無線區域網路換手之適應性頻道掃描演算法

Abstract

近年來由於無線區域網路的廣泛佈建，已成為行動網路服務的一個重要傳輸媒介。然而使用者在存取點間換手動作造成的資料封包延遲及遺失將嚴重影響行動網路服務品質，這個問題對於即時音視訊服務而言更是不容忽視。已有相關研究指出通道掃描佔據大部分換手時間，造成資料封包延遲及遺失，並致力於縮短通道掃描時間。然而這些方法未能考慮即時視訊與語音封包，造成即時通訊的中斷和品質下降，為了改善這樣的問題，部分研究也提出在不影響即時通訊下的通道掃瞄機制，然而在現有的研究中，都未能同時考慮掃描時間或封包遺失或延遲來設計適當的通道掃描策略以適應使用者個別的需求。 有鑒於此，本論文提出了適應性的掃描機制，在滿足使用者對封包遺失及延遲要求的前提下，使整體掃描時間達到最短。由效能評估可以看出本論文提出的方法相較於現有技術而言有效縮短30%以上無線區域網路通道掃描時間(Channel Scan Time)，大幅降低換手動作對即時視訊與語音服務造成的干擾。

Recently, WLAN has been widely deployed over public and private areas and has become one of popular access technologies for mobile Internet services. Handoffs between WLAN access points (APs) that introduce packet loss and delay become one of the critical issues for mobile Internet applications, especially for real-time communications. Previous studies have already indicated that WLAN AP scanning time contributes a significant portion of a handoff delay, and a number of solutions were proposed to address this issue. Unfortunately, the existing approaches either try to minimize the time period of an active scan process, or optimize packet loss and jitter by using passive scan approaches. Solutions considering packet loss, jitter, delay, and also scan latency are not yet investigated. In this thesis, novel scan algorithms that combine active and passive scan strategies for WLAN smooth handoffs are proposed to minimize the scan time without violating the QoS requirements such as packet delay, jitter, and loss for applications. Simulation results demonstrate that the proposed scan strategies achieve faster scan time without introducing packet jitter and delay than the existing solutions.