一個在DQDB網路上等時性槽位再利用的方法

Abstract

DQDB網路是目前支援多媒體通訊的主要技術之一。DQDB網路提供兩種型態 的服務，即同步（等時性）及非同步傳輸服務。非同步傳輸服務中的槽位 再利用法在過去已被深入討論，但前人研究中鮮少提及同步傳輸服務中的 槽位再利用。在本篇論文中，我們提出一種等時性連接的管理協定，以及 其使用之等時性槽位再利用法。前人研究中，有關等時性槽位利用所提出 的方法，係將已使用過的等時性槽位提供給下游站（node）之非同步需求 使用。由於網路效能之考量，最多只有33%的頻寬可分配給等時性服務使 用，亦即，即使網路頻寬達155Mbps，同一時間內最多也約只有15個等時 性連接可以同時設立；然而，這在實際應用中似嫌不足。因此，在不減少 非同步服務可使用的頻寬下，有必要提升網路中可同時設立的等時性連接 數。本篇論文所提出的方法，不僅可將已使用過的等時性槽位釋放給下游 站之非同步需求使用，更可提供給下游或上游其他等時性連接之需求使用 。並且，同時設立的等時性連接只要其間沒有重疊，均可共用一個等時性 槽位，故網路上可同時設立的最大等時性連接數，便與等時性連接之分佈 方式有直接之關聯。經由電腦模擬得知，我們所提出之方法，不僅可大量 提高網路上可同時設立之等時性連接數，並可因此大幅減少等時性連接請 求之失敗率。 In this thesis, an isochronous connection management protocol and its associated slot reuse scheme is proposed. As we know, IEEE 802.6 (DQDB) is one of the emerging techniques to support multimedia communications. There are two types of service in DQDB, asynchronous and synchronous service. Slot reuse scheme for asynchronous service, which increases asynchronous slot utilization, has already been discussed in depth. But only few discussion was on the reuse of synchronous service slots. Previous works on slot reuse for synchronous service provided a method for releasing used video slots for asynchronous requests made by downstream nodes. For performance consideration, synchronous service can take only 33% of bus capacity, which corresponds to only about 15 concurrent video connections when the underlying bus capacity is 155Mbps. Such a small number of connections maybe not enough for practical usage. So there is a need to increase the maximum number of concurrent video connections without reducing the bandwidth available to asynchronous traffics. We propose an isochronous connection management protocol and an isochronous slots reuse scheme which reuses video slots for either synchronous or asynchronous purpose according to the need of downstream nodes. Many isochronous connections can share the same slot as long as there is no overlapping between these sharing connections. Simulation are performed, and the results show that the goal is achieved, and further, connection request failure rate is also vastly reduced.