Data Management Issues

• Data management technology that can support easy data access from and to mobile devices is among the main concerns in mobile information systems.

Mobile database

• Data management technology that enables the use of databases on the mobile computing environment.

• The database is more advanced and challenging than the fixed distributed databases.

Mobile Database Features• Data are available anywhere independent of the

availability of the fixed network connection.– mobile users can virtually access any data, anywhere

and anytime, even in the absence of fixed network connection.

• Databases on both mobile and fixed hosts are sharable in seamless way.– In order to support seamless data sharing among

mobile and fixed hosts, we need to employ distributed computing technologies that should also work properly even in the disconnection-prone environments.

Mobile Environment Model• A mobile environment consists of two distinct sets of entities– mobile hosts and – fixed hosts. • Cell • Mobile hosts are connected by wireless connections to the MSS of the• cell where they currently exist. • A mobile host can move within a cell or between two cells while

retaining its network connection.• Every host and cell in the system is associated with a unique identifier.• A mobile host acts as a data client and a data server at the same time.• A fixed host called location server keeps information of locations of all

mobile hosts located within its management coverage.

Mobile Environment Model

Hierarchical structure of location servers

Peer –to-peer Environment model

• MANET applications can be considered as peer-to-peer, meaning that a mobile unit is simultaneously a client and a server.– Transaction processing and data consistency control become

more difficult since there is no central control in this architecture.

– Resource discovery and data routing by mobile units make computing in a MANET even more complicated.

Mobile Database Characteristics1. The environment where mobile databases are deployed is a mix of two

different networks, i.e., the fixed network and the wireless network. – Fixed network characteristics

• Fixed host location, • relatively high capacity, • high reliability, and • low connection cost.

– Wireless Network characteristics• dynamic network topology, • Relatively low capacity, • low reliability, and • high connection cost.

• Techniques to avoid compromising database performance due to the use of wireless network– Reducing the number of data exchanged via mobile network .– Reducing the response time of accessing data via mobile network .– Providing data cache on mobile host.

Mobile Database Characteristics

2. Limited resources available to mobile users – Mobile hosts will likely bring only the fraction of

data they need to access frequently during mobile.– consistency requirement on databases (both on

mobile and fixed hosts)• Techniques to address the problem – Transaction management for mobile data bases .– Allocation of mobile database replication on the

fixed network.

Mobile Database Characteristics

3. Mobile hosts have low security.– The worst case, for example, our data on the

mobile host would be completely lost if the mobile hosts become the subject of theft.

Data Management Issues

• Data distribution and replication• Transactions models• Query processing• Caching• Database Interfaces• Recovery and fault tolerance• Location-based service• Security

Data distribution and replication

• Higher data availability and lower cost of remote access.

• More efficient access to data.• Higher security.Generally the available replication technologies assume the deployment on fixed distributed environment.

Replication issues

• Increasing the number of replicas will result in the increase of costs for updates and signaling.

• Mobile hosts can move anywhere and anytime.

Transaction Models

• All transactions must satisfy the ACID properties– Atomic– Consistent– Isolated– Durable

• Different concurrent transactions performed should be serializable– two-phase locking or timestamping

Transaction Models

• Mobile transactions are, in general, distributed transactions where some actions are performed in mobile computers and others in fixed hosts.

• ACID properties are hard to enforce, especially when the mobile computers are disconnected. I

Query Processing

• Query processing is affected when mobility is considered.

• It is possible to formulate location dependent queries. For example: “where is the nearest gas station?” or “which are the cinemas that project some film at 8:00pm in this city?” return different values depending on the location of the mobile computer.

Query Processing

• Query optimization methods try in general to obtain execution plans which minimize CPU, input/output and communication costs.

• In centralized environments the cost that affects most is the input/output.

• In distributed environments, communication cost is the most important one.

• In a mobile distributed environment, the communication costs are much more difficult to estimate because the mobile host may be situated in different locations.

• Dynamic optimization strategies are required in this mobile distributed context.

Caching• Caching techniques for query processing can

reduce communication costs dramatically. • Difficult to apply caching techniques in mobile

context because cache contents may change rapidly or get out-of-date due to mobility.

• Updates to the cache memory may not be sent due to disconnections of the mobile unit.

Database Interfaces• Design of query interface such as Query By Icons

that – are convenient to use.– address the features of screen size.– address limitations in memory and battery power and

the restricted communication bandwidth.• How the pen and voice can be used as substitutes

for the mouse and keyboard.• Implementation of a pen-based graphical

database interface on a computer.

Recovery and Fault Tolerance

• Fault tolerance is the ability of a system to perform its function correctly even in the presence of internal faults.

• Faults can be classified as transient or permanent.• A transient fault will eventually disappear without any apparent

intervention.• A permanent fault remains unless it is removed by some

external agency. • While it may seem that permanent faults are more severe, from

an engineering perspective they are much easier to diagnose and handle.

• The intermittent transient faults that recur often unpredictably are the most problematic.

Recovery and Fault Tolerance• The mobile computing environment is characterized by – limited resource availability, – high mobility, – low bandwidth and – frequent disconnection.

• These features make the mobile computing environment more susceptible to transient faults.

• Software faults are the results of problems in software running on PDAs, e.g., mobile crash.

• Transaction recovery deals with the capability of ensuring failure atomicity.

Location based Services

• Determining the location of mobile users is one of the most challenging tasks which must be undertaken in order to enable a location-based service.– Non-GPS positioning techniques• Cell of Origin, • Time of Arrival, • Angle of Arrival, and • and Enhanced Observed Time Difference.

Location based Services

• Global Positioning System– GPS is a relatively mature technology, – Current receiver hardware is smaller, lighter, cheaper and

uses less power than earlier-generation equipment.• GPS limitations

– strong attenuation of the satellite signals by buildings.– GPS does not operate well (or at all) in dense ‘urban canyon’

areas, or inside buildings. Yet, these are often the very areas where demand for location based services is the highest.

– In order to use GPS, the mobile handsets must be modified to integrate GPS receiver chips.

Location based Services

• Assisted GPS– refers to the GPS positioning technique whereby

there is assistance data provided from a special GPS server/base station by the mobile telephony network.

– A-GPS enables GPS positioning even in urban and indoor areas, where the signal is too weak to be acquired using standard signal tracking procedures within the receiver.

Location based Services

• Issues– Diverse mobile mapping standards– Interoperability– Market capacity– User Privacy

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