近年來，因為無線區域網路的便利性和移動性，造成它迅速成為最熱門的資訊研究領域之一，未來的發展趨勢備受眾人矚目，引起產學界的高度興趣， IEEE 802.11是無線網路的一個標準，IEEE 802.11 MAC(Media Access Control) 主要有兩種不同功能的擷取方式，分散協調式功能(Distributed Coordination Function, 簡稱DCF)和集中協調式功能(Point Coordination Function, 簡稱PCF)，可以兼顧到競爭式服務(非同步傳輸)和免競爭式服務(具時限傳輸)，根據 IEEE 802.11 的標準，通常在DCF模式傳送資料，在PCF模式傳送需要即時傳送的資料(如語音)。在現實世界中，和無線網路相關的應用，因為基本的技術發展趨於成熟，而正在蓬勃發展，但由於目前的通訊協定對於語音傳輸的規定有許多不足之處，例如對於優先權與控制存取並沒有做到很妥善的處理及分配，而使得服務品質降低。另外當主機在移動時造成 AP 控制權的轉換(handoff)的情況，主機與 AP 之間也沒有一套很好的協調溝通機制，這是一個等待被解決的老問題。為了給無線網路應用提供一個較好的服務品質，802.11e團隊建立了HCF (hybrid coordination function) 機制，目的就是來改善 IEEE 802.11b 標準的MAC通訊協定。雖然如此，還是有一些尚未解決的問題存在於 IEEE 802.11e 中，無法很有效地改善傳送的效能。如何運用一套完整的機制來控制這些可利用的 MAC 參數，使得在 802.11 無線網路下可以很容易地控制其Qos，仍是我們大家正在努力的目標。在本文中，我們設計了一個提供服務品質多重輪詢的機制，建置在 IEEE 802.11e 標準的平台之上，利用一套演算法依據聲音封包的參數，預測封包的產生時間，並將其順序紀錄在Queuing List中，AP依照Queuing List依序Polling，且在無聲音封包傳送的時間間隔，AP可切換為傳送資料的DCF模式，不但利用802.11e標準中多重輪詢(MultiPoll)的方法，解決之前在 IEEE 802.11e 標準下聲音封包仍然有可能會遭遇到的一些問題，也使得在無線網路下傳送聲音和資料的封包受到 Qos 的保證，最後設計一個模擬的無線環境，模擬及分析系統效能，經實驗證明在 802.11e標準中，我們所設計MultiPoll的方法確實能顯著地提升Qos之效能。 With flexibility and mobility, Wireless Local Area Network (WLAN) has rapidly become one of the most emergent computer research fields. It attracts significant interests both in academic and industry communities. IEEE 802.11, the standard of WLAN, encompasses Distributed Coordination Function (DCF) and Point Coordination Function (PCF) operational modes to process asynchronous and time-bounded traffic respectively. However, the protocol suffers from lacking priority and access control policy so that it is hard to cope with various types of multimedia traffic as well as user mobility (handoff).For applying a higher Quality of Service (Qos) to network applications, the 802.11e Task Group has deployed hybrid coordination function (HCF) to improve the original IEEE 802.11 Medium Access Control (MAC) protocol. The HCF defines two medium access mechanisms, one of them is channel access control. Nevertheless, how to choose the right MAC parameters and QoS mechanism so as to achieve a predictable performance still remain unsolved. In this paper, we propose a Qos Provision Multi-polling Mechanism(QPMM) which is in the PCF mode deploys non-preemptive priority in order to transfer voice packets more efficiently. The voice traffic characterized by packet rate of voice source and the maximum tolerable jitter (packet delay variation) is forecasted. We record the scheduling results in a queue, with which AP (Access Point) can poll and then enable mobile users to communicate with their opposite sites. This occurrence also solves the problem that some voice packets do not suit QoS in IEEE 802.11e standard with multi-polling. During the time-gap while transmitting no voice packets, the scheme changes to DCF mode to transfer data packets. Furthermore we simulate and analyze the performance of the scheme in a WLAN environment. The experimental results show that our approach can dramatically improve the quality of network service.
