带门槛的环形搜索移动性管理策略中移动台所越过的位置区环数

The Number of Location Area Rings Crossed by a Mobile Between Two Call Arrivals in the Ring Search Scheme with a Threshold

位置管理或移动性管理是移动计算研究领域的一个具有挑战性的问题 .我国及其他国家目前正在使用的个人通信网络如GSM、IS 4 1中 ,用两层数据库HLR/VLR支持位置管理 .在这些网络中 ,每当移动台从一个位置区LA(Locationarea)移动到另一个位置区 ,其基本的位置管理策略 (简称“基本策略”)不管是否必要均立即进行位置更新操作 ,因而造成系统资源的极大浪费 ,降低了系统的性能 .本文给出一种带门槛的环形搜索策略 (简称“门槛环形策略”)———将位置区分成环 ,设定一个门槛 (整数 ) ,当移动台越过的位置区的环数没有达到门槛时 ,系统不进行位置更新操作 ,以减少位置管理的费用 .在假定移动台的呼入是一个泊松过程 ,移动台在各个位置区的逗留时间是符合一般概率分布的随机变量的条件下 ,推导出在两次呼入之间移动台处于各位置区环的概率及移动台所越过的位置区环数公式 ,给出了“门槛环形策略”的位置管理费用小于“基本策略”的条件 .

Location management or mobility management is a challenging topic in mobile computing environment.HLR (Home Location Register) and VLR (Visitor Location Register) are currently used in some Personal Communication networks (PCN),such as GSM and IS-41,in many countries to support location management.In these networks,the basic location management scheme,called the basic HLR/VLR scheme below,updates the HLR whenever a mobile crosses the boundary of an LA (Location Area) regardless of whether the updating is necessary.As a result,this scheme produces enormous location updating cost,wastes substantial networks' resources,and reduces networks' performances.A ring search scheme with a threshold,where HLR is not updated until the number of LA rings crossed by a mobile reaches the threshold,an integer,is proposed to decrease the location updating cost.Both the probabilities of a mobile residing in LA rings and the number of LA rings crossed by the mobile between two successive call arrivals in the proposed scheme are derived on the assumption that the incoming calls to the mobile form a Poisson process and that the time of the mobile residing in each LA is a general distributed random variable.Besides,the conditions on which the proposed scheme outperforms the basic HLR/VLR scheme are given.