The PARCTAB Mobile Computing System

The PARCTAB is a personal digital assistant (PDA) that communicates via infrared (IR) data-packets to a network of IR transceivers. The infrared network is designed for in-buildinguse, where each room becomes a communication cell. In contrast to the approach used by other PDAs, most PARCTAB applications run on remote hosts and therefore depend on reliable communication through the IR network. The infrastructure provides reliability as well as uninterrupted service when a PARCTAB moves from cell to cell. The PARCTAB and a supporting infrastructure [1, 4] has been operational since March 1993 at the Computer Science Lab at Xerox PARC. The system currently comprises thirty cells and twenty-five PARCTABs and will expand in the near future. There are three types of software components in the PARCTAB system: gateways, agents, and applications. Gateways implement a datagram service for sending and receiving packets using IR signals. Each tab is represented by an agent. An agent tracks the location of its tab and provides location independent reliable remote procedure calls. The protocols enforce security, preventing, for example, an unauthorized application from taking control of a tab. Applications are built using a library of widgets designed to accommodate the PARCTAB’s low IR-communication bandwidth and small display area. A distinguished application, the “shell”, permits a tab user to start and switch among applications.