.
Consists of:
Location management: tracking mobiles and locating them prior to establishing incoming calls (deliverying pending messages).
Handoff management (a.k.a. automatic link transfer): rerouting connections with minimal degradation of QoS.
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46. Location Management Problem
In static networks, a terminal’s network address serves two purposes:
End-point identifier
Location identifier
Mobility prevents using a single address for both purposes
Both end-point identifier and location identifier are needed.
Location management keeps mapping between an end-point identifier and its location identifier
Basically a directory problem.
Two primitive operations:
Lookup (a.k.a. search/find/paging/locating) operation: is the procedure by which the network finds the location of the mobile.
required when a call (message) to a user is placed (to be delivered)
Update (a.k.a tracking/move/registration) operation: is the procedure by which the network elements update information about the location of the mobile.
required when a user changes its “location”
The information gathered during updating/tracking is used during the locating operation
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47. Location Management: Issues
More precise location needs to be maintained as cell size shrink:
Wide area cells are 10’s – 100’s km in diameter
Macro-cells: 1-10 km
Micro-cells: 100’s m
Pico-cells: under 10 m
Database issues in tracking mobile users:
Maintaining update intensive location information
Strategies to reduce location query latency (such as replication) and traffic (such as caching)
Consistency between replicas; Cache management polices
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48. Location Management: Schemes
Several schemes have been developed which are motivated by fundamental trade-off between search operation cost and update operation cost.
Schemes which try to minimize one cost tend to increase the other cost
Try to optimize the aggregate cost or normalized cost.
Categorization:
Update Scheme: Static or Dynamic
Static update scheme: registration areas
Dynamic update scheme: distance/time/movement based strategy
Locating Scheme: Static or Dynamic
Static location scheme: page all the cells in the network
Dynamic location scheme: expanding ring search centered at last reported location of the the user
Database Architecture: Flat or Hierarchical
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49. Selection of LM Schemes
Cost of location updates and lookups
Maximum service capacity of each location database =
the maximum rate of updates and lookups that each database can service
Space restrictions (size of the location database)
Type and relative frequency of call to move operations (call-to-mobility ratio (CMR))
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50. One-Tier Scheme
A home database, called Home Location Register (HLR) is associated with each mobile user.
The HLR of a user x maintains the current location of x as part of x’s profile.
To locate a user x, x’s HLR is identified and queried.
When a user x moves to a new cell, x’s HLR is updated.
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51. Two-Tier (Basic) Scheme
Visitor Location Registers (VLRs) are maintained in each zone (registration area).
VLR in a zone stores copies of profiles of users not at home and currently located in that zone.
When a call is placed from cell i to user x, the VLR at cell i is queried first, and only if the user is not found there, is x’s HLR contacted.
When user x moves from cell i to j, in addition to updating x’s HLR, the entry of x is deleted from VLR at cell i, and a new entry for x is added to the VLR at cell j.
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52. Two-Tier Scheme: Standards
Many current and proposed standards use this scheme:
Electronics Industry Association Telecommunications Industry Associations (EIA/TIA) Interim Standard 41 (IS 41) - commonly used in North America.
Global Systems for Mobile Communications (GSM) - used in Europe.
Internet Engineering Task Force (IETF) Mobile IP protocol
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53. Enhancements to Basic Scheme
Per User Location Caching (dynamic replication) [Jain et. al., IEEE JSAC 12(8), ‘94]
reduces search (lookup) cost
increases update cost
exploits locality in call pattern
(Static) Replication
Per User Profile Replication [Shivakumar & Widom Mobicom’95]
Working Set Replication [Rajagopalan et. al., Mobicom’95]
Forwarding Pointers [Jain & Lin, Wireless Networks 1, ’95]
reduces update (move) cost
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54. Per User Location Caching
Basic Idea:
Every time user x is called, x’s location (or a pointer to this location) is cached at the VLR in the caller’s zone.
Any subsequent call to x originating from that zone can use this information:
Upon call origination the cache at the VLR of the caller’s zone is checked before querying the callee’s HLR.
Issues:
cache replacement schemes (LRU can be used)
cache invalidation schemes
eager caching or lazy caching
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55. Eager and Lazy Caching Eager Caching Lazy Caching
Every time a user moves to a new location, all cache entries for this user’s location are updated.
The cost of move operations increases for those users whose address are cached.
Lazy Caching
the cached pointer for any given user is updated only on a cache miss
