Application: Intermittently Synchronized Databases

• In some of the applications, a server or a group of servers manages the central database and the clients carry laptops or palmtops with a resident DBMS software to do “local” transaction activity for most of the time.
• The clients connect via a network or a dial-up connection (or possibly even through the Internet) with the server, typically for a short session—say, 30 to 60 minutes.
• They send their updates to the server, and the server must in turn enter them in its central database, which must maintain up-to-date data and prepare appropriate copies for all clients on the system.
• Thus, whenever clients connect—through a process known in the industry as synchronization of a client with a server—they receive a batch of updates to be installed on their local database.
• The primary characteristic of this scenario is that the clients are mostly disconnected; the server is not necessarily able to reach the client. This environment has problems similar to those in distributed and client-server databases, and some from mobile databases, but presents several additional research problems for investigation.
• We refer to this environment as Intermittently Synchronized Database Environment (ISDBE), and the corresponding databases as Intermittently Synchronized Databases (ISDBs).

Data Management Issues

From a data management standpoint, mobile computing may be considered a variation of distributed computing. Mobile databases can be distributed under two possible scenarios:

1. The entire database is distributed mainly among the wired components, possibly with full or partial replication. A base station or fixed host manages its own database with a DBMS-like functionality, with additional functionality for locating mobile units and additional query and transaction management features to meet the requirements of mobile environments.

2. The database is distributed among wired and wireless components. Data management responsibility is shared among base stations or fixed hosts and mobile units.

(All the Distributed data management issues can be applied to Mobile)

• Data distribution and replication: Data is unevenly distributed among the base stations and mobile units. The consistency constraints compound the problem of cache management. Caches attempt to provide the most frequently accessed and updated data to mobile units that process their own transactions and may be disconnected over long periods.
• Transaction models: Issues of fault tolerance and correctness of transactions are aggravated in the mobile environment. A mobile transaction is executed sequentially through several base stations and possibly on multiple data sets depending upon the movement of the mobile unit. Central coordination of transaction execution is lacking, particularly in scenario (2) above. Moreover, a mobile transaction is expected to be long-lived because of disconnection in mobile units. Hence, traditional ACID properties of transactions may need to be modified and new transaction models must be defined.
• Query processing: Awareness of where the data is located is important and affects the cost/benefit analysis of query processing. Query optimization is more complicated because of mobility and rapid resource changes of mobile units. The query response needs to be returned to mobile units that may be in transit or may cross cell boundaries yet must receive complete and correct query results. 
• Recovery and fault tolerance: The mobile database environment must deal with site, media, transaction, and communication failures. Site failure at an MU is frequently due to limited battery power. If an MU has a voluntary shutdown, it should not be treated as a failure. Transaction failures are more frequent during handoff when an MU crosses cells. MU failure causes a network partitioning and affects routing algorithms.

Architecture of Mobile Ad-hoc network

Client/Network Relationships:

·        Mobile units can move freely in a geographic mobility domain, an area that is circumscribed by wireless network coverage.

·        To manage the mobility of units, the entire geographic mobility domain is divided into one or more smaller domains called cells, each of which is supported by at least one base station.

·        The mobile discipline requires that the movement of mobile units be unrestricted throughout the cells of a geographic mobility domain, while maintaining information access contiguity—i.e., movement, especially intercell movement, does not negatively affect the data retrieval process.

Explanation:

• In a MANET, co-located mobile units do not need to communicate via a fixed network, but instead, form their own using cost-effective technologies such as Bluetooth.
• In a MANET, mobile units are responsible for routing their own data, effectively acting as base stations as well as clients.
• Moreover, they must be robust enough to handle changes in the network topology, such as the arrival or departure of other mobile units.
• MANET applications can fall under the peer-to-peer paradigm, 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 more complicated.

Mobile Databases

·         Recent advances in portable and wireless technology have led to mobile computing, a new dimension in data communication and processing.

·         Portable computing devices coupled with wireless communications allow clients to access data from virtually anywhere and at any time.

·         This feature is especially useful to geographically dispersed organizations.

·         Some of the software problems, which may involve data management, transaction management, and database recovery—have their origins in distributed database systems.

Explanation:

·         It is a distributed architecture where a number of computers, generally referred to as Fixed Hosts (FS) and Base Stations (BS), are interconnected through a high-speed wired network.

·         Fixed hosts are general purpose computers that are not typically equipped to manage mobile units but can be configured to do so.

·         Base stations are Mobile Units' (MU) gateways to the fixed network. They are equipped with wireless interfaces and offer network access services of which mobile units are clients.