MOBILE DATABASES

UNIT - V ADVANCED DATABASES

Wireless communication

• Frequency

• Spectrum

• Bandwidth
Cellular communication

• Communication slots

- cell

• Cell site

- transceiver location

• Sector
Channel

- communication path between two devices

• Uplink(reverse) Basestation

• Downlink(forward)

Downlink
Uplink
• Signal fading
Personal Communication Services(PCS)

wireless mobile service with advanced coverage and that delivers

services at a more personal level

Global System for Mobile Communication(GSM)

protocols for second-generation digital cellular networks used by

mobile devices
GSM Architecture
GSM Architecture
1.Mobile Unit
- Mobile Host/ Mobile station
- wireless device
2.Base Station
- consists of a pair of transmitter and receiver
- manages the cell
3.Base Station Controller
- manages the activities of BS
- Frequency administration & handover management
GSM Architecture

4.Mobile Switching Center

- process mobile telephone calls
- MU—>BSC—>MSC—>BSC—>MU
5.Home Location Register
- Database
- stores geographical location of a subscriber, call
processing, billing, service subscription, service
restrictions
GSM Architecture

6.Visitor Location Register

- subset of HLR for a particular cell
- Whenever a subscriber enters a new cell, its current
location is stored in VLR representing that cell.
- The entry is removed when the subscriber leaves that
cell.
- information about a subscriber is replicated in HLR and
VLR to achieve faster location search
GSM Architecture

7. Authenticating Centre
- authenticates subscribers
- it is co-located with HLR, to access user
information for authentication process
- first call —> complete authentication process
- subsequent calls within a time period —> stored info
GSM Architecture

8. Equipment Identity Register

- database which stores information for the
identification of mobile units
- Electronic Serial Number
9. Public Switched Telephone Network (PSTN)
- wired line telecommunication network(landline)
10. Integrated Service Digital Network
- wired line network which provides enhanced digital
GSM Architecture

11. Short Message Entity

- System message service
- text messages
12. Message Center
- stores and forwards short messages to mobile
destination
 LOCATION AND HANDOFF
MANAGEMENT

UNIT - V ADVANCED DATABASES

LOCATION MANAGEMENT

Tasks
1. Location management
2. Handoff

LOCATION MANAGEMENT
Tasks
- Location update
- Location lookup
LOCATION MANAGEMENT

A mobile unit can freely move around in

a.active mode
- MU actively communicates with other subscriber
- may continue to move within the cell
- may encounter a handoff
LOCATION MANAGEMENT

A mobile unit can freely move around in

b.doze mode
- MU does not actively communicate with other
subscribers
- continues to listen to the base station and monitors the
signal levels
c.powerdown mode
unit is not functional at all
LOCATION LOOKUP

S-LS - Source Location Server

D-LS - Destination Location Server
LOCATION UPDATE
Forwarding Pointer Location Management
Scheme
Location Search Using Forward Pointers
Handoff Management

Degradation interval
Handoff Management
Steps in Handoff processing
1.Handoff detection

2. Assignment of channels

3. Transfer of radio link

Handoff Management
1.Handoff detection
Approaches:
a. Mobile-Assisted Handoff (MAHO)
• continuously measures the signal strength from
surrounding base stations
• notifies the strength data to the serving base station
• a handoff is initiated when the strength of a neighboring
base station exceeds the strength of the serving base station
Handoff Management
1.Handoff detection
Approaches:
a. Mobile-Assisted Handoff (MAHO)
• handoff decision is made jointly by base station and
Mobile Switching Center (MSC) or base station
controller (BSC
Handoff Management
1.Handoff detection
Approaches:
b. Mobile-Controlled Handoff (MCHO):
• Mobile Unit (MU) is responsible for detecting a
handoff
• MU continuously monitors the signal strength
from neighboring base stations and identifies if a
handoff is necessary
Handoff Management
1.Handoff detection
Approaches:
b. Mobile-Controlled Handoff (MCHO):
• selects the base station with strongest signal for
initiating a handoff
Handoff Management
1.Handoff detection
Approaches:
c. Network-Controlled Handoff (NCHO):
• BS monitors the signal strength used by MUs,
• If it falls below a threshold value, the BS initiates a
handoff
• MSC instructs BSs to monitor the signal strength
occasionally, and in collaboration with BSs the handoff
Assignment of Channels
Schemes
1. Nonprioritized Scheme
Assignment of Channels
Schemes
2. Reserved channel Scheme
Assignment of Channels
3. Queuing Priority Scheme
Assignment of Channels
4. Subrating Scheme
Radio Link Transfer

