保證WiFi技術服務品質之基於速率分群分散式協調功能

Abstract

WiFi在無線區域網路中成為微型基地台輔助技術的重要角色，提供更多頻寬資源達成經濟效益，WiFi 所採用的IEEE 802.11標準，會依照通道的狀態採用適當的調變方案，因此在多天線的支援下，實體層資料傳輸能提供6.5Mbps至600Mbps等多種速率，然而，其實質系統輸出卻受限於媒介存取控制(Media Access Control, MAC)層機制所採用的分散式協調功能(Distribution Coordination Function, DCF)，儘管在此方案中保障所有裝置長期存取通道次數的機會，使得裝置以相同速率傳輸下能夠有公平的性能呈現，但受限於傳送端與接收端之間的傳輸距離與裝置硬體上的差異，無法保證在相同基礎服務設定(basic service set, BSS)中所有裝置皆能以相同的狀態對通道進行存取，因此現實環境中各種不確定因素造成裝置與裝置間的差異而形成多重速率的傳輸環境，在分散式協調功能中低速裝置使全體裝置傳輸的次數驟降，稱之為性能異常(performance anomaly of 802.11)，在本論文中提出了一個貼近於原始DCF運作方案Rate Based Clustering Distribution Coordination Function(RC-DCF)，以裝置傳輸速率與裝置個數最為依據分配通道頻寬，將各裝置之傳輸獨立於其傳輸速率，解決多重速率環境下，通道分配不均與高低速裝置碰撞之議題，在throughput最大化的同時也能達到無線網路中的QoS的需求。

WiFi plays an important role of supporting micro-cell based station to fully utilize the channel resource in the wireless local area network. IEEE 802.11 standard are being used in WiFi technology and apply a link adaptation algorithm to minimize the packet error rate. IEEE 802.11n supports transmission rate from 6.5Mbps to 600Mbps at physical layer. However, the performance of the system is limited by distribution coordination function (DCF) using in the standard. This MAC scheme provides long-term channel access opportunities among stations. Each station gets same throughput in same transmission rate condition. But there is no guarantee of the same transmission rate that stations are in the different channel condition. Channel state is often dominated by hardware of the mobile stations and the distance between AP and stations in one Basic Service Set (BBS). In the multi-rate environment, the performance of high-rate stations is dramatically degraded because the channel being occupied for a long time by the low-rate station. This phenomenon is so called performance anomaly of 802.11. In this thesis, we proposed a novel MAC scheme based on the transmission rate which provides air-time fairness. Reducing the collision probability between high-rate and low-rate stations to maximize the throughput and achieve the quality of services in wireless network.