IEEE 802.16標準是一種無線都會網路技術，其具有高傳輸速率、較廣的訊號覆蓋與支援多媒體服務等優點。為了滿足不同需求的資料服務，IEEE 802.16標準定義了多種服務品質(Quality of Service, QoS)類別。然而，在IEEE 802.16標準中並沒有定義頻寬分配的排程演算法，因此，如何設計一個好的排程演算法來滿足所有服務品質類別的需求就成為了主要探討與研究的議題。
　　在根據封包剩餘時間的比率限制排程法中[1]，延伸之即時輪詢服務(ertPS)的比率限制是固定的，因此，當ertPS的負載過重時，由於使用掉大部分的頻寬，即時輪詢服務(rtPS)的封包遺失率會變得無法接受。所以，在本論文中，我們將根據目前rtPS的封包遺失率來動態調整ertPS的比率限制。目的是在ertPS性能可以接受的範圍內，犧牲部分分配頻寬來改善rtPS的封包遺失率。第一回合將緊急封包（剩餘時間小於等於一個訊框時間的封包）在當前的訊框排程傳送。第二回合先算出每個佇列的封包平均剩餘時間T，接著根據目前rtPS的封包遺失率調整ertPS與rtPS各別的剩餘時間比率x與y，佇列中ertPS與rtPS的封包剩餘時間各別小於等於排程門檻x．T與y．T，就在當前訊框將它排程，直到所有的即時性服務佇列都被服務完或頻寬已用盡為止。若第二回合排程結束仍有頻寬剩下，則第三回合使用加權公平排程演算法(WFQ)將剩餘頻寬分配給所有的佇列，由於即時性封包已經被先排程過，所以本論文中各個等級的佇列賦予相同的權重。
　　模擬結果顯示，動態調整ertPS與rtPS的排程門檻，會使ertPS的性能在可接受的範圍內變差，以改善rtPS的封包遺失率。

　　IEEE 802.16 Standard is a Wireless Metropolitan Area Network (WMAN). Its advantages are high transmission rate, wide signal coverage, multimedia service support,　.... In order to satisfy a variety of demanded data services, IEEE 802.16 defines several types of Quality of Service (QoS) classes. Nevertheless, in IEEE 802.16, the scheduling algorithms for allocating bandwidth is not defined. Therefore, to design an excellent scheduling algorithm satisfying the QoS requirements of all classes becomes a main research topic.
　　In “Ratio Scheduling Algorithm according to Packet Left Time”, the ratio for extended real-time Polling Service (ertPS) is fixed [1]. Therefore, when the traffic load of ertPS is too high and uses most of the bandwidth, the packet loss rate of real-time Polling Service (rtPS) will become not acceptable. In this paper, the ratio limit of ertPS will be adjusted dynamically according to the current packet loss rate of rtPS. The packet loss rate of rtPS will be improved and that of ertPS will be in the controlled range. In the first round, the emergency packets (the packets must be sent in this frame because of the deadline) will be scheduled in this frame. In the second round, the average left Time T of packets for each class is calculated. Then, the left time ratios of ertPS and rtPS are set as x and y respectively. If the left time of a packet in the queue of ertPS or rtPS is not larger than the scheduling threshold x．T or y．T, this packet will be scheduled in this frame. The packets of all real-time packets will be checked one by one until all packets satisfying the conditions are scheduled or the bandwidth of this frame is exhausted. If there are remaining bandwidth left after this round, Weighted Fair Queue (WFQ) will be used in the third round to allocate bandwidth for the left packets. Since real-time packets are scheduled in the first two rounds, queues of all classes will have the same weight in this round.
　　The simulation result indicates that if we dynamically adjust the scheduling threshold of ertPS and rtPS, the performance of ertPS will become worse in the acceptable range, and rtPS will be improved.

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