• Intracell handoff

• Intercell or Inter-BS handoff

• Inter-BSC handoff

• Intersystem or Inter-MSC handoff

Hard Handoff
• user experiences a brief silence or discontinuity in
communication

• Inter-BSC handoff
• Intersystem or Inter-MSC handoff
Roaming

• mobile moves from one GSM to another system PCS

• location of MU must be informed by the new service
provider to the old service provider.
• two service providers communicates with each other
to complete the location management and the
registration process
 EFFECT OF MOBILITY ON
DATA MANAGEMENT

UNIT - V ADVANCED DATABASES

Data Categorization
Location-dependent data
• data values are tightly linked to specific geographical
location
• 1:1 mapping between the data value set and the region
it serves
Data Categorization
Location dependent query
• LDD and the geographical location of the origin of the
query must be known
• system must map the location with the data to obtain
correct LDD
Ex:
What is the distance from the airport to here?

What is the sales tax of this city?

Data Categorization
Location aware query
• query includes reference to a particular location either
by name or by suitable geographical coordinates

Ex:

What is the distance between chennai and bangalore?

Location Dependent Data Distribution

• Spatial Distribution( dependent on geographical

locations)
• Temporal Distribution

• Spatial replication
• Temporal replication
Location Dependent Data Distribution

• Effect of mobility on atomicity

- log at server, mobile device
• Effect of mobility on consistency
- spatial consistency, temporal consistency
• Effect of mobility on isolation
- mobile unit may visit multiple data regions
- enforce isolation in each region separately
Location Dependent Data Distribution

• Effect of mobility on durability

- durability is regional as well as global
• Effect of mobility on commit
- location-dependent commit
Effect of Connectivity on Transaction
Processing
Modes
• connectivity mode
- MU is continuously connected to the database server
• disconnected mode
- MU voluntarily disconnects from the server after
refreshing the cache and continues to process
workload locally
• intermittent mode
- MU can be disconnected any time by the system or
voluntarily by the user
 Transaction Management
in Mobile Database
Systems

UNIT - V ADVANCED DATABASES

Mobile Database System
Essential Properties
• Geographical mobility
• Connection and Disconnection
• Data processing capability
• Wireless communication
• Transparency
• Scalability
 Mobile Transaction Model

UNIT - V ADVANCED DATABASES

HiCoMo: High Commit Mobile Transaction
Model

• Model for processing aggregate data stored in a data

warehouse which resides in mobile units
• transactions are always initiated on mobile units where
they are processed in a disconnected mode
• results of these transactions are then installed in the
database upon reconnection
HiCoMo: High Commit Mobile Transaction
Model

