| Since 90's, with the rapid development in mobile communication technology and Internet technology, and extensive application in mobile computer, make many mobile nodes be able to build up connections with network in the mobile process freely. Consequently, the concept of" Mobile Computing" and" Mobile Database" emerges and just become a hot research field. Mobile Computing means distributed computing environment where mobile terminal can exchange data with server by wireless and fixed network. It makes computers or other communication equipments be able to deliver data without the link with the fixed physical equipments. Mobile Database means the distributed database of which supports mobile computing environment. It involves many fields such as database technique, distributed computing technique and mobile communication technique etc. Mobile Database System and Operating System integrate together and run in various intelligent imbedded-equipments or mobile equipments, hence Mobile Database is also called imbedded-Mobile Database. Compared with traditional distributed database, mobile database system has the following basic characteristics: (1) Mobility ; (2) Frequent Disconnections ; (3) Network Diversity ; (4) Communications Asymmetry;(5) Limited Resources;(6) Lower Security;(7) Larger Scale. According to above characteristics, researchers put forward four targets of Mobile Database system: (1) Scalability ; (2) Mobility ; (3) Serializability;(4) Convergence.On one hand, because of limit such as power, wireless communication expenses and network condition etc., users switch their terminals on and off regularly. This requires transactions in Mobile Database be able to continue with frequent disconnections or switch to doze mode automatically.On the other hand, in wireless network communication, fixed server node usually has a strong capability of delivery, however, the ability of mobile node is limited, i.e. , downstream bandwidth from server to mobile node is much... |
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