for lazy scheme to work better than basic scheme p  CH/CB where p is the hit ratio, CH is the cost of a lookup when there is a hit and CB is the cost of lookup in the basic scheme.
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56. Replication
To reduce the lookup cost, the location of specific users is replicated at selected sites.
Let
 : cost savings when local lookup succeeds as opposed to a remote query,
 : cost of updating a replica,
Ci,j: expected number of calls made from cell j to i in a unit time.
Ui: expected number of moves by i in unit time
Then a replication of the location of user i at cell j is judicious if
Ci,j Ui.
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57. Per User Profile Replication
Objective: to minimize the total cost of moves and calls, while maintaining
Constraint 1: a maximum of ri replicas for user i, and
Constraint 2: a maximum of pj replicas in the database of cell j.
Replication assignment problem: The profile of user i is replicated at all cells in set R(i) such that the system cost
Ni=1Mj=1,jR(i) (Ui- Ci,j)
is minimized, where N is the number of users and M is the number of cells., and constraints 1 and 2 above are met.
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58. Replication Assignment
Flow Network based solution: s t
users
cells 1 i N
(0,r1)
(0,ri)
(0,rN) M j
(0,pj)
(0,p1)
(0,pM)
(Ui- Ci,j,1)
(UN- CN,M,1)
Edges are
labeled with
(cost,capacity)
tuple.
source
sink
Solving Min-Cost Max-Flow on the Flow Network finds the
required assignment.
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59. Working Set (WS) Replication
Relies on the observation that each user communicates frequently with a small number of sources called its working set.
Copies of location are maintained at the members of its working set.
No constraints are placed on database storage capacity or on number of replicas per user.
Hence, the decision to provide the information of the location of a mobile unit i to zone j can be made independently for each user.
Adapts to user’s call and mobility patterns
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60. Working Set Adaptation
The inequality Q: Ci,j Ui. is evaluated locally at a mobile unit I each time:
1. a call is set up,
2. the mobile unit moves.
In case 1, Q is evaluated only if the caller’s site is not a member of mobile’s WS:
If the inequality holds then the caller’s site becomes member of the callee’s working set.
In case 2, the Q is evaluated for every member of WS; the members for which Q no longer holds are dropped from WS.
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61. Performance of WS Replication
Computation overhead:
in case 1 all four terms of Q need to be reevaluated
in case 2 only the number of moves (Ui) needs to be reevaluated.
Adaptability:
when call-to-mobility ratio (CMR) value is low the WS scheme performs like a scheme without replication.
when CMR value is high, the scheme behaves like a static scheme in which the WS for a user is fixed.
Performance is mainly dependent upon the CMR of individual users (not on num. of users).
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62. Forwarding Pointers
Each time a mobile unit x moves to a new location, a forwarding pointer is set up to its previous VLR to point to the new VLR.
To establish a call, the HLR of callee is queried to find the first VLR in the forwarding pointer chain. This chain is followed to get to the current VLR of the callee.
To bound the time taken for lookup procedure, the length of the chain is bound to a max value of K.
Pointer compression is used to eliminate loops.
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63. Forwarding Pointers (Cont.)
Mobile IP protocol includes pointer forwarding in conjunction with lazy caching.
The forwarding pointer strategy is useful for those users who receive calls infrequently relative to the rate at which they change registration areas.
Benefits of forwarding depends also upon the cost of setting up and traversing pointers relative to the costs of updating the HLR.
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64. Overlapping Registration Areas
Inter-RA hand-off: a user changes cells and RAs
Intra-RA hand-off: a user changes cells within an RA.
Inter-RA hand-off doesn’t happen as long as the hand-off can be intra-RA.
Inter-RA call is when caller and callee are in separate RAs
Intra-RA call is when caller and callee are in same RA.
A non-overlapping cell is serviced by one LR.
A overlapping cell is serviced by multiple LRs.
Reduction of inter-RA hand-offs.
Without overlapping A 1 2 3 B C A
With overlapping 1 2 3 B C
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65. Overlapping RAs (cont.)
Advantages:
Each RA can provide service to more mobiles within their covered area.
Reduces the number of inter-RA handoffs
Reduce the load to update mobile’s HLR.
Disadvantages:
the communication overhead for call-delivery and intra-RA handoff is increased.
the increase in overhead depends upon the underlying network topology.
If this overhead is ignored then the extreme configuration in which each RA has all the cells in the system becomes the “optimal” configuration.
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66. Overlapping RAs (cont.)
Dynamically Resizing RAs
We need to find optimal configuration (allowing overlapping RAs) i.e. configuration which minimizes load on MSSs.
When move and call patterns periodically change, a static scheme may not provide a good solution
