隨著IEEE 802.11的制定與無線傳輸技術的快速發展，無線區域網路已成為行動通訊世界中的一個重要成員，由於具備低成本及易建置性等特性，無線區域網路的應用也日益普及。要達成不間斷的傳輸服務，換手(handover)機制必須通常地設計。然而在目前的換手機制中，需要完整地掃描(scan)周遭的通道，造成過長的掃描延遲，對於即時性的應用有嚴重的影響。除此之外，標準中制定的換手機制是依據訊號強度(RSSI)來選擇換手存取點(access point)，忽略了考慮通道擁塞情形的因素，可能切換到的AP無法提供足夠的傳輸率，因此在本篇論文中，我們將探測請求(probe request)訊框與探測回應(probe response)訊框交換的時間差所提供的資訊，來判斷換手AP忙碌的程度，並調整掃描機制中的通道停留時間，來縮短掃描所花費的時間，以有效的解決目前換手機制中的缺失。我們並將本篇論文所提出的方法，實作於目前無線網卡驅動程式中，以驗證其在換手機制上的效能。 With the announcement of IEEE 802.11 standards and the rapid development of the wireless transmission technology, WLANs (wireless local area networks) have become popular due to its low cost and being easy to set up. To achieve an uninterrupted transmission service, handover mechanisms must be carefully designed. However, current handover mechanisms require completely scanning surrounding channels which can result in long scanning delays and thus severely affect the performance of real-time applications. Apart from this, the handover mechanism in the standard specification chooses the new access point (AP) based only on the information of the received signal strength (RSSI). Since the congestion condition on the new AP is not considered in the selection process, the new AP may not it support the required enough transmission rate if it is heavily loaded. In this thesis, we propose to use the information of the round-trip delay between the probe request and the probe response for handover AP selection. Furthermore, the duration of scanning on a specific channel will be properly designed to reduce the scanning delay. We have verified the proposed method through implementation on the driver of the wireless network card. The experiment results show that the proposed method outperforms the standard mechanism on the handover performance.