IEEE 802.11多重速率無線網路的效能評估與改善

Abstract

由於無線技術的進步,使得現今的無線設備能夠提供較高的傳輸速率,因而無線網路變得很受歡迎. 例如, IEEE 802.11n能提供最高到540 Mbps的傳輸速率. 這些高傳輸速率都是藉由在網路實體層使用不同的調變技術而達到的. 當傳輸頻道的品質發生變化時,使用不同的調變技術或傳輸速率可改善網路的頻寬. 為了要讓無線設備能根據當時傳輸頻道的品質,使用最適當的傳輸速率,我們可用人工的或自動的方式來調整傳輸速率. 但如何自動調整傳輸速率,以往只有在格狀網路中有較多的研究,在IEEE 802.11中不但沒有規定,且在無線區域網路中所作的研究也很少. 在本論文中,我們首先針對802.11在多重速率無線區域網路提出3個問題,即(1)耗電量的管理效能不足(2)對使用不同傳輸速率的無線設備有不公平的頻道時間分配及(3)網路頻寬下降. 然後我們提出了一個封包排班方式,來改進這些問題. 在本論文的第二部份,我們提出了一個可用來評估802.11 DCF在多重速率無線區域網路中頻寬及封包延遲效能的模型. 最後,我們也介紹如何計算一個靜態多跳躍多重速率無線網路中路徑的頻寬,接下來,我們希望能找到一個較實際的無線使用者移動模式,這樣就能計算一個動態路徑的頻寬,進而能設計一個以頻寬為標準的路徑選擇方式. 論文中我們也藉由模擬的結果來驗證所提出的封包排班方式, 效能評估模型及路徑頻寬計算的正確性.

Due to the availability of affordable devices that are capable of transmitting at high data rates, wireless LANs (WLANs) have become increasingly popular. For example, the IEEE 802.11n standards now can support data rates up to 540 Mbps. These high rates are achieved through new modulation schemes that are optimized for the channel conditions bringing about a dramatic increase in throughput performance. Since the choice of which modulation scheme to use depends on the current state of the transmission channel, newer wireless devices often support multiple modulation schemes, and hence multiple data rates, with mechanisms to switch between them. Users are given the option to either select an operational data rate manually or to let the device automatically choose the most appropriate data rate to match the prevailing conditions. Although automatic rate adaption protocols have been studied widely for cellular networks, there have been relatively few proposals for WLANs. In this dissertation, we first showed 3 problems of the 802.11-based WLANs in which the wireless devices have the multi-rate capability: (1) the problem of power management inefficiency; (2) the problem of unfair channel time allocations; and (3) the problem of degraded network throughput. We then proposed a scheduling mechanism, called the Shortest Time First Scheduling, to improve these problems. In the second part of this dissertation, an analytical model, called the Rate-Adaptive Markov Chains, was proposed to study the saturation throughput and delay performance of a WLAN in which the mobile hosts have the multi-rate and automatic rate adaption capability. We also showed how to evaluate the throughput of a path for stationary STAs in multi-rate multi-hop ad hoc networks. In the future, we hope to develop a practical user mobility model such that the expected throughput of a dynamic route can be evaluated. Then, a routing protocol to select the most throughput efficient path from all possible paths can be designed. Simulations are also provided to verify the performance of the packet scheduling mechanism, the correctness of the analytical model and the mechanism used to evaluate path throughput.