IEEE 802.11/802.16e無線區域及都會型異質性網路之換手性能分析

Abstract

近年來，透過IEEE 802.11 無線區域網路(wireless local area networks, WLANs)上網已經是相當的普遍，但是多數應用被限制在沒有換手機制的無線網路內，當很多即時應用快速發展，如VoIP(Voice on IP)等，如何有效降低無線網路換手延遲時間變成是一個很重要的換手議題；另外，近來IEEE 802.16e 無線都會網路(wireless metropolitan area networks, WMANs) 也吸引到很大的關注，因此，相信在未來，IEEE 802.16e 無線都會網路將會結合現有的IEEE 802.11無線區域網路形成一個混合型無線網路，無庸置疑的，智慧型無間隙換手在這樣的混合型網路將會變成一個很重要的議題。 在這篇論文中，我們首先研究IEEE 802.11 無線區域網路的換手問題，從相關換手延遲時間量測的文獻中，發現其中搜尋可用的IEEE 802.11基地台(Access point)的延遲佔了換手絕大部份時間，因此，我們首先提出一個IEEE 802.11換手機制中搜尋基地台所需時間及其成功率的分析模型，我們並發現兩個很重要的設計參數(探測需求封包(probe request frame)及探測回覆封包(probe response frame)的傳送次數)會影響搜尋基地台所需的時間，我們進一步提出最佳化這兩個換手設計參數的演算法，我們的分析結果指出，比起現有標準定義的方法，當一個系統使用最佳探測需求封包(probe request frame)及探測回覆封包(probe response frame)的傳送次數時，可降低30%到40%的有效搜尋時間。 在IEEE 802.11/802.16e無線區域及都會異質性網路之換手問題中，當有兩個系統同時作換手時，有比較短換手時間的系統不一定會有比較高的資料吞吐量，因此，我們針對一個新的換手情境 ``WLAN to hybrid WLAN/WiMax’’，提出同時考慮跨系統(vertical)及同系統(horizontal)換手的動態網路選擇機制去決定連線的網路，在提出的機制中，我們要求使用者等一個最大網路選擇時間以至於可以最大化可傳輸的資料量，另外，我們亦提出飽和網路選擇時間去最佳化傳輸資料量及換手時間的所形成的擇衷(trade-off)問題。經過我們的分析結果指出，當鄰近無線網路有5~25個使用者時，使用我們提出的機制可以提昇至少10%的平均資料傳輸量。

In recent years, the IEEE 802.11 wireless local area networks (WLANs) have become very popular. However, most application scenarios are limited to hot spot areas without handoff. As many real-time application, e.g. voice over IP(VoIP) grow rapidly, one of the burning issues for WLAN is to reduce handoff latency. Furthermore, IEEE 802.16e wireless metropolitan area networks(WMAN) also attract a great deal of attentions. Thus, in the future, we can expect the appearance of the hybrid IEEE 802.11 WLANs and IEEE 802.16e WMANs system. Clearly seamless and smart handoff in this hybrid network will become an increasingly important issue. In this thesis, we first investigate the handoff issue for the IEEE 802.11 WLANs. Some studies have indicated that the latency in searching available channel in neighboring AP dominates handoff latency. Therefore, we develop an analytical model to calculate the channel search latency and its success probability for the IEEE 802.11 WLANs. We find that the number of the probe requests and that of probe responses are two key design parameters influencing handoff channel search time. Thus, we develop a method to determine the optimal numbers of probe request and probe response. The numerical results demonstrate that a system with the optimum probe request and probe response can reduce the effective search time by 30% to 40% compared to the legacy IEEE 802.11 WLANs. As for the handoff issue in the hybrid IEEE 802.11 WLANs and IEEE 802.16e WMANs system, the system which has shortest handoff latency may not have the highest throughput. Thus, we develop a dynamic network selection scheme to determine the connecting system in a new ``WLAN to hybrid WLAN/WiMax’’ handoff scenario, in which we consider both the vertical and horizontal handoffs. The proposed scheme requests the station to wait an additional network selection time ``tw’’ before the selection to maximize the amount of delivered bits during a dwelling time. We formulate an optimization problem to find the maximum network selection time for maximizing the delivered information bits. In addition, we also find the saturation network selection time for optimizing the delivered bits and handoff latency during the dwelling time. From the numerical analysis, in the case with 5~25 stations in target WLAN, the proposed scheme can improve the delivered bits at least by 10% compared to that without awaiting the scenario.