Introduction Autonomous robots operating in real environments need to be able to interact with a dynamic world populated with objects, people, and, in general, other agents. The current generation of autonomous robots, such as the ASIMO robot by Honda or the QRIO by Sony, has showed impressive performances in mechanics and control of movements; moreover, recent literature reports encouraging results about the capability of such robots of representing themselves with respect to a dynamic

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... world, of planning future actions and of evaluating resulting situations in order to make new plans. However, when multiple robots are supposed to operate together, coordination and communication issues arise; while noteworthy results have been achieved with respect to the control of a single robot, novel issues arise when the actions of a robot influence another's behavior. The increase in computational power available to systems nowadays makes it feasible, and even convenient, to organize them into a single distributed computing environment in order to exploit the synergy among different entities. This is especially true for robot teams, where cooperation is supposed to be the most natural scheme of operation, especially when robots are required to operate in highly constrained scenarios, such as inhospitable sites, remote sites, or indoor environments where strict constraints on intrusiveness must be respected. In this case, computations will be inherently network-centric, and to solve the need for communication inside robot collectives, an efficient network infrastructure must be put into place; once a proper communication channel is established, multiple robots may benefit from the interaction with each other in order to achieve a common goal. The framework presented in this paper adopts a composite networking architecture, in which a hybrid wireless network, composed by commonly available WiFi devices, and the more recently developed wireless sensor networks, operates as a whole in order both to provide a communication backbone for the robots and to extract useful information from the environment. The ad-hoc WiFi backbone allows robots to exchange coordination information among themselves, while also carrying data measurements collected from surrounding environment, and useful for localization or mere data gathering purposes. The proposed framework is called RoboNet, and extends a previously developed robotic tour guide application in the context of a multi-robot application; our system allows a team of robots to enhance their perceptive capabilities through coordination obtained via a hybrid communication network; moreover, the same infrastructure allows robots to exchange information so as to coordinate their actions in order to achieve a global common goal. The working scenario considered in this paper consists of a museum setting, where guided tours are to be automatically managed. The museum is arranged both chronologically and topographically, but the sequence of findings to be visited can be rearranged depending on user queries, making a sort of dynamic virtual labyrinth with various itineraries. Therefore, the robots are able to guide visitors both in prearranged tours and in interactive tours, built in itinere depending on the interaction with the visitor: robots are able to rebuild the virtual connection between findings and, consequently, the path to be followed. This paper is organized as follows. Section 2 contains some background on multi-robot coordination, and Section 3 describes the underlying ideas and the motivation behind the proposed architecture, whose details are presented in Sections 4, 5, and 6. A realistic application scenario is described in Section 7, and finally our conclusions are drawn in Section 8. Related Works In the past years, various classifications of multi-robot systems have been proposed. Dudek, et al., for instance, have proposed a taxonomy on communication mechanism and their cost to highlight that different multi-robot systems have very different capabilities (Dudek et al., 1996) . The taxonomy proposed by Dudek takes into account some criteria, such as the number of robots in the collective, the maximum distance between robots such that communication is still possible, the communication topology, the composition of the collective, and the computational model of individual robots. Some of the works presented in literature achieve coordination among robots through distributed control, as in the case of the Alliance architecture (Mataric, 1997) , where a robot increases the utility measure for the task that it is currently accomplishing while it decreases it for all other tasks; each robot then observes the behavior of its team-mates and selects the fastest achievable task. On the other side, the MARTHA project (Alami et al., 1998 ) assumes a centralized control to coordinate a team of autonomous robots for transport application in structured environment. Parker in (Parker, 2003) presented a review of the main topic areas of research regarding multi-robot systems: biological inspired robot teams, communication, architectures and task planning, localization and mapping, object transport and manipulation, motion coordination, reconfigurable robotics, learning. A large amount of research has been dedicated to the issue of communication in multi-robot systems. Several studies have been conducted to assess the benefits provided by communication on the performance of a robot team. Balch and Arkin conducted experiments with robots equipped with LED indicators signaling the state they were in (Balch & Arkin, 1994) . The results indicated that communication considerably improves system performance. Simple communication strategies are preferable, because more complex approaches do not significantly improve results. Mataric (Mataric, 1998) used communication to share data between robots in order to compensate for the limitations of direct sensory modalities. The proposed networking