利用符記指定及緩衝區管理之非同步傳輸交換網路擁塞控制方法

Abstract

非同步傳輸模式(ATM)已經成為寬頻整合服務數位網路(B-ISDN)的主幹， 它擁有傳統的同步傳輸模式所缺乏的效率與彈性，藉著統計式多工的方法 去共享未使用到的頻寬來滿足對延遲敏感的語音與視訊及突發性的數據傳 輸的需求。近年來由於多媒體通訊的急速發展，其中之大量且具有時限的 資料常會造成網路擁塞，因而一些急迫且重要的資料無法及時且正確地送 達，使得網路效益大為降低。本篇論文提出一個在非同步傳輸模式網路 中，當網路流量變大時，能提高網路效益的方法。在非同步傳輸模式網路 中，所有的資料均被切成固定大小的晶格 (Cell)來傳送。早期的擁塞控 制方法如隙漏水桶法(Leaky Bucket)是在緩衝區填滿的時候，將新到達的 晶格捨棄，而這些被捨棄的晶格很多是不能或缺的重要資料，這樣做法導 致了大量的資料必須重送，以致於往往又不能滿足資料的即時性，使得重 送後之資料無法使用。我們提出的方法，是在隙漏水桶法內技巧地把數量 有限的安全符記(Green Token)指定給重要的資料，在擁塞發生時儘速傳 送且避免捨棄有安全符記的晶格，使得這些重要的資料能正確且迅速地送 達終點，減少大量的資料重送且兼顧了及時性的要求。這樣一來使網路的 效益提高，另一方面也提高了網路上應用軟體的效能。 ATM (Asynchronous Transfer Mode) will be the backbone of B-ISDN (Broadband Integrated Services Digital Networks), which is more flexible than traditional switching technique. Through statistical multiplexing,the unused bandwidth can be shared to satisfy the requirements of delay-sensitive and loss-sensitive services. As the application of modern multimedia communications grow, the high-volume and bursty traffic usually cause congestion in the management in ATM networks is proposed. In ATM networks, all information is packetized into a fixed size "cell". Usually, the admitted bandwidth of a call is less than its peak rate, so when congestion happens, the traffic exceeding the admitted bandwidth may be discarded. In leaky bucket mechanism, both high priority and low priority cells may get green tokens fairly, as a result the high priority cells suffer the same loss rate as low priority cells do. It is a pity that the loss of high priority cellsa rise a lot of retransmission. In addition,retransmitted data may be invalid for the requirement of a certain real-time services. So we propose ascheme to make sure that high priority cells can get green tokens and transmit green cells previously. At burst level, we assign green token to high priority cell as much as possible. At cell level, we transmit cells with green tokens previously in order to ensure low loss rate and end-to-end delay requirement. Combining both burst-level and cell-level congestion control schemes, this approach could improve the system performance significantly.