WiMAX提供QoS的設計讓語音服務的執行在WiMAX上多了一份保證，並且在IEEE所制訂的 802.16-2005標準裡，為了將即時語音服務和即時影像服務更能清楚區分，將原本已提供的四種QoS服務類型，加入第五種稱為ERT-VR的服務型態，讓WiMAX所提供的QoS服務更加完善。 不同於有線網路的排程設計只需要考慮到瞬時流量大小和佇列的狀態。無線網路因為受通道狀況影響會導致多重路徑的訊號衰減和都卜勒效應，設計無線網路的排程時必須將通道狀況列入考慮。況且IEEE 802.16-2005是屬於移動式無線網路，這使得使用者在移動中接收到SNR值隨時間也隨地點不同而不斷變更，導致通道狀況不容易掌握。正因如此，了解PHY層資訊並正確且迅速的上傳至MAC層作跨層式排程設計的確有其必要性。 IEEE802.16e規格裡將E-model 中考慮語音品質的重要參考因素抖動率列為參數之一，讓VoIP的品質更加貼近人們的需求。本文重點在探討ERT-VR加入抖動率保證與原本RT-VR沒有提供抖動率保證的語音品質比較。但對於UGS而言，其往後在WiMAX主要應用並非因此淘汏，因為可利用於單向傳輸的廣播服務或是提供固定頻寬的T1服務，當然即時服務中不可或缺的傳輸影像將由RT-VR服務型態負責，最後本文研究重點ERT-VR服務型態則是達到即時語音服務。 在IEEE 802.16-2005中各個服務型態將各兼其職，配合本文所設計的跨層排程機制發揮最佳的頻寬使用率，以達到WiMAX上所提供QoS的參數要求。 The design of QoS in WiMAX provides better quality in real-time services over wireless network. The new scheduling service, named ERT-VR, in IEEE 802.16e standard provides the jitter guarantee parameter to enhance the quality of real time applications. Unlike the fixed network, which only considers the instantaneous traffic and queue condition, the factors of multipath and doppler in radio environment will affect the transmission quality. Therefore, the channel condition shall be considered for the scheduling scheme in IEEE 802.16e network. The IEEE 802.16e network is a kind of the wireless technologies, the SNR may be changed quickly by time and place especially in mobile condition. In this paper, we propose a cross-layer scheduling in order to feedback from the information of PHY layer quickly and accurately to the MAC layer. The E-model is a common way to evaluate whether the data network is well performed in carring VoIP traffic. In E-model, the important element of equipment impairment factor is jitter. And, the IEEE 802.16e standard also regards jitter as one of the important factors in ERT-VR scheduling service. In this paper, a jitter guanarteed scheduling scheme for ERT-VR service is proposed. During the simulation, the UGS scheduling service is assumed to be unidirectional real-time service such as radio service, or provide fixed bandwidth service such as T1 service. And, the RT-VR scheduling service is regarded as an important role in video transmission such MPEG. The ERT-VR scheduling service will carry VoIP service in IEEE 802.16e network. Our simulation results show that all of the above scheduling types can meet appropriate qualities in accordance with their characteristics and QoS requirements and the bandwidth utilization can also be improved.
