Research On Cache Strategy In Mobile Computing Environment

With the development and combination of computer technique and wireless communication technique, a new computing environment called mobile computing is appearing and developing. The diversity of services delivered over wireless channels has increased people's desire in ubiquitously accessing these services from their mobile devices. However, a ubiquitous mobile computing environment faces several limitations such as low bandwidth of wireless network, limited energy resources of mobile node, the asymmetry of up-down communication link and frequent disconnection between the server and mobile devices. For facilitating users to access information, considering the specificities of mobile computing environment, research on data management in mobile computing is carried out.Caching frequently accessed data is an effective technique to improve the network performance because it reduces the network congestion, the query delay, and the power consumption. When caching is used, maintaining cache consistency becomes a major challenge since data items that are updated on the server should be also updated in the cache of the mobile devices. Disconnection between mobile client and server can hardly be avoided due to the inherent features of mobile computing environment, which leads to more complexity of maintaining cache consistency between cache of mobile node and server.Firstly, this thesis introduces the concept and characteristics of mobile computing, as well as researches on cache consistency, discusses typical mobile computing environment, working status of mobile node cache, focuses on cache consistency, classification of cache invalidation strategies and evolution of existed cache invalidation strategies. Secondly, a cache invalidation scheme called Selective Adaptive proposed by Haidar Safa is improved. The enhanced sheme overcomes the shortcoming of SAS that it causes contents in cache not consistent with those in the server, and focuses on maintaining cache consistency after a long time disconnection. When mobile node disconnects from server for a very long tiom, it doesn't clear all the cache, but improves the reusability of cached data in mobile node, according to characteristic of server's data and depending on server's help, in order to use cache as much as possible. The enhanced strategy combines timestamp and counter to dynamiclly adjust broadcasting window size. Finally, the performance of the enhanced strategy is evaluated and compared with one strategy of existed startegies by NS-2. Results showed that a significant cost reduction can be obtained with the proposed strategy when measuring performance metrics such as delay, bandwidth, and energy, especially under the condition of long disconnection.