One of the promises of ubiquitous computing is the ability to tie heterogeneous computing elements together in a manner where the whole is larger than the sum of the individual parts. Thus, Internet services, file servers (PC's), personal digital assistants, sensors, and embedded computers (in appliances ranging from light switches to automobiles) will interact with one another based on application requirements and proximity. The metric of proximity is interesting because it can be either physical, where computers interact with one another by virtue of being in the same approximate location (for example, in a Smart Room), or semantic, where events in one region of the world/network affect interested parties in another (for example, a stock price hitting a target level or an alarm clock coordinating with traffic sensors, weather reports, airline delays, and a user's calendar). One key challenge to coordinating computers in the above manner is the scalable dissemination of service data and metadata to a potentially large population of clients. We discuss a number of examples below.
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