Our Approach: Allow RAs to be dynamically adapted.
Periodically resize RAs to minimize MSS load:
Resizing criterion: load reduction due to lesser number of inter-RA handoffs > increase in load due to more expensive call delivery and intra-RA handoffs.
If resizing criterion is ignored then each RA will grow to maximum size.
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67. Overlapping RAs (cont.)
Negative effect of underlying conventional star topology on signaling overhead under overlapping RAs
Backbone Network
MSS/LR
Cells
RA B
RA A
RA C a b
Even though mobiles a and b belong to the same RA, any
calls between them would need to go through two MSSs.
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68. Overlapping RAs (cont.)
Inclusion and Exclusion Boundary
In order to facilitate orderly growth and shrinking of RAs, an MSS only includes and excludes cells from its RAs current boundary.
Two types of boundary:
Internal Boundary
External Boundary BS MH
MSS
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69. Overlapping RAs (cont.)
Inclusion/Exclusion Decision
The decision to include or exclude a candidate cell is based on whether the resulting configuration will have a lower expected load on MSS.
For a given system configuration A, mobility pattern M, and call C, SystemLoad(A,M,C) is the combined signaling load (in terms of message time complexity) as a result of all the handoffs due to M and call-deliveries due to C: SystemLoad(A,M,C) = Load(k,M,C).
In case of inter_RA handoffs and call-deliveries we spilt the signaling overhead equally between the two MSSs involved.
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70. Overlapping RAs (cont.)
What changes when cell x is included in RA r?
Handoffs to cell x from cells of RA r become intra-RA handoffs.
Handoffs from cell x to rest of RA r performed by users already registered in r become intra-RA handoffs.
Calls to x from cells of r are now intra-RA calls.
Calls from users of r that are in x to rest of r are now intra-RA calls.
Mobility of users in r that move out of cell x into a new RA is now inter-RA mobility.
Inter-RA calls of users in r that call from cell x is inter-RA call loading to r.
Call the decreasing part of the load Costin(x,r) and the increasing part Costex(x,r).
At intervals T each MSS/LR r computes I_Boundary(r) and E_Boundary(r) and for each cell x in the two sets computes Costin(x,r) and Costex(x,r). By comparing the two values, it decides if it is worth keeping excluded, keeping included, including or excluding the cell c.
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71. Dynamic Updating
Most schemes have fixed locations (i.e. the boundaries of registration areas) where the mobiles update.
Users that move around boundaries cause a lot of registrations.
[Bar-Noy95] Solution: introduce dynamic update schemes that don’t depend on location of mobile.
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72. Dynamic Updating (cont.)
Time-based
User updates location at intervals of time T independent of actual location.
Movement-based
User updates location after crossing M hops (cells) from last updated location.
Distance-based
User updates location after being distance D from last updated location.
Two metrics to evaluate schemes:
update rate (# of updates/sec)
search area (# of cells/search)
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73. Dynamic Updating (cont.)
Time-based versus Movement-Based
Update rate
If user crosses less than M cells per time T, then time-based makes more updates, otherwise time-based makes less updates
If M is average hops traversed per time T, then two schemes have same rate of updates.
Search area
Search area in time-based is the cells that can be reached from last updated location at max user speed in time T.
Search area in movement-based is cells that can be reached in M or less hops from last updated location
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74. Dynamic Updating (cont.)
Movement-based versus Distance-based
Update rate
If D=M, distance-based will do at most as many updates as movement-based
Search area
Search area is same in both schemes (cells that can are at distance D or less from last updated location)
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75. Look-ahead update
[Tsai97] Proposed a look-ahead update scheme based on the distance-based scheme of [Bar-Noy95].
Mobility is modeled as a “normal walk” where the mobile tends to keep the direction of movement
Look-ahead update scheme: in distance-based scheme don’t update current location, but update a location Δ hops ahead
Under normal walk mobility model, user is more probable to cross standard circle (solid) before crossing look-ahead circle (dotted). Therefore look-ahead saves updates.
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76. Dynamic Hierarchy
[Ho97] Proposed a hybrid scheme with different hierarchy levels.
Directory Registers: are a inter-mediate level of hierarchy between VLRs and HLRs.
For each user there is the consept of Local DR, the DR that is above the VLR.
HLRs may point to VLRs (direct pointer) or to LDRs (indirect pointer). Hence, the two hierarchies:
Two levels: HLRVLR
Three levels: HLR LDR VLR
Scheme includes Forwarding Pointers Of users various DRs to other DRs or LRs
Local DR
MSCa
Remote DR1
Remote DR2
MSCb
MTa
MTb
HLRa
HLRb
Indirect HLR pointer
Direct HLR pointer
Direct DR remote pointer
Indirect DR remote pointer
CSE535 Mobile Computing

