無線區域網路中改善換手機制效能之研究

Abstract

無線區域網路(wireless local area network, WLAN)中，行動節點在無線存取點(access point，AP) 之間進行換手(handoff)時所產生的延遲時間，將嚴重地影響即時性(realtime)網路應用的服務品質，例如網路語音電話(voice over Internet protocol，VoIP)。主要的原因是由於過長的換手時間，將造成VoIP封包遺失甚至換手失敗。先前的研究顯示換手時所需花費的時間，主要是在於通道掃描(channel scanning)的階段。因此，在本論文中，將針對通道掃描設計排程演算法(scheduling algorithm)，以使得行動台在進行換手程序時，不但VoIP封包遺失率可滿足服務品質(quality of service, QoS)之要求，並可有效地降低換手失敗的機率。本論文所提出之排程演算法主要考慮語音電話雙向使用者之通話狀態，並利用不發音時(silent period)安排通道掃描順序。本論文將採用電腦模擬的方式，以分析排程演算法之相關效能指標，其中包含VoIP封包遺失率(packet loss rate)以及換手成功機率(handoff success probability)。本論文所獲得之結果，預期將可作為設計無線區域網路系統時之重要參考。To guarantee seamless handoffs between access points (APs) is one of the most decisive factors in providing real time applications such as voice over internet protocol (VoIP) in IEEE 802.11 networks. It is because in wireless local area networks (WLANs) there is no specific control channel for executing a handoff process and a mobile station (MS) has to search for new APs from channel to channel by temporarily disconnecting its association with the old AP. While previous studies are mainly focused on how to reduce the handoff latency, in this paper a channel scanning algorithm is proposed to schedule suitable time for performing a handoff process efficiently. In the proposed algorithm, the voice activity is taken into consideration, and the handoff process is performed during silent periods. Extensive simulations are conducted to investigate the performance of the proposed algorithm in terms of the handoff success probability and the packet loss rate, and the results obtained in this paper can serve as a good reference for the system design.