行動電信網路中即時計費之研究

Abstract

全球行動通訊系統(UMTS)為第三代行動通訊(3G)的主流規格之一。第三代行動通訊規格組織(3GPP)第五版中提出了IP多媒體子系統(IMS)，以提供多媒體服務。若要成功推廣IMS服務，如何正確及合理地收取封包費用成為行動電信業者關心的議題。建置IMS服務需要精確的計費管理系統以及有效率的服務提供機制，因此，3GPP第五及第六版提出了一個基於IP的即時計費系統(OCS)。透過即時計費機制，電信業者可以更彈性地針對每筆交易處理帳戶餘額以及網路資源授權等。

3GPP第八版提出了長期演進技術(LTE)，此技術為UMTS的演進。由於LTE提供更高的效能以及更低的延遲時間，服務品質(QoS)控制成為有效資源管理的重點項目之一。QoS控制的要求包括：在不同QoS等級中的使用者必須能夠以不同的費率計費，且電信業者必須能夠處理資料傳輸並提供合適的QoS給使用者。QoS控制以及彈性計費的議題都在策略與計費控制系統(PCC)中提及。根據事先定義的PCC規則，UMTS/LTE管控IMS應用服務使用之IP網路資源(如預留之頻寬)。在PCC機制中，計費決策受到許多因素影響，其中包含：服務型態、使用量、以及提供的QoS等。

本論文探討即時計費及策略控制之議題。在本篇論文的第一部份，我們首先研究即時計費中OCS的點數保留機制。OCS之設計需要降低信號控制訊息的交換，以及增加系統處理計費的效能。若分配給某應用服務之點數在服務結束之前消耗完畢，則此服務需要再向OCS請求更多點數。在請求點數的同時，封包傳遞會被暫停，直到此服務向OCS取得點數為止。為了避免服務之暫停，我們提出了點數提前保留機制，也就是在點數耗盡之前先向OCS發出點數請求。我們發展出數學模型和模擬實驗，以準確分析影響系統效能之各項指標。根據本研究的結果，我們提出的方法能為行動業者提高即時收費系統的效能。

本論文的第二部份中，我們設計並實做LTE中的PCC系統。本系統根據應用服務中的資訊來做PCC決策，而這些PCC決策規則可以由使用者定義。當一個服務請求符合某個決策規則時，此PCC系統會根據QoS資訊向OCS提出計費請求。

在論文的第三部份，我們開發了基於PCC系統創建新應用服務的服務平台。藉由本平台提供的標準的網路服務(Web Service)應用程式介面(API)，應用程式開發者可以在不需知道電信網路的細節(如PCC協定)的情況下創建新的應用服務。我們以群組計費系統(GAS)應用服務為例，描述在本服務平台上如何輕易的創建新的計費應用服務。

本論文提出的研究成果，可以在電信網路中作為未來研究即時計費及策略控制之重要參考依據。

Universal Mobile Telecommunications System (UMTS) is one of the major standards for the third generation (3G) mobile telecommunications. The 3G Partnership Project (3GPP) Release 5 introduced the IP Multimedia Subsystem (IMS) to provide multimedia services. 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 IMS services requires effective charging management system and efficient service delivery mechanism. Therefore, the 3GPP Release 5 and 6 proposed the IP-based Online Charging System (OCS) to incorporate data applications with real-time control and management. Through online charging, an operator can ensure that credit limits are enforced and resources are authorized on a per-transaction basis. The 3GPP Release 8 introduced Long Term Evolution (LTE) that is a set of enhancements to UMTS. Since LTE offers higher throughput and lower latency, Quality of Service (QoS) control becomes an important issue for cost-effective resource management. It is desirable that the subscribers who require different QoS levels should be charged with different rates, and telecom operators negotiate the data transfer and offer appropriate QoS for the subscribers. The QoS control and flexible charging issues are addressed in the Policy and Charging Control (PCC). Based on the predefined PCC rules, the UMTS/LTE manages and controls the IP network resources (e.g., the allocated bandwidth) to the services. The factor that affects charging decision includes the service type, the amount of usage and the provisioned QoS. In the first part of the dissertation, we study the online credit reservation in the OCS. Specifically, the design of OCS needs to reduce the signaling overhead and improve the system performance. If the assigned credit units are consumed before the session is completed, the session needs to request more credit units from the OCS. During the credit request operation, packet delivery is suspended until extra credit units are granted from the OCS. To avoid session suspension, we propose the credit pre-reservation mechanism that reserves credit earlier before the credit is actually depleted. Analysis and simulation experiments are conducted to investigate the effects of input parameters. As a result of our study, the mobile operator can achieve high performance in the online charging management. Second, we investigate the design and the implementation of the PCC system in LTE. According to the session information, the PCC makes policy decisions, where the policy rules can be formulated based on user-defined information. When a service request meets a policy rule, our PCC system initiates a charging operation toward the OCS based on the QoS information. In the third part, we develop an IMS service platform for new service creation based on the PCC system. Through the standard-based web service APIs furnished by the service platform, application developers can create services without knowing the details of the telecommunication network such as PCC protocols. We use Group Accounting System (GAS) as an example to illustrate how a new charging application can be easily created and provided in our service platform. These research results presented in this dissertation can be viewed as a useful foundation for further studies in policy and charging in mobile telecommunications.