77. Multiple-level Hierarchical Scheme
[Bejerano98] define a multiple-level hierarchy of overlapping Location Areas:
If LAs are viewed as circles of radius r, then the outer r/2 part (periphery) overlaps with the inner parts (cores) of neighboring LAs, and the inner r/2 part (core) overlaps with the outer parts (peripheries) of neighboring LAs.
At each level n, there is twice the number of the (n+1)-level Location Areas and half of the (n-1)-level Location Areas.
Logarithmic number of levels
For every user, there is at least one LA at each level that has a “location pointer” to a LA to the next lower level.
CSE535 Mobile Computing

78. Multiple-level Hierarchical Scheme (cont.)
Update policy:
At each level, starting from lowest, if the user moves between two cells that are not in the same LA, the move is “updated” to the LA in the above level as well.
A movement update goes up to the LA that embraces both ending and starting cells of the user’s movement.
Search Policy:
If there is no downward pointer, then the search is propagated upward until a LA has a downward pointer of the user.
The downward pointers are followed until the user is reached.
Scheme shows very good average and best case costs, but very bad worst case.
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79. Summary of Variations to 2-Tier Scheme
Method
Variations
Used When:
Caching
When x is called by y, cache x’s location at y’ zone.
Eager caching
Cache update overhead occurs at moves
Large LCMR
Lazy Caching
Cache update overhead occurs at calls
Replication
Selectively replicate x’s address at the zones from which it receives the most calls
Per-user profile Replication
Additional constraints are set on the number of replicas per site and on the number of replicas per user
Working Set
Adaptive Distributed: the replication sites are computed locally at each mobile host
Forwarding Pointers
When x moves, add a forwarding pointer from its old to its new address.
Restrict the length of the chain of forwarding pointers
Small LCMR
CSE535 Mobile Computing

