An efficient location cache scheme for three-level database architecture in PCS networks

Recently, hierarchical architecture for location databases has been proposed in order to accommodate the growing number of personal communication systems users. With the three-level hierarchical database architecture, which is compatible with the current cellular mobile systems, newly developed additional databases, including the regional location database (RLR), are positioned between the HLR and the VLRs. We propose an efficient cache scheme, the double T-thresholds location cache scheme, which could reduce the network and database costs to lookup a portable using the three-level architecture. This scheme extends the existing T-threshold location cache scheme, which is effective only under the two-level architecture of location databases currently adopted by IS-41 and GSM. The idea behind our proposed scheme is to use two pieces of cache information, VLR and RLR, which serve the called portables. These two pieces are required in order to exploit not only the locality of a registration area (RA), but also the locality of a regional registration area (RRA), a wide area covered by the RLR. We also use two threshold values in order to determine whether the two pieces are obsolete. In order to model the RRA residence time, the branching Erlang-∞ distribution is introduced. The cost analysis presented in this paper shows that the double T-thresholds location cache scheme significantly reduces the network and database costs for most patterns of portables.