Mobile Networks and Applications 7, 315–328, 2002  2002 Kluwer Academic Publishers. Manufactured in The Netherlands. Keep Your Data Safe and Available While Roaming YOLANDA VILLATE CSSI Department, University of Zaragoza, Maria de Luna 3, 50018 Zaragoza, Spain ARANTZA ILLARRAMENDI CLS Department, University of the Basque Country, Apdo. 649, 20080 San Sebastian, Spain EVAGGELIA PITOURA CS Department, University of Ioannina, GR-45110 Ioannina, Greece Abstract. The possibility of accessing and/or receiving local or remote data anywhere and at anytime constitutes an important advantage in many business environments. However, when working with mobile devices, users face many problems, such as: (1) devise exposure problems – mobile devices are more vulnerable and fragile than stationary devices, because they can be easily stolen, lost or damaged, (2) media problems – wireless communications are often unstable, asymmetric and expensive, and (3) availability problems – mobile devices stay disconnected for long periods of time. To alleviate these problems, we present a service, Data Lockers, which offers to its users first, the possibility of keeping their data in a secure and safe space in a proxy, thus alleviating the device exposure problem. Next, data stored in a data locker are available even when the mobile device is disconnected, thus providing a solution to the availability problem. Finally, specific tasks are carried out at the fixed network on behalf of the mobile user, in this way relieving the media problem. The architecture of the Locker Rental Service is based on mobile agents. These agents, and the locker, stay always close to the location of the user, traveling to meet the user wherever the user moves, therefore, allowing users to use anywhere-anytime, ubiquitous persistent storage space located at the fixed network. Keywords: mobile computing, multi-agents systems, data storage, wireless services 1. Introduction The sheer rapidity of the spread of both wireless and In- ternet technologies are favoring a new telecommunication revolution. In the near future, the general-purpose devices of today will be complemented with new easy-to-use, low- maintenance, portable, ubiquitous, and ultra reliable task- specific devices [4]. Even if future devices may not be as resource-limited, they will still be constrained in terms of size, power consumption, and intermittent connectivity. To confront such constraints, it is desirable to define a frame- work for mobile computing where services can be accessed, or tasks can be accomplished, ubiquitously and without the need of the direct intervention of the user. To this end, we are developing the ANTARCTICA 1 System [7] that pro- vides a set of data services that enhance the capabilities of Mobile Units (MU) and offers new possibilities to mobile users. The architecture of ANTARCTICA is based on the Client/Intercept/Server model [18,19] that incorporates mod- ules and agents both at the mobile devices and at intermedi- ary elements, or proxies, situated at the fixed network. Each proxy is called a Gateway Support Node 2 (GSN). 1 ANTARCTICA: Autonomous ageNT bAsed aRChitecture for cusTomized mobIle Computing Assistance. 2 The Gateway Support Node name is borrowed from the General Packet Radio Service (GPRS). We take GSM and GPRS as cellular network model for our work. The goal of this paper is to describe the design and imple- mentation of a service central to ANTARCTICA: the Locker Rental Service. This service incorporates mechanisms that al- low mobile users to rent storage space, called lockers, at the GSN. The service provides a variety of types of lockers so users can choose the one that better fits their needs and pref- erences. The Locker Rental Service offers several advantages to the users of mobile units: 1. Storage space. A mobile client can deposit in the locker all required data and results obtained for it. The locker con- stitutes an ubiquitous and persistent storage space kept al- ways close to the user, available at anytime and from any- where. 2. Data protection. The data stored in the locker are pro- tected against unauthorized accesses and modifications as well as any unexpected failures. This characteristic makes the locker an appealing option for maintaining a secure backup copy of some files of the MU, such as configu- ration files or files containing sensitive information, that need to be protected against losses. 3. Higher presence at a lower cost. An agent manages au- tonomously the locker contracted by the mobile client, so the user can stay disconnected for longer periods of time. The agent can retrieve and manipulate data stored in the locker or store new data in it. 4. Wireless communications optimizations. The space in the locker can be used to store data until it is possible or de-