Are “Disks in the Air”

Mobile computers a n d wireless networks are emerging technologies which will soon make ubiquitous computing a reality. I n the wireless environment, mobile clients may often be disconnected f rom stationary server machines or inay have only a low-bandwidth channel f o r sending messages to servers. This environment raises new challenges for the support of daiabase applications f o r three reasons: 1) the limited storage capacities of mobile machines, 2) the inabili ty to accurately predict the future data needs of many data-intensive applications, and 3) ihe need to provide clients with new or updated data values in order t o ensure consistent data access. One way (perhaps the only way) to address these challenges is t o provide the stationary server machines with a relatively highbandswidth channel over which to broadcast portions of the database. Wireless networks are but one cornpelling example of the inore general class o f asymmetric communication environments, which also includes CATV networks and information distribution services. We have proposed a mechanism called Broadcast Disks to provide database access in asymmetric cornmunication environments. B y appropriate organization of the broadcast program, an arbitrarily fine-grained memory hierarchy can be created. Such a hierarchy raises fundamental new issues f o r pre-fetching and client cache management. I n this short paper we present a brief overview of asymmetric environments, our novel approach t o organizing broadcast d i sks a n d some in i t ia l simulation results.