在無線區域網路中使用擴展性混和協調功能控制型通道存取方式保證服務品質

Abstract

在IEEE 802.11e的媒體存取控制中定義了一個新的通道存取機制，Hybrid Coordination Function(HCF)，分配傳送機會(TXOP)給符合服務品質(QoS)需求的工作站。IEEE 802.11e規格定義的HCF控制型通道存取控制(HCCA)參考排程器(Sample Scheduler)，傳送機會的計算是採用平均資料速率對於變動位元速率(Variable Bit Rate)的封包並不適用。一種預測和最佳化控制型通道存取方式(PRO-HCCA)被提出用來處理變動位元速率的封包，把訊務流(Traffic Stream)的延遲限制(Delay Bound)列入考慮，相較於參考排程器，能保證較佳的服務品質。在預測和最佳化控制型通道存取方式排程中，每個訊務流是獨立被輪詢(Polling)，導致有較多的輪詢額外負擔(Overhead)。因此，我們提出了一個修改的機制，每個工作站獨立被輪詢，接著使每個工作站分別有不同的輪詢週期，模擬結果指出我們的方法不但可以達到服務品質的需求，對於頻寬的使用效率上也比預測和最佳化控制型通道存取方式來的好。

The Medium Access Control of IEEE 802.11e defines a novel coordination function, namely, Hybrid Coordination Function (HCF), which allocates Transmission Opportunity to stations taking their quality of service (QoS) requirements into account. A sample scheduler was provided for HCF Controlled Channel Access (HCCA), a contention-free channel access function of HCF. The sample scheduler is not suitable for VBR traffic because delay bounds are not considered in TXOP allocation. A prediction and optimization-based HCCA (PRO-HCCA) scheduler, which takes delay bounds of different traffic streams into consideration, has recently been proposed to handle VBR traffic. PRO-HCCA was shown to provide much better QoS guarantee than the sample scheduler. The granularity of PRO-HCCA is per traffic stream which causes scalability issue. Besides, each traffic stream is polled individually in every service interval, which implies considerable overhead. Therefore, we present a modified scheme that has per-station granularity and thus is more scalable than PRO-HCCA. To reduce polling overhead, we further modify the scheduler so that different stations can have different polling periods. Numerical results show that our proposed schemes meet QoS requirements and utilize bandwidth more efficiently than PRO-HCCA.