IEEE 802.16無線寬頻網路下行通道封包排程機制之設計

Abstract

IEEE 802.16 無線都會網路已被視為新一代革命性的無線寬頻接取技術。在最新釋出的標準中(IEEE 802.16-2005)，共定義有五種服務型態分別支援不同的服務品質(Quality of Service)以滿足更多元化的應用服務，以供現在及未來所需。然而，標準中並未對IEEE 802.16網路中封包級的排程(packet-level scheduling)做任何描述，而在提供服務品質的保障之系統實作中，這反而是最關鍵的技術之ㄧ。 在本研究中，我們乃提出了一個針對IEEE 802.16網路下行通道的封包排程機制，這個機制可實現於基地台中並能夠與IEEE 802.16的MAC層協定相容。為了考慮無線通道中叢集姓通道錯誤(burst error)以及位置相關通道錯誤(location dependent error)的特性，我們設計一個新穎的封包排程演算法，命名為信貸式演算法(Credit-based Algorithm)，以保障即時性服務與非即時性服務兩者的服務品質。在此，「保障」包含了即時性服務封包因未被準時送出而被丟棄的機率，以及非即時性服務平均的有效流量(effective throughput)。信貸式演算法引進了「信用」的概念，用「信用」來表示一個資料流(flow)再系統中的保護容量。本研究進而使用一個借貸分配程序，來分配遭遇暫時性叢集通道錯誤(temporary burst error)的資料流過度消費的「信用」給其餘的資料流支用。我們進行了一連串的模擬實驗來評估我們所提出的封包排程機制的效能。根據模擬實驗的結果，我們所提出的封包排程機制確實能有效提升系統容量的使用效率，且能保障即時性服務封包的延遲時間，並將剩餘的系統容量以有公平效率的方式分配給非即時性服務。故能同時滿足兩種服務型態的服務品質需求。

IEEE 802.16 wireless MAN is expected to be a revolutionary wireless broadband access technology. Five service types supporting different Quality of Service are defined in the latest release of the standard (IEEE 802.16-2005) to satisfy a variety of applications supported in nowadays or in the future. However, the standard does not specify the packet-level scheduling in IEEE 802.16, which is one of the crucial technologies to provide the guarantee of QoS of the system level. In this study, a packet scheduling scheme in the downlink channel for IEEE 802.16 is proposed. The scheme can be implemented in the base station and compatible to the MAC protocol of IEEE 802.16. Consider the properties of location-dependent and burst error of wireless channel, we design a novel packet scheduling algorithm named the Credit-Based Algorithm to guarantee QoS for both real-time and non-real-time services. The guarantee includes the packet loss rate due to delay violation for real-time services and the average effective throughput for non-real-time services. The Credit-Based Algorithm introduces the concept of credits to represent protected capacity of a flow in the system. We then employ a debt allocation procedure to re-allocate the overspent credits consumed by flows suffering temporary burst error to other flows. Simulation experiments are conducted to evaluate the performance of our proposed scheme. Based on the simulation results, our packet scheduling scheme is shown to improve the efficiency of the usage of system capacity significantly, and guarantee the packet delay for real-time services while allocating the remained capacity to non-real-time services more fairly and efficiently. Thus, the goal for satisfying different QoS requirements for real-time and non-real-time services simultaneously is achieved.