無線區域網路之即時服務排程演算法

Abstract

IEEE 802.11無線區域網路已發展多年，隨著服務品質的需求，IEEE 802.11e根據原IEEE 802.11媒體存取控制層提出改良與增進的控制機制。無線區域網路的媒體存取機制可分成中央輪詢和分散競爭兩種：中央輪詢式媒體存取藉由一中央存取點(AP)負責分配無線網路頻寬；在分散競爭的存取環境下，各使用者(Station)競取共享頻寬。本論文主要探討前者，除了介紹標準文獻所提出的簡單排程演算法外，尚提出一以計時器為基底之演算法。經由完整的模擬結果而有以下三結論：一，中央輪詢式存取的頻寬使用效率優於分散競爭而可支援較多的使用者；二，相較於平等對待每一個使用者的簡單排程演算法，所提出的演算法提供區別性服務，在單考慮即時服務下，可支援較多使用者外；在考慮即時與非即時服務下，亦可增加非即時服務傳輸流量；三，二演算法本身都具有可調整之交易(tradeoff)參數，本研究除模擬其影響並交互比較，其結果亦顯示出所提之演算法相較於簡單演算法之優越性。

With the growing QoS requirements in Wireless Local Area Network (WLAN), it is crucial to emphasize the enhanced Medium Access Control (MAC) layer in the 802.11e. This thesis adopts the centralized polling-based scheduler mechanism rather than the distributed contention-based channel access to allocate the precious shared resource for each service. Besides of pointing out the equal treatment of the simple scheduling algorithm for all kinds of services, this thesis proposes a timer-based scheduling algorithm which is based on the concept of the Earliest Deadline First to improve the bandwidth utilization and support the differentiate QoS demands. Based on the complete comparisons by the simulation, the polling-based mechanism is superior to contention-based access on the bandwidth efficiency, which results in the higher capacity. Furthermore, the proposed algorithm provides better bandwidth utilization than the simple algorithm in the capacity and the data throughput under the QoS requirements. In addition, the influence of the adjustable tradeoff parameter for each algorithm itself is revealed.