UMTS即時計費協定之研究

Abstract

在第三代無線通訊系統Universal Mobile Telecommunications System（UMTS），當使用 者（User Equipment；UE）經由整合封包無線電服務General Packet Radio Service（GPRS） 網路開始使用預付式服務時，閘道GPRS 支援節點Gateway GPRS Support Node（GGSN） 會向即時計費系統Online Charging System（OCS）請求預支費用。此計費控制的方法乃 由GGSN 和OCS 在Gy 介面使用Diameter 計費協定中的計費控制請求Credit Control Request（CCR）與計費控制答覆Credit Control Answer（CCA）信息來達成。 當GGSN 的預支費用罄，GGSN 會執行預留費用操作，向OCS 請求更多的費用。此時， 進行中的服務會先被暫停，直到從OCS 獲得費用才能繼續。而延遲處理服務中傳送的 使用者封包，將會降低服務的品質。為了解決這個問題，本計畫提出一個名為「提早預 留費用」的機制Credit Pre-reservation Mechanism（CPM)。這個方法定義了一個邊界參 數δ。當GGSN 預留的預支費用少於δ時，GGSN 將會提早執行預留費用操作，使得在此 操作執行時，GGSN 還是可以繼續處理使用者封包。 在CPM 機制，如果δ的值設太小，GGSN 的預支費用很快會用完，而進行中的服務就會 在執行預留費用操作時被暫停。但如果δ的值設太大，則一個進行中的服務會預留太多 費用，那麼OCS 的費用就會被很快消耗。這樣，新產生的服務要求可能就無法被接受， 而正在執行的服務也可能被強迫終止。在本研究計畫中，我們描述了數學推導和模擬兩 種模型來量測CPM 的效能表現。除此之外，我們將在GGSN 節點以及IP 多媒體子網路 之節點建置以Diameter 協定為基礎的CCR 及CCA 信息及執行程序。並實作CPM 機 制。此外，我們也使用TTCN-3 實作Diameter 之一致性測試工具，進而測試及分析計費 系統與各服務節點間運作時之效能。

In Universal Mobile Telecommunications System (UMTS), when a User Equipment (UE) originates a prepaid session through the General Packet Radio Service (GPRS), the Gateway GPRS Support Node (GGSN) will request an amount of prepaid credit from the Online Charging System (OCS). The credit control is achieved by exchanging Diameter Credit Control Request (CCR) and Credit Control Answer (CCA) messages through the Gy interface between the GGSN and the OCS. When the credit units are depleted at the GGSN, a reserve units operation is executed to request additional credit units. The session must be suspended until extra credit units are granted from the OCS. Delayed processing of user packets at the GGSN may seriously degrade the quality of service (QoS). To resolve this issue, we propose the Credit Pre-reservation Mechanism (CPM). In the CPM, we define a threshold δ. When the remaining credit c of the session is not larger than δ, the GGSN conducts the RU operation to request extra credit from the OCS. During the RU operation, the GGSN continues to process the user packets. In the CPM, if δ is set too small, then the credit units are likely depleted and the service must be suspended during the RU operation. If δ is set too large, many credit units are reserved in the active sessions, and the credit in the OCS is consumed fast. Therefore, a newly incoming session has less chance to be served, and an in-progress session is likely to be force-terminated. In this work, we will propose both analytic and simulation models to investigate the performance of the CPM. We also implement the Diameter charging protocol in the GGSN nodes and other IP multimedia service nodes, and the CPM mechanism in the application server. We will implement the Diameter conformance test tool based on Testing and Test Control Notation version 3 (TTCN-3). Through the conformance testing and the analysis, we will investigate the performance between the OCS and the service nodes.