全光分波多工封包交換都會環狀網路之高效能服務品質保證媒介存取控制技術

Abstract

下一代全光都會型網路(Metropolitan Area Networks; MANs)旨在低成本有效率地運用先進的光封包交換技術(Optical Packet Switching; OPS)支援各類型要求高頻寬之網路應用程式以及訊務特性趨於動態變化之網路應用程式。此篇論文提出一高效能服務品質保證媒介存取控制機制，應用在我們建立的高效能分波多工光封包交換都會環狀網路(High-performance OPS Metro WDM slotted-ring Network, HOPSMAN)實驗平台。HOPSMAN的設計為一可擴展性架構，所以網路節點數目可不受光波道的數目限制。HOPSMAN網路中包含數個服務節點，額外配備時槽除訊器，具備時槽除訊功能，以高效率低成本的方式增加頻寬利用率。HOPSMAN最重要的設計為其獨一無二的媒介存取控制機制，稱為機率式定額與額外配額(Probabilistic Quota plus Credit; PQOC)；而後，我們又在其上加入服務品質保證(Quality of Service; QoS)的功能，稱為機率式定額與額外配額服務品質保證(Probabilistic Quota plus Credit with QoS Assurance; PQOC/QA)。藉由機率式定額傳送資料的方式，本媒介存取控制機制可以高效率且公平的使用頻寬。根據服務節點的數量，目標節點的訊務量分配方式，我們分析計算出該定額分配量。除此之外，為應付極具動態變化的都會型網路的訊務量，本媒介存取控制機制引進一時間控制機制的額外配額方式來公平使用多餘的頻寬。更甚者，為了支援服務品質保證以及解決分波多工網路固有的存取問題，PQOC/QA利用簡單且具彈性的標記方法來執行時槽預訂。為了更適應動態的即時訊務(VBR)，我們不像以往其它的研究著重於估算動態訊務量；取而代之，PQOC/QA簡單地採用平均連線速率頻寬保留的方法在環狀網路上的每個循環(cycle)預定保留頻寬，彈性地建立即時連線以傳送動態的即時訊務。另外，根據M/G/m 排隊理論的分析，我們發展了一個獨特的概算方式求得平均建立連線等待時間。本分析的伺服機數量為系統預先定義的即時訊務的最大可接受定額數，服務時間包含一指數分佈之長度及外加一常數值。關於M/G/m 排隊理論，除了少數的服務分佈可以得到準確結果之外；以往，針對具某些特性的一般服務分佈的概算分析，只能達到10%的相對誤差值。但我們針對本系統內特定的服務分佈，我們提出一概算方式，所求得的結果與模擬實驗結果完全吻合。更甚者，經由深入的模擬結果，藉由本篇論文所提出的媒介存取控制機制，即使在各種負載或大量突發訊務之下，HOPSMAN可以達到更為優異的系統輸出，低延遲，以及卓越的即時訊務表現，達到高即時訊務輸出，以及極低的VBR延遲及延遲變化量。

Future optical Metropolitan Area Networks (MANs) have been expected to exploit advanced Optical Packet Switching (OPS) technologies to cost-effectively satisfy a wide range of applications having time-varying and high bandwidth demands and stringent delay requirements. In this thesis, we present a high-performance real-time medium access control scheme for our experimental high-performance OPS metro WDM slotted-ring network (HOPSMAN). HOPSMAN has a scalable architecture in which the node number is unconstrained by the wavelength number. It encompasses a handful of nodes (called server nodes) that are additionally equipped with optical slot erasers capable of erasing optical slots resulting in an increase in bandwidth efficiency. In essence, HOPSMAN is governed by a novel medium access control (MAC) scheme, called Probabilistic Quota plus Credit (PQOC), which is further enhanced with QoS assurance, called Probabilistic Quota plus Credit with QoS Assurance (PQOC/QA). The proposed MAC scheme embodies a highly efficient and fair bandwidth allocation in accordance with a quota being exerted probabilistically. The probabilistic quota is then analytically derived taking the server-node number and destination-traffic distribution into account. Besides, the MAC scheme introduces a time-controlled credit for regulating a fair use of remaining bandwidth particularly in the metro environment with traffic of high burstiness. Moreover, PQOC/QA adopts slot-basis reservation through a simple and flexible marking mechanism to support QoS and to resolve the intrinsic access problem in WDM network. Instead of focusing on estimation of the bandwidth requirements, PQOC/QA sets up real-time connections by employing constant mean rate reservation on each cycle of the ring and effectively accommodates bursty real-time traffic (VBR). Furthermore, we develop a novel approximation to acquire the accurate results of the expected connection setup queueing delay by means of an M/G/m queueing analysis. In the analysis, the maximum admissible quota of real-time traffic is regarded as the number of servers and the service time has a duration that follows an exponential form with an added constant. In M/G/m queueing analysis, the accurate results have only been attained for a limited number of special service distributions, while most of the proposed approximation only maintained a less than 10% relative error for certain properties of service distributions. Our approximation results, which are derived under the particular general service distribution in our system, show that the mean setup queueing time is in profound agreement with the analytic result. Additionally, extensive simulation results show that HOPSMAN with the proposed MAC scheme achieves exceptional delay-throughput performance and remarkable real-time traffic performance (high statistical multiplexing gain for real-time traffic, exceedingly low VBR delay and jitter) under a wide range of traffic loads and burstiness.