臺灣博碩士論文加值系統

802.11 IEEE標準已經使低價與有用的無線區域網路（WLAN）得以實現，我們可以預見無線區域網路將會成為第四代行動通訊系統（4G）的重要成員。在無線區域網路中，無縫換手（seamless handoff）是一個非常重要的議題，完整的換手延遲時間被區分為以下三部份：掃瞄延遲、認證延遲與重新聯結延遲，其中掃瞄延遲遠大於其餘二者，而現存的釵h研究成果便是發展快速換手策略來減少掃瞄延遲。本論文提出一個新的快速換手策略，用來降低802.11系列（802.11-based）無線網路中的掃瞄延遲，此策略稱為平手掃瞄（DeuceScan）。在平手掃瞄中，我們利用預先掃瞄(pre-scan)和時空圖（spatiotemporal graph）提供時空資訊，讓行動主機(mobile host)可以準確地決定要換手到哪一個無線存取點(access point)，並且可以有效地降低媒體存取控制層（MAC）的換手延遲。由模擬結果可知，我們提出的平手掃瞄比現存的其他方法擁有更好的效能。

The 802.11 IEEE Standard has been enabled low cost and effective LAN services (WLAN). It is
widely believed that WLAN will become a major portion of the fourth generation cellular system
(4G). The seamless handoff problem is very important design issue. The entire delay time of a handoff
is divided into probe, authentication, and reassociation delay time, and the probe delay occupies
most of the handoff delay time. Existing results investigate the fast handoff schemes to mainly reduce
the probe delay. This paper presents a new fast handoff, called DeuceScan, scheme to reduce
the probe delay for 802.11-based WLANs. A spatiotemporal approach is developed in DeuceScan
scheme to utilize spatiotemporal graph to provide the spatiotemporal information for making better
handoff decisions to exactly search for the next AP (access point). It efficiently reduces the MAC
layer handoff latency. Our DeuceScan scheme is a pre-scan approach, and two factors of signal
strength and variation of signal strength are both considered in our DeuceScan scheme. Finally,
the simulation results illustrate our performance achievements in handoff delay time and paket loss
rate, compared to existing fast handoff schemes.

Contents
1 Introduction 1
2 Preliminaries and Basic Ideas 4
2.1 Spatiotemporal Graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2 Deuce Procedure with Signal Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3 Deuce Procedure with Variation of Signal Strength . . . . . . . . . . . . . . . . . . . . . . . 9
3 DeuceScan: Deuce-based Fast Handoff Scheme Using Spatiotemporal Graph 12
3.1 Construction of Spatiotemporal Graphs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 The DeuceScan scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
3.3 Adding a Centralized Server . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4 Simulation Results 18
4.1 Variables alpha and beta . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.2 Handoff latency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.3 Packet loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
4.4 Link quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
5 Conclusions 24

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