Advances in networking, sensors, medical devices and smart phones have made it feasible to monitor and provide medical and other assistance to people either in their homes or outside. Aging populations will benefit from reduced costs and improved healthcare through assisted living based on these technologies. However, these systems challenge current state-of-the-art techniques for usability, reliability, and security. In this paper we present the PAS open architecture for assisted living, which allows independently developed third party components to collaborate. Furthermore, we incorporate cell phones in PAS as the local intelligence in order to enhance the robustness and ubiquity. We discuss key technological issues in assisted living systems, such as software architecture layout, power preserving, security and privacy; and results from our pilot study in a real assisted living facility are presented.