Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/91070
標題: 大型有線與無線網路架構下之壅塞控制器設計
Design of Congestion Controllers in Large-Scale Wired/Wireless Networks
作者: 徐秉民
Ping-Min Hsu
關鍵字: Congestion Control;Disturbance Rejection;Delayed System;Input-Output Finite-Time Stability;Robust Stability;壅塞控制;抗雜訊設計;時間延遲系統;輸入輸出有限時間穩定;強健性穩定
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摘要: 
This dissertation studies congestion control in large-scale wired/wireless network with transmission control protocol (TCP) under random early detection (RED); it presents i) a methodology capable of time-delay compensation by compensating a novel pseudo disturbance, named as extended network disturbance (END), and ii) a robust control method compensating time-delay effects by rejecting nonlinear extended network disturbance (NEND) in networks. The former method is constructed under a large-scale model, capturing averaging TCP data transmission behavior of the large-scale wired/wireless network and derived from the fluid-flow point of view. It's quite common that the network system exhibits a time-varying time delay, which is the major source of causing system instability. Therefore, we require a delay compensator established on the basis of END, which characterizes the time-delay effect on system stability. That is, compensating END further fulfills the compensation task. The END-based congestion controller aimed at compensating END is further proposed to guarantee input-output finite-time stability (IO-FTS) of the large-scale model; it has been verified on NS2. However, the considered dynamics may still exhibit steady-state errors or queue oscillation. This gives rise to the demand of further compensation strategy.
The controller is developed based on analysis results of robust asymptotic stability for linear time-varying (LTV) uncertain systems. Robust input-output finite-time stability (IO-FTS) of the LTV uncertain system over a bounded time interval is first analyzed by proposing a theorem characterizing its sufficient conditions. The possible issues of steady-state errors and state oscillation are resolved by proposing an improved strategy in view of the averaging behaviors of state variables. To proceed, a novel type of robust stability—namely robust asymptotic stability with respect to state mean values—for LTV uncertain models is addressed. The system will possess this kind of stability if its state mean varies toward an equilibrium point. Based on the results of this stability, robust asymptotic stability is further analyzed by identifying the oscillating behavior. The congestion control problem is next tackled by using the above analysis results.
Not only the time-varying congestion delays but omission of the difference between queue length/window size and its average may decrease the queue management efficiency. The former destabilizes the network while the latter leads to queue oscillation in practical applications. Despite the END-based methodology is proposed to release the first cause, it only works efficiently around an operating point. To resolve, we propose the robust congestion controller focusing on the time-delay rejection and queue oscillation elimination using the idea of nonlinear extended network disturbance (NEND), which is used to characterize the time-delay effect in the network. The control method with NEND rejection is established by implementing the results related to the robust asymptotic stability analysis; it has also been verified on the network simulator (NS2) showing their applicability.

此論文提出兩種適用於運行傳輸控制協議(transmission control protocol, TCP)與隨機早期偵測(random early detection, RED)策略之大型有線或無線網路架構的壅塞控制策略。第一種為利用擴展性網路干擾訊號(extended network disturbance, END)的概念設計時間延遲補償器,並設計與之相結合的壅塞控制架構。第二種為利用非線性擴展性網路干擾訊號(nonlinear extended network disturbance, NEND)來設計抗時間延遲特性,並應用至主動式佇列管理機制之建構上。為了完成壅塞控制器的設計,本研究利用流量觀點建立用以描述大型網路架構的封包傳輸行為之流量模型。於此模型中可知,不穩定的佇列壅塞會造成傳輸延遲時間的時變特性,並可能破壞網路系統的傳輸穩定性。因此透過將時間延遲產生原因建模為END,藉END補償來達到時間補償的效果,進而設計出可確保系統具有輸入輸出有限時間穩定(input-output finite-time stability, IO-FTS)特性的壅塞控制器。然此時系統可能依然存在著穩態誤差與狀態震盪現象。
為了解決前述缺點,本論文接著探討如何針對大型網路系統設計強健性壅塞控制器。為了完成此設計,必須先思考如何針對一個線性時變(linear time-varying, LTV)系統設計強健控制器。文中先就其輸入輸出有限時間穩定特性加以探討。然如前所述,此系統依然會具有穩態誤差與狀態震盪。為了克服此缺失,本文提出了一種新的穩定性特徵,名為對應系統狀態期望值之強健漸進穩定性。當系統的狀態期望值最終收斂至所需的平衡點時,即可說此時系統具備了此種穩定特性。基於此概念,系統的強健穩定特性可藉辨識其是否具備震盪行為來完成所需探討。
一般來說,網路傳輸系統的穩定性會被以下兩因素加以破壞:一者為時變時間延遲;二者為所忽略的實際與平均狀態值之誤差。前者會破壞系統穩定性而後者會導致壅塞控制效率下降。雖然前述之END控制策略可有效降低前者之影響,但其只適用於系統平衡點附近行為。有鑑於此,本文接著利用NEND之概念設計全域控制策略。先透過NEND將系統轉換成非時變,但具備外在干擾訊號輸入之系統。接著利用前述強健穩定分析結果設計所需的壅塞控制器,進一步使系統具備強健漸進穩定特性。最終於網路模擬器(network simulator version 2, NS2)中驗證文中所有策略的控制成效。
URI: http://hdl.handle.net/11455/91070
其他識別: U0005-0411201313520700
Rights: 同意授權瀏覽/列印電子全文服務,2015-11-07起公開。
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