80. Hierarchical Schemes (HS)
Extend two-tier schemes by maintaining a hierarchy of location databases.
Location database at higher level contains location of users located at levels below it.
Usually hierarchy is tree structured:
Location database at a leaf serves a single cell and contains entries for all users registered in that cell.
A database at an internal node maintains location of users registered in the set of cells in its subtree.
location information can be either
pointer to an entry at a lower level database or
the user’s actual current location.
CSE535 Mobile Computing

81. Hierarchical Location Scheme
[Jain, ICC ’96] 1 2 5 6 3 4 7 8 9 10 11 12 13 14 15 16 17 18
user x x
entries for user x when
the location database
maintain pointers
maintain actual location
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82. Updates/Lookups with Pointers
LCA(i,j): least common ancestor of i and j.
When user x moves from cell i to j, following entries for x in databases are updated:
1. along the path from j to LCA(i,j), and
2. along the path from LCA(i,j) to i.
When a caller located at cell i places a call for a user y located at cell j, the lookup procedure:
1. queries databases starting from node i and proceeding upwards the tree until the first entry for x is encountered (at LCA(i,j)).
2. Then the lookup procedure proceeds downwards following the pointers to node j.
CSE535 Mobile Computing

83. Update/Lookup Example
upward phase:
till entry for callee
is found
LCA(10,13) x
downward phase:
follow the pointer x
LCA(9,10) 1 2 3 x x 4 5 6 x 9 x 10 7 8 9 10 11 12 13 14 15 16 17 18
user x
(moved from cell 9
to cell 10)
a node in cell 13
calls user x
CSE535 Mobile Computing

84. Lookup/Update with Actual Location
When user x moves from cell i to cell j:
record for x is deleted from all the databases from node i to LCA(i,j), and
record for x is updated to indicate the current location to be cell j in all the databases from root node to leaf node j.
When a user x from cell i places a call to user y in cell j the lookup procedure queries database at node i proceeding upwards till node LCA(i,j).
Compared to pointers case, in this case:
1. updates are more expensive operation, and
2. lookups are less expensive operation.
CSE535 Mobile Computing

85. Update/Lookup Example
LCA(10,13) 9 10 x 9
LCA(9,10) 10 1 2 3 x x 4 9 10 5 6 x 9 x 10 7 8 9 10 11 12 13 14 15 16 17 18
user x
(moved form cell 9 to
cell 10)
a node in cell 13
calls user x
CSE535 Mobile Computing

86. Caching in Hierarchical Scheme1 2 5 6 3 4 7 8 9 10 11 12 13 14 15 16 17 18 s t
forward bypass
pointer
reverse bypass
user y (callee)
search procedure
user x (caller)
Forward bypass pointer is an entry at an ancestor of caller’s cell,
say s, that points to an ancestor of callee’s cell, say t.
The reverse bypass pointer is from t to s.
In simple caching both s and t are leaf nodes.
In level caching s and t can belong to any (possibly different) levels.
CSE535 Mobile Computing

87. Forwarding Pointers in Hierarchical Scheme
old enteries
new enteries
Lookup cost for calls
initiated from any cell in
this subtree is increased 1 2
level m
Forwarding Ptr 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
user x new location
user x old location
Reduces the update cost in case of move from cell i to cell j, instead of
updating all databases on the path from j through LCA(i,j) to i, only the
databases up to a level m are updated and a forwarding pointer is set
from a node s to node t, where s is the ancestor of i at level m and t is
ancestor of j at level m.
CSE535 Mobile Computing

88. Hierarchical Schemes: Summary
Method
Description
Caching
When x at zone i is called by user y at zone j, cache at a node on the path from j to LCA(i,j) a pointer to a node on the path from I to LCA(i,j) to be used by subsequent calls to x from zone j.
Replication
Selectively replicate x’s location at internal and/or leaf database.
Forwarding Pointers
When x moves from cell i to cell j, instead of updating all databases on the path from j to LCA(i,j) and from LCA(i,j) to j, update all databases up to level m and add a forwarding pointer at the level m ancestor of I to point to the level m ancestor of j.
CSE535 Mobile Computing

89. Hierarchical vs. Two-Tier Scheme
No pre-assigned HLR
Support Locality
Increased number of operations (database operations and communication messages)
Increased load and storage requirements at the higher-levels
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90. Location Management: Summary
Location management is a rich research topic; the following represents 3D space of possible solution [ What(granularity), Where (availability), When (currency)]:
Availability
nowhere
at all sites
At selective sites (e.g., at frequent callers)
Currency
Never update
Always update (at each movement)
Granularity
Exact location
the whole network
some partition
CSE535 Mobile Computing

