Dissemination-based data delivery using broadcast disks

Mobile computers and wireless networks are emerging technologies which promise to make ubiquitous computing a reality. One challenge that must be met in order to truly realize this potential is that of providing mobile clients with ubiquitous access to data. One way (and perhaps the only way) to address these challenges is to provide stationary server machines with a relatively high-bandwidth channel over which to broadcast data to a client population in anticipation of the need for that data by the clients. Such a system can be said to be asymmetric due to the disparity in the transmission capacities of clients and servers. We have proposed a mechanism called broadcast disks to provide database access in this environment as well as in other asymmetric systems such as cable and direct broadcast satellite television networks and information distribution services. The broadcast disk approach enables the creation of an arbitrarily fine-grained memory hierarchy on the broadcast medium. This hierarchy, combined with the inversion of the traditional relationship between clients and servers that occurs in a broadcast-based system, raises fundamental new issues for client cache management and data prefetching. In this article we present a brief overview of asymmetric environments and describe our approaches to broadcast disk organization, client cache management, and prefetching.