一個ATM網路之多階式擁塞控制方法

Abstract

本論文中，提出了一種在非同步傳輸模式下，保證不流失重要資料的多階 層擁塞控制的方法。非同步傳輸模式已由 CCITT建議為發展寬頻整合服務 數位網路的一種傳輸交換模式，它沒有傳統頻道觀念的結構限制，它將所 有要求連線資料切成固定大小的 cells，以統計式多工的方法送到同一傳 輸線上，它能更有效率及彈性的利用傳輸線的頻寬，並且相當能滿足及時 資料之流量不均、大量突發性資料的特性。此外，簡化了交換節點功能， 在交換節點不做流量控制、錯誤偵測及錯誤更正等動作，以滿足更快速的 交換。為了得到這些效益，網路也面臨了新的問題。由於網路無法用 RR/ RNR 訊息控制網路負載，因此容易使網路產生擁塞的情況，降低網路的效 率，早期提出的擁塞的方法，乃當緩充區填滿了之後，把新到達的資料丟 棄，但在高速網路上使用這種方法，則可能有大量資料流失，大量的重送 資料，這將降低網路的效率，根據不同階層的控制，我們提出一種依密集 資料長度和緩充區大小關係的連結許可控制，配合其它階層的控制，可以 使得代表重要資料的 cells，不會有被丟棄的可能，一方面可提高網路效 率，另一方面可提高網路的可靠性及服務品質。 In this thesis, a multiple level congestion control mechanism is proposed for ATM congestion control, this method guarantees that high priority data cells will never be discarded. ATM( Asynchronous Transfer Mode ) is expected to be adopted by the International Consulative Committee for Telephone and Telegraph( CCITT ) as the multiplexing/switching technique for B-ISDN. In ATM, calls are multiplexed on a cell( fixed 53 bytes ) level, it does not have the traditional rigid channel structures. Using statistical multiplexing, it has more bandwidth efficiency and flexiblity. It also fits the bursty nature of the traffic on ATM. To take these benefits, newproblems are presented. Since the network cannot control its load with RR( Receiver Ready )/RNR( Receiver Not Ready ). The network may be congested when many traffic sources transmit their data in the same time. The congestion control mechanisms existed, simply discard the new coming cells when the buffer is full. In a high speed network, a great amount of data may be dropped when the buffer is full, and it is infficient if we retransmit these lost data. We proposed an admission control scheme which decides whether a new call request is accepted or not according to the buffer size and burst length of the call. By counting the sum of burst length of all calls to be passing through the node, if it does not exceed a threshold of buffer size, it is accepted, otherwise it is rejected. Combined this admission control with the control mechanisms in the other level, we can make sure that high priority cells will never be discarded due to a buffer overflow, as a consequence,this method improves the efficiency , realiability and QOS( Quality Of Service ) of the network.