DQDB 都會網路上免確認式週期性資料傳輸機制

Abstract

分散式佇列雙匯流排(DQDB)為一高速大都會網路，可同時提供一般性資料 與等時性資料之傳輸． DQDB 利用非連結性的佇列裁定(QA)來傳送一般性 資料，而利用連結性的預先裁定(PA)來傳送等時性資料．在預先裁定方法 中，建立一個連結需傳送多量的控制訊息，十分費時費事．其次如何將頻 寬有效地分配給二種不同性質的資料傳送法使用，也未在DQDB中明確的規 範．因此本論文提出一個有效率的分散式控制法來完成等時性資料傳輸， 此控制法名為免確認式控制機制(CFAC)，主要是利用分散在各節點上的計 數器組來達成資料傳輸控制．本方法有二個特點，首先是透過少量控制訊 息的交換，便能建立或終止連結；其次是提供頻寬管理者，以達成等時性 資料傳輸對於傳輸品質(QOS)之要求． 頻寬管理者隨時監視二種資料傳送 法的頻寬使用情況，並機動地調整頻寬使用量．調整方式主要根據一個佇 列模式，以目前頻寬使用量與未來頻寬需求量為輸入參數，計算而得．最 後，我們將經由系統模擬所得的結果，比較 CFAC 與 DQDB-PA 的效能． 比較的範圍包括：連結建立之延遲時間(CALL SETUP DELAY)，連結建立被 阻斷之機率(CALL BLOCKING PROBABILITY)， 一般性資料傳輸平均延遲時 間 (MEAM PACKET DELAY) 以及系統整體輸出率 (AGGREGATE SYSTEM THROUGHPUT)． Distributed Queue Dual Bus (DQDB) is a high speed Metropolitan area Network (MAN) providing both data and isochronous services using the connection-less QueuedArbitrated(QA) and connection- oriented Pre-Arbitrated (PA) access schemes, respectively. Essentially, DQDB-PA adapts aconnection management procedure requiring laborious exchanges of control messages. Besides, it leaves the subject of how bandwidth is allocated for each type of traffic unresolved. This thesis thus proposes a new access mechanism, called Confirmation-Free Admission Control (CFAC), aimed at providing an efficient and distributed admission control and connection management for isochronous traffic in DQDB networks networks. CFAC allows isochronous slots to be accessed in a distributed and fair fashion through the use of three sets of counters at each node. Primarily, connections are accepted and terminated by means of minimal control message exchanges. In addition, CFAC is augmented with a bandwidth manager. The manager dynamically measures and allocates the bandwidth to isochornous data based on a queueing model according the current and imminent isochronous traffic. As a result, the bandwidth manager assures isochronous traffics of the Quality of Service (QOS) in terms of blocking probability. Finally, the thesis demonstrates simulation results to compare the performance of CFAC and DQDB-PA in terms of a set of QOS indexes including call setup delay, call blocking probability, mean packet delay, and the aggregate system throughput.