非同步傳輸模式交換網路輸出佇列狀態之分析與呼叫建立控制方法之研究

Abstract

多年來，人們致力於改善通訊品質及頻寬以順應日增的通訊需求。然而，今日由於多媒體資料包含許多訊號源，且各個訊號源之型態又不相同，因此，現今的傳輸交換面監了更為複雜的問題。但問題亦隨著寬頻整合服務數位綱路的引入而獲得解決。而在實現寬頻整合服務數位綱路技術中，非同步傳輸模式由於其高效率及富彈性而成為最受歡迎之技術。 在非同步傳輸模式綱路中，傳輸速度遠高於傳統使用的綱路。然而其亦面臨一些問題。例如，假若綱路正處於極度擁塞狀態，亦即，過多從訊號源送出的細胞由於擁塞而被丟棄時，此時往往難以回復至正常的狀態。因此，找出一個能使綱路正常受控的方法顯得十分重要。關於擁塞預防的其中一個重要主題為呼叫建立控制（Call Admission Control），其控制準則為須基於一些可信參數，如：服務品質參數（QoS）等。 我們在此提出一個計算瞬時細胞遺失率的方法，並用計算出之結果來決定是否接受新的呼叫。原始的細胞遺失率為根據目前綱路狀態所測出，而當新呼叫要求建立連線時，細胞遺失率的增加量將經由特定的數學分析而計算出來。 我們先建立一些數學模型來描述非同步傳輸模式綱路，以方便分析細胞遺失率。令人驚喜的是分析出的數學公式十分簡單，這將使系統的計算量減少，而使呼叫建立控制之決策更為迅速。

For years, efforts have been made to improve the communication quality and bandwidth for the increasong traffic volumn of the communication needs. However, today the transmission and switching for multimedia data have met big problem since it contains a large variety of traffic sources, with many sources different characteristics. With the introduction of B - ISDN, the problem seems to be solved well. Among the techniques used to implement the B - ISDN architecture, the ATM (Asynchronous Transfer Mode) is found to be the most promising protocol due to its high efficiency and flexibility. In an ATM network, the transmission speed is much higher than that in the traditional networks. But some problems are also met in the network. For example, if the traffic state of the network is at serios condition that is, too many cells generated from the transmitted sources are discarded due to netowork congestion, then in most cases it is hard to recover the netoork to normal state. Therefore, a good way for the network system to keep the network under control is very important. One of the important topic for congestion avoidance is the CAC (Call Admission Control). The admission decision should be made based on some convincing factors, such as Qos. Here we will propose a method for estimating instantaneous cell loss rate and thus use the result to determine whether or not to accept a new call. The original cell loss rate is measured based on the current traffics tate of the ATM network. And the increment in the instantaneous cell loss rate when a new call asks for connection is then calculated based on some mathematical analysis. We use some mathematical models to describe the ATM network, so that analysis on instantaneous cell loss rate could be proceeded well. What is amazing is that the result of the estimated formula is quite simple. This would do help to the call admission control since the calculation work for the system to make the decision could be much lessen.