Caching and prefetching are two effective ways for mobile peers to improve access latency in mobile environments. With short-range communication such as IEEE 802.11 and Bluetooth, a mobile peer can communicate with neighboring peers and share cached or prefetched data objects. This kind of cooperation improves data availability and access latency. In this chapter the authors review several cooperative caching and prefetching schemes in a mobile environment that supports broadcasting. They present two schemes in detail: CPIX (Cooperative PIX) and ACP (Announcement-based Cooperative Prefetching). CPIX is suitable for mobile peers that have limited power and access the broadcast channel in a demand-driven fashion. ACP is designed for mobile peers that have sufficient power and prefetch from the broadcast channel. They both consider the data availability in local cache, neighbors’ cache, and on the broadcast channel. Moreover, these schemes are simple enough so that they do not incur much information exchange among peers and each peer can make autonomous caching and prefetching decisions.
Mobile broadcast is a scalable data dissemination model for mobile computing (Acharya & Alonso, 1995; Imielinski, 1997; Tan, 2000). In mobile broadcast, a server broadcasts data objects on a wireless channel and (a large number of) mobile peers get their required data objects by tuning into the broadcast channel and retrieving the data objects when they appear. Data broadcast differs from traditional point-to-point access in that the broadcast channel is open to all mobile clients and one transmission of a data object on the broadcast channel can satisfy the needs of potentially many clients. Mobile broadcast is especially suitable for data dissemination in asymmetric communication environments where the client to server ratio is large and there is a high degree of commonality among client interests. Information interesting to the majority of the clients is more suitable for broadcast. Many projects and systems are based on the data broadcast technology (Acharya & Franklin, 1995; Acharya, 1997; Acharya, 1998; Altinel, 1999; Hughes, 2008; Gifford, 1990; Imielinski, 1997; Microsoft, 2008; Zheng, 2005). They are sometimes referred to in the literature as Dissemination-Based Information Systems (DBIS) (Franklin, 1996).
Mobile peers in broadcast environments sometimes suffer from long access latency (the time elapsed from the moment a client has a query for a data object to the point when the client gets the data object), especially when the broadcast cycle is long due to large volume of data or limited broadcast channel. When the broadcast cycle is long, a mobile peer has to wait a long time before their required data objects appear on the broadcast channel.