• Transaction which are initiated and processed at

mobile units are called HiCoMo.
• Nested transaction model
HiCoMo: High Commit Mobile Transaction
Model
Transaction Transformation Function
1.Conflict detection
- conflict in transaction —> Abort
2.Base transaction generation
No conflict —> generate, execute base trans.
3.Alternate base transaction generation
• Integrity const. violation —> Abort
• Try updates after redistribution of errors
Moflex Transaction Model
- Flexible transaction model
Components
Moflex transaction T ={M, S, F, D, H, J, G)
M - compensable on non-compensable sub transactions
S - success dependency, Tj has SD on Ti ,
Tj after Ti commit
F - failure dependency, Tj after Ti fail
D - external dependency, Execute Ti after satisfying ED
Moflex Transaction Model
Components
H - Handoff control rules, execution of trans. ,
continue, split-resume, split-restart
J - acceptable join rules, determine the execution of
subtransactions
G - acceptable states of T
Moflex Transaction Model
States
• not submitted for execution - N
• currently under execution -E
• successfully completed - S
• failed - F
Kangaroo Transaction Model
- captures both data and movement of mobile units
- split transaction model
- enforces ACID property
- Kangaroo Transaction<— sub-transactions(Joey Trans)
- JT —> local to base station
- Mobile unit migration —>BS takes control of exec. of
trans.
Kangaroo Transaction Model
Transaction execution modes
- split mode (default mode)
- compensating mode
Compensating mode
- Failure —> all executions are stopped
- previous commits —>compensated
• JT are serializable in compensating mode
Kangaroo Transaction Model
Transaction processing
- MU initiates KT, unique identity for a KT
- BS creates JT, one JT for a KT
- Handoff —> JT, split into JT1, JT2
- Its executed sequentially
 Mobile Database Recovery

UNIT - V ADVANCED DATABASES

Database Recovery

• recover a database from transaction or system

failures
• restore the database to a consistent state
• task of a recovery protocol —> identify the right
operation for recovery
Database Recovery

Operations
• Roll forward or Redo
• Roll backward or Undo

• Transaction log —> committed values of data items,

modified values of data items
• Write Ahead Logging (WAL)
Recovery Protocols

• Undo -Redo

• Undo - No Redo

• No Undo - Redo

• No Undo - No Redo

- shadow copy
LOG MANAGEMENT IN MOBILE DATABASE
SYSTEMS

• Log - sequential file —> information needed for recovery

Where to save the log?

• Mobile Switching Center

• Base Station

• Mobile Unit

* BSs are comparatively better candidates for saving an

Mobilaction

• mobile transaction model

• capable of processing location-dependent data in the

presence of spatial replication

• composed of a set of subtransactions —> Fragments

Effect of Mobility on Logging

• mobilaction —> executed at MUs, BSs

• Fragment visits multiple MUs —> log, scattered at

more than one BS

• Recovery needs log unification

Centralized logging-Saving of log at a
designated site
• Base station —> logging site

• all mobile units from all data regions save their log at one BS

• each mobile unit generates the log locally

• at suitable intervals, copy its local log to the logging base

station

• Failure —> local recovery manager acquires the log from the

base station and recover the mobilaction

Centralized logging-Saving of log at a
designated site
Limitations
• very low reliability
- BS failure—> stop logging process,
- Transaction processing will stop until BS recovers
• Logging may become a bottleneck

* Suitable for a lightly loaded system with little MU

movement
Home logging
• MU stores its log at the base station it initially registers
• MU —>roam, all logging will still be at its base station
Limitations
• If fragments are processed by different mobile units with
different base stations, log unification is needed
• May not work for spatial replicas (LDD)
• Logging location is not distributed —> poor availability,
excessive traffic
At a designated Base Station

• MU stores its log at the base station

• MU —>roam, all logging will still be stored at its
base station
• MU at a remote BS, log—>travel through a chain of
BSs
• Should not be any communication failure
At all visited Base Stations

• MU - saves the log at the base station of the cell it is

currently visiting
• MU —>roam, entire application log is stored in
multiple base stations
• Recovery—> all log portions are unified to create the
complete log
At all visited Base Stations

i. Lazy Scheme
- logs are stored on the current base station
- mobile unit moves to a new base station, a pointer to
the old base station is stored in the new base station
- pointers are used to unify the log distributed over
several base stations
At all visited Base Stations

i. Lazy Scheme
Log unification schemes
a. distance-based scheme
b. frequency-based scheme
At all visited Base Stations

ii. Pessimistic Scheme

- entire log is transferred at each handoff from old to
new base station
- recovery is fast
- each handoff requires large volumes of data
transfer
 Mobile Database Recovery
Schemes

UNIT - V ADVANCED DATABASES

A Three-Phase Hybrid Recovery Scheme

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