Asymptotic Analysis of a Peer Enhanced Cache Invalidation Scheme

A major factor in determining the effectiveness of caching in wireless networks is the cache coherency scheme which maintains consistency between mobile stations (MSs) and the server. Because the wireless channel is inherently a broadcast medium, a cache coherency scheme in which the server broadcasts cache invalidation reports (IRs) containing data update information is suitable. However, one key issue that IR based techniques must resolve is the intermittent disconnectedness of MSs. Since connecting to the network uses power and because power is a limited resource, MSs connect to the network only when necessary. During periods when the MS is disconnected, IRs are missed, which may result in the MS’s cache becoming invalid. This means that upon reconnection, the MS will have to purge its cache if it missed an IR while it was disconnected. In this paper, we propose a Peer Enhanced Cache (PEC) invalidation scheme. This scheme utilizes the ad-hoc mode of operation which is now available with new generation wireless interfaces. PEC exploits the peer-to-peer capability that is enabled by ad-hoc mode by having MSs store IRs on behalf of other MSs. If an MS misses an IR due to disconnection, it can retrieve the missed IRs from its peers. PEC can be used in conjunction with any cache invalidation method since it is an orthogonal scheme. We develop a mathematical model to derive the throughput and hit rate of PEC when it is used in combination with the previously proposed Amnesic Terminal (AT)[1] cache invalidation scheme. Our results show that incorporating PEC significantly improves both the throughput and the hit rate.