System support for mobile, adaptive applications

Imagine that, while on a business trip to Paris, you decide to take a few extra days to sample the city's museums. When you buy your museum pass, you are given a virtual tour guide device-a small PDA that can deliver information about specific pieces in the museum system as well as general information about the city. This device communicates via wireless networks. In or near a museum, the device has access to a high-speed micro-cellular network; in the rest of the city, it makes use of the GSM infrastructure. The user can ask this device to elaborate on specific sites or pieces, display related information, or perform geographically-based queries. These requests are satisfied by applications such as a customized Web browser and a video playback application. When within range of a museum's high-quality network, the displayed information is of excellent quality: images are at high resolution and color, and video is delivered in full motion. However, when the user strays away from the high-bandwidth network, each application degrades the quality of the data it delivers so that it arrives in a timely fashion.