ATM網路中頻寬的配置和服務品質的保證

Abstract

非同步傳輸模式(ATM)將會是未來通訊網路的主幹,並且用於多媒體( multimedia)資料的傳輸,它利用統計式多工(statistical multiplexing) 的方式來傳遞固定長度的晶格(cell).非同步傳輸模式最強的功能就是其 處理突然大量擁至資料(bursty traffic)的能力,而且這些突然而至的大 量資料對我們的傳輸品質(Quality of Service, QOS)有嚴重的影響.在這 篇論文中提出了一個方法,不只保証傳輸的服務品質,也兼顧到非同步傳輸 網路的利用率.晶格遺失率(Cell Loss Probability,CLP)與晶格延遲時 間 (cell delay time)是估計傳輸品質的兩個基本參數,在我們的方法中, 我們利用不同等級(priority)的晶格以及使用擠出法的緩衝區 (push- out buffer)達到對估計晶格遺失率(Estimated Cell Loss Probability, ECLP)的控制.在這裡的估計晶格遺失率就是晶格遺失率的上限值.另外利 用監督功能中的截止值可用來控制晶格在路徑上的延遲,而且由於估計晶 格遺失率的即時特性(real-time property),我們利用動態調整的方式將 它固定在一個定值.最後,這個方法被應用在入口控制(admission control),利用其固定頻寬(equivalent bandwidth)配置的方式來簡化其 判斷是否接受或拒絕一個呼叫的作業流程,在這篇論文中,估計晶格遺失率 與延遲時間的可控制性,都會被討論.這個方法將使得需要配置的頻寬為最 小,同時保証傳輸的服務品質,並使得網路的利用率達到最高. Asynchronous Transfer Mode (ATM) will dominate future communication networks. To support the transmission of multimedia data traffic, it uses statistical multiplexing of fixed length cells. The major strength of ATM is its ability to handle bursty traffic. Bursty traffic has a significant effect on the QOS (Quality Of Service) of a connection. The thesis develops a scheme for guaranteeing QOS and improving bandwidth utilization in ATM networks. There are two major QOS measurements: cell loss probability and cell end-to-end delay. In our scheme, we use different priorities of cells and a push- out buffer to control the ECLP (Estimated Cell Loss Probability) which bounds the end-to-end CLP (Cell Loss Probability). An additional threshold value in the traffic policing function is used to enforce the end-to-end cell delay. With the real-time property of ECLP, we also propose a dynamic threshold method for bounding ECLP. The scheme is applied to admission control which greatly simplify the decision whether to accept or reject a new call by using equivalent bandwidth assignment. The controllability of ECLP and delay time in ATM is demonstrated. The system also makes minimal allocation of bandwidth to satisfy QOS and thus increases the overall network utilization.