We develop and analyze a simple, elegant medium access control (MAC) protocol for use in transmitting real-time data in point to point ad hoc wireless local area networks (WLANs). Our enhancement of IEEE 802.11, real-time MAC (RT-MAC), achieves dramatic reductions in mean delay, missed deadlines, and packet collisions by selectively discarding packets and sharing station state information. For example, in a 50 station network with a normalized offered load of 0.7, mean delay is reduced from more than 14 seconds to less than 45ms, late packets are reduced from 76% to less than 1%, and packet collisions are reduced from 36% to less than 1%. Regression models are developed from simulation data to describe network behavior in terms of throughput, mean delay, ratio of late packets, and ratio of collisions. Stations using RT-MAC are interoperable with stations using IEEE 802.11.

Caching of remote data in a mobile client's local storage can improve data access performance and data availability. Traditional approaches are page-based, without taking advantage of the semantics of cached data. It is difficult for a client to determine if a query could be answered entirely based on locally cached data, forcing it to contact the database server for additional data. We propose a {\em semantic caching mechanism\/} which allows data to be cached as a collection of possibly related blocks, each of which is the result of a previously evaluated query. We investigate mechanisms for transforming projection-selection queries to reuse cached data blocks. This avoids transmitting unwanted data items over low bandwidth wireless channels. Cache replacement techniques based on the semantics of cached data are also proposed. We describe the design of our prototype and study its performance.