The Study On Broadcast Technology Of Mobile Database

Recently, with the developments of mobile computing technology & wireless LAN and the ceaseless creations and upgrades of information products, such as PDA and palm PC, mobile database technology has become a hotspot on the fields of embedded system, database system and mobile computing, and has been widely used.Mobile database is a database system which is embedded in a mobile device. It keeps touch with centric database server with wireless method. It downloads data from the server, and uploads the local modifications to the server.Broadcast is one of the key technologies of mobile database. The server can support a large scale of clients, save communication bandwidth and server processing resources, and make the clients get the newest data via broadcast.First, this thesis talks about the multi-disks broadcast scheduling algorithm on the server side. In this algorithm, data with different access probabilities are broadcasted with different frequencies. The author simulates four scheduling algorithms: flat scheduling, random scheduling, skewed scheduling and multi-disks scheduling, and analyses the simulation results. Based on multi-disks algorithm, the author brings forward a method to obtain broadcast frequencies, and introduces the idea of priority.Not only the server side, the client side should also be paid attention to in the broadcast technology. The client usually has some cache. The thesis talks about two cache replacement strategies: P1X & LIX. How to keep the synchronization of cache and the server is a problem that matters. The author talks about 3 methods to resolve the problem of inconsistency: invalidation, propagation and timestamp.There are upstream channels from the clients to the server in many practical application environments. The client may send a request to the server via a upstream channel, and then wait for the response for the request. Based on the relation of the way the client obtain a data item and broadcast, the author introduces concepts of loose coupling and tense coupling, and talks about the cut-off point algorithm for tense coupling.Finally, the author brings forward four-level disks scheduling algorithm based on multi-disks algorithm, and simulates the split-disk strategy in this algorithm. Based on the analysis of deficiency of cut-off point algorithm, the author also brings forward the review-period scheduling model which can build up initial broadcast and dynamically adjust the contents of broadcast with the elapse of time.