| 摘要: | 透過無線界面進行分封交換數據服務的GPRS系統可提供行動使用者無遠弗屆的存取服務及提供更有效率的資源利用率。為了進一步支援多媒體服務，適當的服務品質管理功能必須被實作出來及詳加定義。特別地，為了能在GPRS系統之上支援即時性應用，符合點對點延遲、封包漏失機率、頻寬要求的機制扮演了很重要的角色。此外，為了支援典型的數據服務，縮減數據存取的延遲時間則是另一個關鍵議題。 在這篇論文中，為了能夠同時支援即時性應用及典型的數據傳輸，我們致力於縮減無線界面的傳輸延遲時間以及即時性應用的訊框丟失機率。而且，為了提供足夠的頻寬給即時性應用，即時性應用將有較高的優先權去存取無線資源。既然在目前的蜂巢式系統中，骨幹網路的處理能力要比無線界面強上許多，而且在無線界面上的延遲時間是點對點延遲的主要原因，所以我們假設骨幹的延遲的時間為一常數而專注於研究無線界面的延遲時間。我們在基地台子系統上，針對GPRS使用者的存取要求設計了一個新的排程方法─AI-EDF去降低延遲的成本。由於在相同的延遲要求下，我們的方法可以支援較大的系統負載，因此可以增加系統的容量。 另外，既然GPRS系統的多重存取控制機制是slotted-aloha，一筆即時性應用的存取要求可能會和其它使用者的存取要求碰撞。為了降低碰撞次數及避免封包丟失機率，我們採用了一種即時性應用的緩衝控制機制去避免碰撞發生。更進一步地，我們分析出在無線界面上送出一個即時性應用存取要求的最佳間隔時間。 為了驗證我們的演算法的效能，我們採用OPNET作為我們的模擬工具。在我們的模擬環境中，四種實際的應用型態如email、fleet management、WWW及VoIP 及它們的混合情形被我們採用來進行研究。最後，模擬結果證明了我們的促進方式比其它種選擇可以提供較多的容量以及縮減點對點的延遲時間。 GPRS (General Packet Radio Service) based on packet-switched transmission over the air interface can provide a ubiquitous data access service to mobile users and offer more efficient resource utilization. For further multimedia services, the proper Quality of Service functions must be implemented and well addressed. Specifically, for supporting real-time applications over GPRS system, the scheme to fit in with end-to-end delay, loss rate and bandwidth requirement plays an important role. Besides, in order to support typical data service, reducing data access delay is another key issue. In this thesis, we devote to decreasing the transmission delay time and real-time loss rate on air interface for supporting real-time and data applications simultaneously. Also, for supplying enough bandwidth to real-time service, real-time application will be given higher priority for channel resources. Since current backbone bandwidth ability is much more powerful than air interface in the deployed cellular system and the delay time on air interface is the main reason of the End-to-End delay, so we assume the backbone delay as a constant value and research on air interface delay. We design a new scheduling method AI-EDF (Air Interface Earliest Deadline First) aimed at GPRS access requests on Base station subsystem (BSS) to decrease delay cost and to increase system capacity owing to supporting higher system load under the same delay requirement. Since GPRS MAC access control mechanism is slotted-aloha, a real-time access request may collide with other access requests. In order to reduce collision and avoid packet loss, we adopt a real-time buffer control mechanism (RTBC) to avoid collision. Further, an optimal interval time to send out a real-time access request on air interface is analyzed. In order to verify our algorithm performance, we adopt OPNET as our simulation tool. In our simulation environment, four kinds of real traffic type and their combinations including email, fleet management, WWW, and VoIP application are studied. Results show that our improvement can offer more capacity and reduce end-to-end delay compared to the other alternatives. |
