非同步傳輸模式網路中擁塞狀況的避免

Abstract

在以非同步傳輸模式（Asynchronous Tranfer Mode, ATM）為交換技術的網路中，如何利用擁塞控制方法以有效率且分平的方式來分配網路資源，達到在合理的成本之下滿足使用者的服務品質（Quality-Of-Service）要求，是非常重要的。網路必須對使用者加以監督與控制使其在傳送資料時能遵守其在進入網路時與網路所建立的使用限制。這些使用限制包括傳送資料的平均速率、最高速度與最大突發量資料長度等參數。假如使用者沒有服從這參數則網路必須對這些違規的傳輸資料加以限制。目前在ATM中所有的流量管理（policing function）方法中以隙漏水桶法（leaky bucket）為最著名。隙漏水桶法乃是執行於網路周圍，但其有可能在沒參考封包的重要性之下將重要的封包加以丟棄或標示。這樣將會使被丟棄的重要封包重新傳輸以致降低網路的效率。 在本論文中，我們提出了一個新的流量管理方法，稱為具有優先權（prioity）控制的一般化隙漏水桶法，來解決這一個問題。這個方法結合了傳統的隙漏水桶法與優先權的控制來達到與使用者資料相關的流量控制。本方法執行於每一個虛擬連接上，並且依照封包的優先權將封包標示以達最佳的頻寬利用率與限制使用者傳送資料的最高速度。同時我們在使用者與網路界面上與網路中的交換節點上分別發展了模擬的模型。模擬的結果顯示我們所提出的具有優先權控制的一般化隙漏水桶法是一個能有效防止網路發生擁塞狀況的流量控制方法。

It is necessary that an ATM-based network be eauipped with a congestion mechanism to effectively and fairly allocate shared network resources such as transmissson bandwidth and buffer capacity in a manner that provides satisfactory quality-of-service (QOS) for all network users with acceptable implementation costs. Traffic from users may need to be monitored and enforced to cmply with traffic parameters that could be established at the time of its admission to the ATM network. These parameters may include average and peak bit rates, and maximum burst lengths. If the user dose not conform to the parameters, actions must be taken for the violated traffic. In currently proposed ATM policing functions, the leaky bucket mechanism is the most popular approach. However, it is carried out at the network edge, which opens the possibility of cells being discarded or marked without reference to the actual massage. This may lead to degradation of the network performance. In this thesis, we propose a new policing funciton, called the generalized leaky bucket leaky bucket mechanism with priority control, to solve this problem. This scheme combines the traditional with priority control to provide a service-dependent marking. The proposed scheme operates on a per virtral connection basis. It marks the cells according to their priorpity so as to achieve an optimistic bandwidth utilization and limit the peak rate at which the cells enter the network. Simulation models for both the user-network interface and the intermediate switching node are developed. Simulation results show that the generalized leaky bucket with priority control is an effective approach to prevent congestion from occurring.