Object Tracking System for Mobile Terminals: Architecture, Protocol and Its Evaluation

technologies. Our solution, which has been developed in a joint collaboration between NTT DoCoMo and DoCoMo Communications Laboratories Europe GmbH, covers two aspects of tracking in a hierarchical architecture: local and global tracking, leading to high scalability and cost efficiency compared to conventional, centralized solutions. For tracking services in local areas, e.g., tracking children in a school, we investigate the feasibility of Peer-toPeer (P2P) search strategies in an ad hoc network *2 environment based on Institute of Electrical and Electronics Engineers 802.11 (IEEE 802.11) *3 capable distributed nodes. For global tracking, a second tier structured P2P system interconnects all local tracking systems and allows a global query resolution. P2P is a networking technology, which maintains an overlay network among distributed nodes in a self-organizing manner. Our two-layered system consists of a local location tracking systems and a global location tracking system. These systems use P2P technology to look up and deliver shared resources. In comparison to conventional data gathering [1], large scale tracking of mobile objects is challenging with respect to data management and localization. Data management for mobile objects is characterized by a high number of tracked objects and a high frequency of location changes. Conventional solutions as shown in Figure 1 do not scale well due to heavy network load arising from updates sent to one central server, or they suffer from high inaccuracy if the update interval is too long (Update type in Fig.1(a)). These weaknesses can be avoided by flooding object queries to all nodes (Flooding type in Fig.1(b)). However, depending on the query frequency, this may cause unacceptable network load, leading to network instability and unavailability of data. We describe a decentralized data management solution based on P2P concepts using a Distributed Hash Table (DHT) that combines the characteristics of both of the above extremes (updating and flooding). A DHT is a data structure which provides the following functionality: given a key it returns a data item (e.g. location of target object) associated with the key. DHT refers to a hash table which is distributed among a set of computers and does not need any central servers. The network nodes run a DHT system, into which tracked objects or sensors insert their data. The DHT enables large scale data distribution and retrieval without Object Tracking System for Mobile Terminals: Architecture, Protocol and Its Evaluation