In the past decade, there have been numerous efforts in ubiquitous computing, making computational resources or communication more widely available. We believe that it is time to move to a global-scale ubiquitous computing system that is securable, administered by multiple independent administrators and integrates off-the-shelf hardware and software. We are developing such a system based on the Session Initiation Protocol (SIP), with Bluetooth devices for location sensing and Service Location

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... otocol (SLP) for service discovery. We also introduce context-aware location information to augment device discovery and user communication. The system builds on our CINEMA infrastructure and can support a range of activities, from home-based settings to collaboration between distant sites. -694-3/03/0006 ...$5.00. administrators and operators and integrate off-the-shelf hardware and software. Thus, scaling requires more than just having a large number of systems. We are developing such a scalable ubiquitous communications and computing system. The system incorporates core tenets of ubiquitous, pervasive and context-aware computing, in particular: Multimedia: We believe that communications incorporates all types of media, from continuous media to application sharing, and we consider multimedia support a core component of a ubiquitous computing environment. Device integration: Our system integrates mobile devices such as programmable active badges, PDAs and laptops with resources embedded into the environment, such as large displays, video projectors, high-resolution video cameras, loudspeakers, stereos and lights. Active multimedia sessions can be moved from one device to another and can be split across devices, so that a multimedia conferencing session might be controlled by a PDA, video shown on a wall-hanging plasma display and audio acquired through an echo-canceling microphone, each with its own network interface. Event-based: We consider that events offer a useful abstraction for tying together diverse systems while requiring modest knowledge about their properties. We chose the SIP event model [29] as a core component of our system, as it scales to large numbers of users spread across administrative domains. Location-aware: Location is one of the key contexts that determines the types of devices that are available and how communication should be conducted to minimize disruption to the user. Rather than just providing geographic location information, we add higher-level information that describes the category of a place, such as "theater" or "public transport" and its properties (Section 3.2). Other user context, such as the number of people in a room, active conversations or how recently a device has been used, also influence system behavior. Privacy-conscious: We aim to give users maximum control over their incoming communication and the amount of information about their context that is revealed to others. Invisible to user: Wherever possible, we delegate system behavior to user-defined policies rather than requiring direct user interaction. Policies can be generated dynamically by presence and location information. The system is designed to support a range of activities, from home-based settings to collaboration between distant sites. It is