Diminishing capacity requirement for mobility database in wireless ATM

In wireless ATM, Visitor Location Register (VLR) is the database that supports mobility management. In case the number of roaming users exceeds the VLR supports, the arriving users cannot receive any service. To solve the problem, the Random Replacement (RR) policy randomly selects a victim record in VLR to be replaced with the record of the arriving user. The quality of services for the victim user will certainly drop. Therefore, the proportion of affected users is not allowed to exceed a tolerable threshold. Apparently, VLR size is the key factor of the threshold. The cost for maintaining a VLR increases significantly as its size grows. So, how to utilize the capacity efficiently is an important issue in VLR planning. We propose a Second Chance Replacement (SCR) policy to reduce the requirements for VLR capacity. As compared with the RR policy, the discrete-event simulation results show that the SCR policy can save 2.5% to 28% VLR size under various QoS thresholds.