非同步傳輸模式網路中可利用位元速率服務的流速控制

Abstract

在廣域之整合型服務數位網路發展中，ATM是最具有希望的傳輸技術，它 能夠支援不同品質要求及不同資源需求的應用，目前ATM Forum定出一種 能有效率的傳送資料的服務—ABR服務，同時也發展出流速控制來做為ABR 服務的控制方法，因此流速控制對ABR服務的效率來說，有著關鍵的影響 性。本篇論文分五個部份來探討流速控制的效果及設計新流速控制的機 制: (I) 在來源端個數固定的情況下，流速控制的效能分析；(II)在來源 端個數變動的情況下，流速控制的效能分析；(III)在晶格層級及呼叫層 級具有爆發性的情況下，流速控制的效能分析；(IV)ER與EFCI交換器之相 互運作的研究；(V)TCP與流速控制—雙層控制的問題。最近有很多論文針 對流速控制來做分析及模擬，然而，這些論文所作的假設並不合適，包括 在交換器中緩衝器無限的容量、來源端個數固定、以及來源端持續有資料 要傳送等假設。因此本論文中我們先考慮交換器中緩衝器容量有限的情況 ，接著來源端個數變動的情況也加以考量，最後也考慮來源端的資料具有 爆發性的情形。我們提出"最佳區域"的概念，用來決定流速控制中一些重 要參數，同時我們提出三種改進方法：固定門檻、動態門檻、修正之ICR ，在來源端會出現及消失的真實情況下，這些改進能降低晶格流失的機率 及提高頻寬的使用。而在晶格層級和呼叫層級的爆發性研究中，經由分析 ，可證實晶格層級的爆發性遠比呼叫層級的爆發性來的有影響性。在不同 型態交換器之相互運作的研究中，了解到CMM這種交換器具有較佳的效果 ，同時ER交換器應該優先置放在瓶頸、最危急、及靠近來源端的位置。最 後在TCP及流速控制—雙層控制的情況下，應該使用"使用或丟棄"的策略 ，以避免過高的ACR及過低的交換器使用率。這些結果在使用流速控制時 ，將會提供相當大的幫助。 ATM (Asynchronous Transfer Mode) is the most promising transfertechnology for implementing B-ISDN (Broadband Integrated Service Digital Network). It supports applications with distinct QoS requirements such as delay, jitter, and cell loss and with distinct demands such as bandwidth and throughput. ATM Forum defined a service class known as ABR (Available Bit Rate) service to support data applications economically. Also an end- to-end adaptive control mechanism called closed-loop rate-based flow control is applied to this service. Hence the rate-based flow control plays an important role for efficient traffic management of ABR service in ATM networks. This thesis mainly includes five parts which investigate the effect of the rate- based flow control and design the new rate-based control scheme: (I) a performance analysis of a rate-based flow control mechanism under a fixed number of sources, (II) the effect of a rate-based control mechanism under a variable number of sources, (III) a performance analysis of a rate-based flow control mechanism under burstiness at call and cell levels, (IV) the interoperability of EFCI (Explicit Forward Congestion Indication) and ER (Explicit Rate) switches, and (V) the dual control problem of TCP over ABR services. Recently several analyses and simulations have been conducted for rate-based control schemes. However, some assumption of these analyses are improper, including infinite buffer at the switch, a fixed number of sources, and non-bursty greedy sources. Hence in the thesis, we consider the case that buffer at the switch is finite. Further, the condition of a variable number of sources is examined. Finally we consider the case that the source traffic is generated with a bursty behavior.We provide the concept of "best area" which helps us to determine some important parameters. Also we develop three enhancements, namely, static threshold, dynamic threshold, and revised ICR (Initial Cell Rate). These enhancements can reduce cell loss probability and raise utilization for the realistic situation where the new sources may arrive and the existing sources may depart. The effect of burstiness at cell and call levels can be observed. It isconfirmed analytically that the impact of burstiness at the cell level is obviously more influential than that at the call level. The research about interoperability of various switch schemes shows some observations which are not shown in the homogeneous environment. We understand that CMM (Charny Max-Min) is better in the homogeneous and heterogeneous environments and that ER switches should be placed at the bottleneck, most critical point and near-source location. Finally we suggest to implement the use-it-or-lose-it policy in ABR flow control to alleviate the problems of unused high ACR (Allowed Cell Rate) and switch queue underflow in the scenario of TCP over ABR where there are two flow control mechanisms working at the same time. These results provide much insight into the effect of using the rate-based flow control.