The dawning of the 21st century has seen unprecedented growth in the number of wireless users, applications, and network access technologies. This trend is enabling the vision of pervasive ubiquitous computing where users have network access anytime, anywhere, and applications are location-sensitive and context-aware. To realize this vision, we need to extend network connectivity beyond private networks, such as corporate and university networks, into public spaces like airports, malls, hotels, parks, arenas, and so onthose places where individuals spend a considerable amount of their time outside private networks.In this article we argue that wireless LAN technologies are the ideal mechanism for extending network connectivity to these public places, and enabling location and context-aware applications in them. However, implementing and deploying public area wireless networks (PAWNs) present a number of practical challenges, including network security, privacy, authentication, mobility management, and provisioning of key services. We discuss these challenges as a general problem for PAWNs, and then describe a PAWN we have designed, implemented, and deployed called CHOICE that addresses them. We describe the architecture and components of CHOICE, the. service models it supports, and the location services and context-aware applications we have implemented and deployed in it. 40 1070-9916/02/$17.00 0 2002 IEEE IEEE Wireless Communications Feburary 2002 42 IEEE Wireless Communications 9 Feburary 2002 -=;-;L . .
A bstructAlerts refer to the delivery of user-subscribed infornzation to the user. As the number of alert services and the types of information delivery devices increase, a new model that allows users to manage alert delivery and avoid alert ovevfow is needed. The unique dependability challenge in the management of alerts is in the proper use of redundancy to achieve timeliness and reliabilitywithout being unduly intrusive or cumbersome.We describe the design, implementation, and user experience of an alert service architecture, called SIMBA. SIMBA utilizes Instant Messaging with acknowledgements as the universal, reliable alert delivery channel, with emails being the fallback channel. All alerts that a user subscribes to are first directed to the user's MyAlertBuddy, which allows centralized delivery preference customization and acts as a personal alert router to protect the privacy of user addresses. Delivery modes, each of which involves multiple user addresses to accommodate communication failures, are supported as an abstraction f o r specihing personalized dependability levels. A working implementation of the SIMBA system, which integrates five different types of alert services, is described. Challenges and techniques in maintaining a highly available MyAlertBuddy to avoid single-point of failure are discussed. The concept of exception-handling automation is introduced for enhancing the robustness of applications that drive third-par5 communication client software through automation interfaces.
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