無線區域網路以高斯近似為基礎之變動位元速率訊務允入控制演算法

Abstract

為了提供即時性訊務的服務品質保證，IEEE 802.11標準制定團隊引入『混合協調功能』(Hybrid Coordination Function)的通道存取機制，其中又分成兩種模式，以競爭來獲得通道存取權利的方法稱為Enhanced Distributed Channel Access (EDCA)；另一種則為非競爭的通道存取模式稱為 Hybrid Controlled Channel Access (HCCA)。然而，在HCCA排程器中，允入控制與分配傳送機會(TXOP) 的參考設計只適用於傳送固定位元速率(CBR)的訊務，對於傳送變動位元速率(VBR)的訊務來說，則可能會發生嚴重的封包遺失(Packet Loss)。

在這一篇論文中，我們提出了一個簡單的允入控制演算法，其基本設計的思維在於利用高斯分佈(Gaussian Distribution)來近似變動位元速率的訊務。電腦模擬的結果指出我們提出的允入控制演算法，可以來確保擁有服務品質保證的站台(QoS-Enhanced Station)在傳送訊務的過程中，封包遺失的機率在事前保證的範圍以內。再者，當有站台一次要求傳送多個變動位元速率之訊務流時，我們提出的方法可以因為獲得多工增益(Multiplexing Gain)而更有效率的分配傳送機會(TXOP)。

For Quality of Service (QoS) requirements of real-time traffic, IEEE 802.11 working group introduces a QoS-aware channel access mechanism, called Hybrid Coordination Function (HCF), which consists of contention-based Enhanced Distributed Channel Access (EDCA) and contention-free HCF Controlled Channel Access (HCCA). The TXOP allocation and the admission control units of the HCCA reference scheduler are only appropriate for constant bit rate (CBR) flows. It may result in serious packet loss for variable bit rate (VBR) flows.

In this thesis, we propose a simple admission control algorithm which adopts Gaussian distribution to approximate VBR traffic. Numerical results obtained from computer simulations show that our proposed algorithm can effectively and efficiently allocate Transmission Opportunity (TXOP) durations to QoS-enhanced stations (QSTAs) to guarantee a predefined packet loss probability. Moreover, our proposed scheme can easily handle multiple VBR flows of the same QSTA to get the advantage of multiplexing gain.