A new type of system architecture is described that uses both duplex ct)mmunication and wide-area simplex commtmication to implement a single service.A working community inlbrmation system based on this architecture is discussed fiom a systems perspective, with an emphasis on the unique way in which processing is distributed among a confederation of shared servers and private personal systems.In the community infimnation system, each personal system maintains a local, user-defined subset of the databases stored on the shared servers.I)atabase updates are transnfitted to the personal systems via a broadcast packet radio system.This design allows many queries to be processed completely at users' personal machines, and thus reduces the reliance on shared servers.A unifying design principle is that the system is seen as a collection of independent shared and personal databascs, as opposed to a single monolithic database.Query routing is used to hide the system's division into component databases from a user.

A new type of information system is described that combines personal computers, broadcast data communication, and bidirectional communication. The system is designed to use broadcast communication whenever possible to deliver information to personal computers, which are used for data storage, indexing, and retrieval. This paper starts with an overview of the system, and then discuss the problem of reliable digital broadcast communication in some detail. A parameterized broadcast protocol is described, and we show how to choose protocol parameters based on observed channel error characteristics. A flexible encryption-based protection system is included in the protocol. We discuss the implementation of the system on contemporary personal computers. A broadcast system based on these ideas is now operating in Boston area homes.