在本論文中，我們介紹號碼可攜以及現今用來實現它的方法。在每一個行動網路中都會建立自己的資料庫(database)，用來記錄做過號碼可攜的用戶的資料，包括行動電話號碼、原屬的行動電話網路(NRH network)、現在歸屬的行動電話網路等等。
現今有兩種方法用來實現號碼可攜，DR和IR。DR是各個業者把每一個網路中做號碼可攜的號碼全部紀錄在資料庫中，當屬於自己網路的用戶打給那些號碼，會經由尋問資料庫找到受話端所屬的網路。IR並不用紀錄每個網路的號碼可攜用戶，而只需要記錄從自己網路移到別的別的網路的號碼。當屬於自己網路的用戶打給號碼可攜的號碼，會透過受話端號碼原來所屬的網路去詢問資料庫找到受話端現在所屬的網路然後幫忙傳遞並建立通話。DR必須耗費大量的成本建立龐大的資料庫，而IR雖然可以節省資料庫的成本，但它必須透過額外詢問NRH網路來得知受話端號碼的資料，造成額外網外通訊的成本。我們提出DR-RRD，用來改善上面兩個缺點。我們讓資料庫中只存常通話的號碼可攜用戶，很少通話甚至沒通話過的號碼則不必存在資料庫中。我們利用取代的方法來達到這個目的。
最後，我們討論DR、IR、DR-RRD所花費的成本，由不同的參數設定找到一個邊界，並討論這些參數的意義，讓我們所提出的方法能比DR或IR省更多的成本。

In this thesis, we introduce number portability and its call routing mechanisms, DR and IR. We establish a number portability database to record these ported numbers. DR must consume much cost to establish a very huge database. IR must communicate with NRH network to obtain the information of ported users.
We propose DR-RRD to improve the above two drawbacks by using replacement algorithms. DR-RRD only record ported users with high call arrival rate. Because of unknowing the distribution of call arrival rates of ported users, we propose four replacement algorithms in order to achieve the above purpose. Through the simulation, we compare these replacement algorithms and discuss their advantages and disadvantages. DR-RRD can save more memory for database without depending on NRH network. We mention short message service and signaling protocol for number portability with DR-RRD.
We will discuss cost, database memory and signaling routing cost, between DR, IR, and DR-RRD. At last, we will analyze all parameters to find a bound, which let DR-RRD be best mechanism with the least cost.

Bibliography
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[2] Yi-Bing Lin, “Number Portability for Telecommunication Networks,” in Proc. IEEE Network, January/February 1999.
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