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Fault-Tolerant Location Tracking in Mobile Networks with Quorum System
This dissertation presents some schemes for fault-tolerant location tracking in the mobile networks. One of the key issues in the design of the mobile systems is the efficient management of location information. The current IS-41 and GSM schemes use a two-tier system of home location register (HLR) and visitor location register (VLR) databases. In a two-tier system, the success of a call requires the HLR and the callee’s current VLR to be failure-free. A VLR failure affects incoming calls to mobile hosts in the VLR’s location area. Meanwhile, if the HLR fails, it cannot direct calls to a mobile host from other hosts that are not in the same location area. Thus, tolerating the failures of the location registers (LRs) is important. In this dissertation, we propose some distributed and centralized schemes, which tolerate the failures of the location registers. Meanwhile, with quorum’s salient set property and region-based approach, our schemes store/retrieve the MH location information in the location registers of a quorum set of the local region as much as possible to avoid long delays caused by the possible long-distance of VLR and HLR. Thus, they yield better Quality of Service (QoS). In this dissertation, firstly, we propose the Legion structure that can be used to construct schemes for distributed applications, such as location tracking, information dissemination, mutual exclusion, etc. We also present a new and simple distributed quorum-based location management scheme, LegRing, which is developed based on the Legion structure. With a small quorum size and the symmetric property, the LegRing scheme can be extended to a fault tolerant and load balanced location tracking algorithm. Also, it is applicable to distributed mobile platforms with any arbitrary number of nodes. Furthermore, we propose a new fully distributed fault-tolerant location management algorithm, which is based on the quorum system. In the literature, many location management schemes have been proposed by researchers, which bent their efforts towards reducing the system cost. Nevertheless, our distributed location management scheme addresses the issue of service quality. With small quorum size and symmetric property, our scheme has salient features that have fault-tolerance for servers’ crashes, load balancing among location servers, and fast query response. With these features, our scheme offers better Quality of Service (QoS) in mobile networks. In this dissertation, we also propose a centralized scheme, which tolerates the failures of the VLRs and HLR at the same time, without adding or changing any hardware of the systems. Moreover, our proposed scheme has lower HLR access rate, which can reduce the cost and delay in updating and querying. Experimental results show that our scheme can improve the performance of updating and querying in comparison with the traditional two-tier scheme. Moreover, we propose a new scheme with cellular quorum construction to tolerate the failures of the HLR and VLRs in two-tier centralized networks. Based on the intersectional property of the update quorum and query quorum, the location information is disseminated to VLRs of the update quorum’s set and can be extracted from one of them by using the query quorum even though one or more location registers fail. Thus, without adding or changing the hardware of the systems in the centralized cellular quorum networks, our scheme provides fault tolerance for the system. Meanwhile, with region-based approach, our scheme is not only fault-tolerant but also connection establishment effective. In summary, this dissertation investigates the problem of fault-tolerant location tracking in the mobile networks. Based on the Legion structure, we developed schemes both for the distributed and centralized systems. Our schemes address on the issues of fault-tolerance and service quality. Experimental results also show that our schemes have good performance compared to some current schemes.
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