IMS 之通話控制研究

Abstract

全球行動通訊系統 (UMTS) 為第三代行動通訊的主流規格之一。UMTS 的核心網路包含線路交換領域 (Circuit-Switched Domain) 與封包交換領域 (Packet-Switched Domain)。為了在行動通訊系統上整合網際網路技術，3GPP在封包交換領域上提出了 IP 多媒體子系統 (IMS)，該子系統利用 VoIP 技術提供網際網路多媒體服務。

在 UMTS 與網際網路的整合過程中，通話控制成為最關鍵的研究議題之一。隨著 IMS 的演化，許多新穎的通話控制功能相繼被提出。本論文探討 IMS 通話控制對三個主要應用服務的影響：緊急電話、隨按即說 (PoC) 以及語音通話連續性 (VCC)。

IMS 緊急電話服務允許緊急應變中心透過位置輪詢機制來持續追蹤發話者的位置，此機制具有追蹤誤差與重複詢問的問題。我們針對 IMS 的位置追蹤進行效能分析，並提出可即時追蹤與不會重複詢問的 主動位置回報機制。

隨按即說服務提供類似 "walkie-talkie" 無線對講機的群組通訊模式，並利用發言權控制機制來仲裁發言權。我們在 IMS 平台上實作了隨按即說服務，並分析發言權控制機制的效能。研究結果顯示，藉由緩衝機制~(讓未獲發言權的請求稍作等待)，隨按即說服務可以在維持相同發言權獲得率的情況下，支援到無緩衝機制時兩倍數量以上的群組成員。

語音通話連續性服務允許使用者在通話過程中切換領域。然而，領域轉換可能會產生大量的信令交換與資源保留，造成冗長的轉換延遲時間。針對此議題，我們提出在 IMS 平台上的通道保留機制，以達到快速的領域轉換。研究結果顯示，通道保留機制的優異效能在使用者行為愈不規則時成效愈顯著。

在前述 IMS 平台上衍生出來的三個研究子題中，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。本論文的研究成果可提供行動電信業者各項通話控制與應用服務之參數建議。 實驗結果證實了我們提出的方法能有效提升通話控制的效能。

Universal Mobile Telecommunications System (UMTS) is one of the major standards for the third generation (3G) mobile telecommunications. In UMTS, the core network consists of two service domains: the circuit-switched (CS) and the packet-switched (PS) domains. To integrate with the Internet, the Third Generation Partnership Project (3GPP) proposed the IP Multimedia Core Network Subsystem (IMS) in the PS domain to provide Internet services through the Voice over Internet Protocol (VoIP) technology.

In integration of UMTS and Internet, call control has become one of the most fundamental issues in telecommunications. As IMS evolves, more new call control features are supported. This dissertation investigates how the IMS call control affects three major applications: emergency call, Push-to-talk over Cellular (PoC), and Voice Call Continuity (VCC).

IMS emergency call provides the capability for the emergency center to track the caller's location through a polling procedure. The problems of existing mechanism include mis-tracking and redundant query. We study the performance of location tracking in the IMS, and then propose the Active Location Reporting (ALR) scheme which reports the caller's location in real time without redundant query.

PoC provides a "walkie-talkie"-like group communication method. The speak permission is arbitrated through a talk burst control mechanism. We implement a prototype in the IMS and study the performance of the talk burst control mechanism. Our study indicates that by including the buffer mechanism for PoC (where an ungranted request can be buffered in the queue, and may be granted later), more than twice clients can be supported while maintaining the same granting probability of a request.

VCC allows a user to switch from one domain to another during a call. Domain transfer may produce a large number of message exchanges and resource reservation that result in long switching latency. To resolve this issue, we propose the Bearer Reservation with Preemption (BRP) scheme in the IMS to support fast domain transfer. Our study indicates that when the user behavior (either in terms of call holding time or movement pattern) is more irregular, the advantage of the BRP scheme becomes more significant.

We also develop analytic models and simulation experiments to investigate the effects of input parameters. Our study provides suggestions to set up the parameters for mobile operations. Numerical experiments indicate that the proposed schemes significantly improve the performance of IMS call control.