91. References
[Akyildiz97] J. S. M. Ho and I. F. Akyildiz. Dynamic Hierarchical Location Management in PCS Networks. IEEE/ACM Transactions on Networking, 5(5):646–660, October 1997.
[Bejerano98] Y. Bejerano and I. Cidon. An Efficient Mobility Management Strategy for Personal Communication Systems. pages 215–222, MobiCom 98, April 1998.
[Bar-Noy95] A. Bar-Noy, I. Kessler, M. Sidi, Mobile Users: To Update or not to Update? ACM/Baltzer Wireless Networks Journal. Vol 1, no 2, 1995, pp
[Tsai97] I-F Tsai, R-H Jan, The Lookahead Strategy for Distance-Based Location Tracking in Wireless Cellular Networks, Mobile Computing and Communications Review, Vol 3, No 4, 1997
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92. Location Querying

93. Challenges
Managing queries that involve the location of moving objects is challenging because
location is a fast changing data
result of such queries may depend on the location of the user posing it, e.g. all the trucks that are within 1 mile of truck ABC123 (which needs assistance).
queries may not directly include location, but may require tracking mobile objects e.g. queries that involve data produced and located at mobile hosts.
In general, queries may be initiated by either static or mobile users and may include databases located at both static and mobile sites.
CSE535 Mobile Computing

94. Types of Location Queries
Location queries with transient data
transient data: whose value changes while the query is being processed e.g. a moving user asking for nearby hospitals.
Continuous queries e.g. a moving car asking for hotels located within a radius of 5 miles and requesting the answer to the query to be continuously updated.
Issues include:
when and how often should continuous queries be re-evaluated?
the possibility of partial or incremental evaluation.
CSE535 Mobile Computing

95. Handling Imprecise Location Data
Maintaining precise location of a user is very expensive in terms of communication cost.
Profile Partitioning is a technique for reducing the update volume [Imielinski & Badrinath VLDB’ ‘92]:
Partition: glues together cells between which the user relocated very often and separates cells between which the user relocates infrequently. 4 6 6 1 4 4 6 6 1 P2 P1
The numbers on the edges indicate the number of times the user relocates between the end locations connected by the edge during certain interval of time.
CSE535 Mobile Computing

96. Profile Partitioning
User do not have to inform the location server about each and every change of their location
location server maintain which partition the user is currently in.
the user informs the location server whenever it moves from one partition to another.
Supports bounded ignorance:
the real position of the mobile and the position which is know are always in the same partition.
“I do not know where you are exactly at this time but I know that you are in downtown Boston.”
Search is always within a partition
CSE535 Mobile Computing

97. Modeling Fast Changing Values
Issues:
margin of error
how to find better (or more precise) value
cost of finding precise value finding procedure
Distinction is made between actual and stored value of a dynamic attribute (such as location, temperature)
Partitions help in defining margin of error
Following locating methods provide exact values at additional cost
paging/broadcasting to all cells in partition
maintain a list of locations, ordered by likelihood of user being at a given location; search each location in the list order
maintain forwarding pointers
CSE535 Mobile Computing

98. Additional Attributes & Methods
The following additional attributes are associated with a mobile object MO:
Partitions: {P1,P2, ...} where Pi: set of cells and Pi Pj=, ij.
Correctors: method to be used for finding exact location of MO
Predictors: method/set-of-rules to be used to find default location of MO (in case MO has not notified location server about exceptional behavior).
The following methods are defined on MO:
LOC: MO.LOC returns the stored location of MO
ERR: MO.LOC.ERR returns the partition which is stored in MO’s profile and contains MO.LOC
loc: MO.loc returns the actual location of MO
involves additional considerable communication along with database access.
CSE535 Mobile Computing

99. An Example Additional attributes of user John
Object: John
Partitions: {Cell1,Cell2}, {Cell3,Cell4,Cell5}
Correctors: Paging with pointers
Predictors: {John is at home, in Cell2 after 6 P.M.}
John.LOC : returns stored location of John, say Cell1.
John.LOC.ERR: returns {Cell1,Cell2}
John.loc:
If invoked before 6pm: pages Cell1 and Cell2 to determine the exact location of John and returns that location.
If invoked after 6pm: pages Cell2 (John’s home) first before paging Cell1.
CSE535 Mobile Computing

