行動通信網路之高效能即時計費機制設計

Abstract

隨著網際網路的蓬勃發展及通信技術的急速演進，利用網際網路提供高質素的多媒體服務已成為行動市場之主流趨勢。為了在行動通信系統上整合網際網路技術，第三代行動通訊規格組織（3GPP）制定了全球移動通信系統（UMTS）完全IP（All-IP）架構的規格。其中，3GPP第五版提出了一個處理網際網路多媒體服務的核心網路子系統（IMS）。透過IMS平台，行動業者能夠有效率地開發及整合包括語音、視訊與數據等組合的多媒體服務。然而，在IMS服務正式上市並獲取利潤前，行動業者必須先處理關於收費系統的各項議題。

若要成功地向用戶推廣IMS多媒體服務，行動電信業者必須能正確及合理地針對服務內容的價值收費。為了能夠有效率地開發及建置IMS服務， 3GPP第五版與第六版提出了一個具有開放性和通用性的即時計費系統（OCS）。透過即時計費的管理機制，業者可以更彈性地即時處理網路資源授權、客戶帳戶餘額以及線上批價等等。

行動服務的即時收費受許多因素影響，其中包括使用者的收費模式，服務類型，資源分配與服務品質等等。因此，即時計費系統的設計必須要達到兩項要求。第一，增加系統處理計費事項的準確性；第二，盡量減低控制訊息的交換。在這篇論文的第一部份，我們首先提出一個能在IMS架構上整合預付式網際網路服務的伺服器。當預付式語音服務與訊息服務同時提供時，其中一項重要議題是要在不中斷進行中的語音服務下，提供即時訊息的傳送服務。在本論文中，我們提出一項控制即時訊息傳送的策略，並分析其對整個系統之效能影響。

在本論文的第二部份，我們研發出能在即時收費系統上提供高效能、低成本的餘額分配管理機制。我們探討兩個重要的即時收費程序：餘額保留機制與餘額重新授權機制。我們首先針對預付式使用者，解決了當餘額不足時之處理程序。在這個機制中，當使用者之可用餘額低於系統預設的臨界值時，即時計費系統會向使用者發出餘額過低的警示訊息，以便提醒使用者盡快補充其預付帳戶內之金額。業者在使用這個機制時，必須謹慎地設定這個臨界值參數。一方面要避免因餘額不足而被迫中斷使用中的服務的機率，另一方面要降低警示訊息之發送頻率。我們探討了這個臨界值的設置對即時收費系統之各種影響，並向業者提供適當之參數設定建議。為了進一步提高即時收費系統的效能，我們研究由服務品質改變而引起之餘額重新授權程序。本論文以一個動態改變的臨界值機制，有效地降低餘額重新授權程序中所交換的控制訊息數目。

針對以上三個研究題目，我們分別發展出數學模型及模擬實驗，以準確分析影響系統效能之各項指標。根據本論文的研究結果，我們為行動業者提供各項即時收費機制之參數建議。實驗結果證實了我們提出的方法能夠提高整個即時收費系統之效率。

Introduction of the 3G mobile system has driven the Internet into new markets to support mobile users. To integrate IP with wireless technologies with the right business model, the 3rd Generation Partnership Project (3GPP) proposed UMTS all-IP architecture that evolved from Global System for Mobile Communications (GSM) and General Packet Radio Service (GPRS). 3GPP Release 5 introduces the IP Multimedia Core Network Subsystem (IMS) on top of the PS service domain to provide multimedia services. Before the benefits of IMS and all-IP can be realized, a fundamental issue must be addressed is charging/billing, which is one of the most important activities in telecommunications.

In order to successfully promote IMS services, how to charge packet data service accurately and reasonably has become a major concern of operators. The deployment of the IP multimedia services requires effective charging management system and efficient service delivery mechanism. Therefore, 3GPP Releases 5 and 6 propose the convergent and flexible IP-based Online Charging System (OCS) to incorporate data applications with real-time control and management. Through online charging, the operator can ensure that credit limits are enforced and resources are authorized on a per-transaction basis.

In mobile data services, there are many factors that affect charging, for example, the service type, the amount of usage and the provisioned QoS. The design of online charging system needs to reduce the signaling overhead and improve the system performance. In the first part of the dissertation, we first study how to develop an IMS application server to integrate existing IP-based prepaid services in the UMTS environment. When both voice and messaging are simultaneously offered, a potential problem is that the delivery of a prepaid message during a call may result in force-termination of that call due to credit depletion. To address this issue, we describe a strategy to determine if a prepaid message can be sent out during a call session.

In the second part of the dissertation, we investigate how to provide efficient credit quota management with lower signaling overhead and higher accuracy in the OCS. Specifically, we study the credit reservation and credit re-authorization mechanisms in the OCS, which are two most essential procedures in online charging management. First, we study the online credit reservation procedure for prepaid users in the UMTS online charging system. When the remaining amount of prepaid credit is below a threshold, the OCS should remind the user to refill the prepaid account. It is essential to choose an appropriate recharge threshold not only to avoid forced termination for the in-progress service sessions but also to decrease the recharge signaling overhead. Then we focus on the online credit re-authorization procedure of the OCS. The main objective is to reduce the traffic signaling overhead for credit re-authorization due to frequency QoS changes. We propose a threshold-based scheme that can significantly reduce the number of re-authorization message exchanges during a session.

We also develop analytic models and simulations experiments to evaluate the performance of the proposed schemes. Based on our study, the mobile operator can achieve high performance in the online charging management by configuring appropriate parameters.