無線區域網路通道之存取與QoS排程機制

Abstract

於無線網路□, 媒體存取控制為仲裁移動站之間使用無線通道的一項機制。此機制最主要的目的是讓網路能夠提供最大的效能和高品質服務。網路的効能會受到許多因素所影響。當移動站傳資料之前，會先執行一個隨機延遲程序。一個計設良好的延遲程序可以有效地降低資料碰撞的機率，進而提升網路的效能。我們提出了一個多錬結的延遲方法。它能使網路達到更高的效能，並且讓無線通道被公平的使用。此方法利用多個延遲錬結，每個延遲錬結分別適用在不同的網路擁擠狀態。移動站可以藉由使用不同的延遲錬結，以降低碰撞的機率。在多重跳躍網路□，移動站隱藏問題也會對網路的效能造成影響。我們在無線通道□以傳送雜訊以佔據無線通道的方式來避免移動站隱藏問題的發生。此方法把來源移動站和目地移動站的資料，分別放在不同的無線通道□傳送，並在這兩個無線通道□，傳送雜訊以防止移動站隱藏問題的發生。在多媒體網路□，frame dropping rate可以用來衡量一個網路是否提供高品質服務。我們對IEEE 802.11e的transmission opportunity (TXOP)機制，做了一些改善。增加了一個deadline constraint，限制移動站在TXOP□，只能傳緊急的資料，以避免別的移動站的緊急資料，無法在時限內傳出去。根據這個機制，我們提出一個deadline constraint scheduling algorithm。它能有効地降低frame dropping rate並且增加網路的效能。我們對以上所提的方法，都有評量其效能，並且和文獻上的方法做比較。並做了一些數學上的分析，和討論如何選取參數，使網路達到最大效能。

In wireless networks, medium access control (MAC) plays a significant role in coordinating mobile stations’ access to the wireless channel. This coordination is desirable in order to achieve the goals of maximizing channel throughput and supporting quality of service (QoS). The throughput of a MAC protocol is influenced by several factors. In most WLANs, a backoff procedure is executed before frame transmission begins. This procedure helps to prevent frame collision and increase the throughput. We propose a multi-chain backoff algorithm that employs multiple backoff chains to provide a higher throughput while maintaining fair channel access. In a multi-hop ad hoc network, the hidden terminal problem also has a significant impact on the channel throughput. In this respect, we propose a jamming-based MAC protocol which separates source stations’ traffic from destination stations’ traffic into different channels to prevent the hidden terminal problem. For multimedia applications, frame dropping rate is another useful metric to measure how well a scheduling algorithm can support real-time applications. We propose a deadline-constraint scheduling algorithm for IEEE 802.11 WLANs. The scheduling algorithm imposes a deadline constraint on each TXOP to make urgent frames to be transmitted before their deadlines. The performance of the above access scheme and scheduling algorithm are evaluated and compared to prior arts. For some of them, we also analyze their performance and show how to choose proper parameters to obtain optimal throughput.