100. Query Processing
Queries differ in the complexity of location constraint:
“Find me a doctor near the campus” has
one non-location based constraint, and
another unary constraint on location.
“Find a gas station, fast-food restaurant, and grocery store such that all of them are on the same highway and in the above order.”: involves ternary constraint (between) plus three unary constraints on individual location (“on the highway”).
Goal: Minimize the communication cost to answer the queries (in presence of imprecise knowledge about locations of users).
CSE535 Mobile Computing

101. Query Processing: Problem
Main problem arising in query processing in presence of imprecise knowledge about locations of users is:
How to minimize the communication cost to “find out” the missing information necessary to answer the query?
General query in “pseudo SQL” form:
SELECT x1.loc, .... , xm.loc
FROM Users
WHERE (x1.loc=l1 ...  xm.loc = lm)
C(l1, ... , lm)W(x1, ... , xm)
C(l1, ... , lm) is an m-ary constraint on locations l1, ... , lm.
W(x1, ... , xm) is a constraint on individual object locations.
Users: class which stores all instances of users in the system
Query wants to evaluate a predicate over actual locations of users, while the only locations available are {x1.LOC, .... , xm.LOC} with the associated “error” determined by ERR.
CSE535 Mobile Computing

102. Query Processing Strategies
Naïve strategy:
Find objects a1, …, am such that W(x1, ... , xm) is satisfied.
Determine exact locations a1.loc, .... , am.loc by paging partitions a1.LOC.ERR, .... , am.LOC.ERR.
Check if the results satisfies C(a1.loc, .... , am.loc).
Naïve strategy has high communication cost
In terms of number of cells paged
Computation is less expensive that communication.
Optimization Strategies (to reduce #paging msgs.):
Paging can be avoided completely if it can be determined that C(a1.loc, .... , am.loc) is true (or false) for all combinations in the Cartesian product a1.LOC.ERR  …  am.LOC.ERR.
Only partial paging may be required in some cases e.g. after determining exact location for a1 and a2 it may be determined that the constraint is true/false irrespective of location of other objects.
CSE535 Mobile Computing

103. Query Processing: An Example
Give the names of doctors (possibly mobile) located near John’s current location:
{d: Near(d.loc, John.loc)Doctors(d)}, where
Near(y,x): true if x is a neighbor of y; false otherwise.
Possible strategies:
Page both John.LOC.ERR and d.LOC.ERR
First page John.LOC.ERR and then determine if it is necessary to page d.LOC.ERR, or
First page d.LOC.ERR and then determine if it is necessary to page John.LOC.ERR
Strategy 2 or 3 are better than 1; But which is better?
depends upon the profiles and locations of John and doctors
Cost is characterized using Classification Tree
CSE535 Mobile Computing

104. Classification Tree (CT)
Used for determining (expected) minimal cost strategy to evaluate the query
Strategy  sequence in which partitions are paged.
A CT for strategy with variables x1, …, xm :
Consists of two special types of terminal nodes:
POS: indicates constraint is true
NEG: constraints is false
Has edges labeled with conditions:
(xi.loc=li); lixi.LOC.ERR
Associates with each node N a predicate path(N) which is conjunction of all conditions along the path from root to N.
A path terminates with POS (NEG) node iff the conjunction of all the conditions along the path implies (does not imply) the constraint C(x1.loc, ... , xm.loc).
CSE535 Mobile Computing

105. Determining Evaluation Cost
Cost of a node = the number of outgoing edges which do not terminate in NEG node
this is the cost of paging; since we do not need to page locations which we can determine lead to failure.
Cost of a path = the sum of the cost of nodes along the path.
Expected cost of using the strategy associated with a CT = the sum of costs of all paths weighted by the associates probabilities.
corresponds to the expected number of paging messages which will have to be sent in order to determine if x1.loc, ... , xm.loc satisfy the query.
The problem of finding a tree corresponding to the optimal strategy is NP-complete.
CSE535 Mobile Computing

106. Example (Cont.) Classification tree U = d.LOC.ERR V = John.LOC.ERR
NEG
POS
L10
L11
L12 L3 L7 d d d
NEG
POS
NEG
POS
NEG
POS L4 L3 L7 L3 L7 L3 L7 L4 L4 L4
Strategy 2
Strategy 3
CSE535 Mobile Computing