競爭式無線網狀網路之服務品質保證

Abstract

近來，以載波感測多重存取碰撞避免協定(CSMA/CA)為基礎的無線網狀網路(wireless mesh network)受到相當多的矚目，因為其能提供一個都會型規模的寬頻網際網路接取服務，且其成本低廉不需要使用太多網路線材去建置這樣的大型網路。一個無線網狀網是由幾個網狀基幹網路(mesh backhaul network)組成，其中的每個網狀節點(mesh node)形成自己的網狀客戶網路(mesh client network)，某些網狀節點提供匝道的功能使其網狀基幹網路能與網際網路互相連通。因此，每個網狀節點都可以當作接取點，使得其底下的行動使用者能夠透過網狀基幹網路互相溝通或是存取網際網路服務。 現今，隨著許多應用程式的蓬勃發展，除了傳統的盡力式(best effort)應用程式以外，未來不同傳輸品質的多媒體應用程式也即將出現，因此為了滿足多媒體應用程式的品質需求，無線網狀網路必須能夠提供不同等級傳輸服務品質(quality-of-service)之功能，使得所需要的資源能夠被保證。然而到目前為止，要在以載波感測多重存取碰撞避免協定為基礎的無線網狀網路上提供不同等級傳輸服務品質之功能仍然是一個挑戰。這是因為除了要滿足服務品質的需求，還必須要考量到共享的無線媒介(shared wireless medium)上的空間競爭(spatial contention)。在本論文裡，我們從兩個方面來探討傳輸服務品質繞路(QoS routing)在網狀基幹網路的問題。 (1) 符合頻寬需求之傳輸服務品質繞路問題。 (2) 最大頻寬之傳輸服務品質繞路問題。 另外一方面，當一個網狀客戶網路上有利用不同傳輸速度的行動裝置存在的話，一個效能異常(performance anomaly)的問題將會發生。這個異常的問題會發生的原因是在於載波感測多重存取碰撞避免協定的頻道存取機制會使得所有的用不同速度傳輸的行動裝置有相同的長期頻道存取機率。因此，對於那些利用較低速度傳輸的行動裝置而言，其長期所佔用的頻道時間將會大於那些利用較高速度傳輸的行動裝置。當一個利用較低速度傳輸的裝置佔用頻道時，因為其傳輸的時間較長，所以就會造成那些利用較高速度傳輸的裝置的吞吐量(throughput)降低，這樣的問題會對網狀客戶網路上的傳輸品質造成相當大的影響，所以我們提出了下面的問題: (3) 在網路有多重速率的傳輸時的吞吐量最佳化方法。 本論文之其中一個目的是要研究在以載波感測多重存取碰撞避免協定為基礎的網狀基幹網路上的傳輸服務品質繞路問題，使得其上的多媒體應用程式都能被保證其所需的資源。另外一個目的在於提供吞吐量最大化的方法以減輕效能異常問題所造成的傷害。我們將進行大量的實驗模擬來評估所設計協定之效能。

Recently, CAMA/CA-based wireless mesh networking technology has received much attention, because they can provide metropolitan-scale wireless broadband Internet access networks, and enable a cost-effective scalable deployment with less wired lines. A wireless mesh network consists of several mesh backhaul networks where any two mesh nodes in a mesh backhaul network communicate with each other by wireless radio in a multi-hop manner. Some mesh nodes also act as gateways for connecting the mesh backhaul network to the Internet. Each mesh node in a mesh backhaul network forms its own mesh client network, where its associated mobile users can use it as an access point to communicate with other users or access Internet services through the mesh backhaul network. With the diversity of applications evolved until today, in addition to traditional best effort applications, multimedia applications with different quality requirements will be available in the near future. In order to provide quality delivery to multimedia applications, it is imperative that wireless mesh networks have to support quality-of-service (QoS) so that their required resource can be guaranteed. Thus far, providing QoS in a CAMA/CA-based mesh backhaul network is still a challenge, due to the spatial contention among shared wireless medium. In this dissertation, we focus on the QoS routing problems in a mesh backhaul network with the following two aspects. (P1) QoS routing with bandwidth requirement. (P2) QoS routing with maximum bandwidth. On the other hand, in a CSMA/CA-based mesh client network, a performance anomaly problem will incur when there are multiple applications transmitted with different channel bit rates. The anomaly problem arises because the basic CSMA/CA channel access method guarantees that the long-term channel access probabilities of the mobile devices transmitting at different data rates are equivalent to one another, and hence the long-term channel occupancy time for those devices transmitting at a lower data rate will be larger than those devices transmitting at a higher rate. When one device transmitting at a lower data rate captures the channel, it will last for a long time and hence penalize the total throughput of those devices transmitting at a higher data rate. Since the problem will bring about great impact on the quality transmission in a mesh client network, the following problem is identified. (P3) A throughput optimization method when multi-rate traffic is presented. In this dissertation, one of objectives is to study the QoS routing problems in the CSMA/CA-based mesh backhaul network such that the required resource of multimedia applications can be attained. The other objective is to alleviate the impact of the performance anomaly in the CSMA/CA-based mesh client network by maximizing the total throughput. To evaluate their performance, extensive experiments shall be carried out.