IEEE 802.11無線網路下具網路拓樸知覺無縫換手的設計與實作

Abstract

本篇論文提出了一套具網路拓樸知覺的「目標基地台搜索與量測」機制，以縮短IEEE 802.11無線網路基地台換手過程的延遲。 近年來，以熱點型式出現的IEEE 802.11無線網路已經成功地部署在公眾場所，大量服務也隨之應運而生，最受矚目的莫若於VoIP等即時性多媒體應用服務。然而，目前普遍實作的IEEE 802.11無線網路，Client端只有在目前通訊受到威脅、即將無法再繼續進行通訊的前一刻，才開始重新找尋目標基地台進行換手。在這樣的換手模式下，Client端在執行換手程序之前會進行一段較完整的目標基地台搜索，導致整個換手過程時間過長，不符合VoIP等即時性應用服務低延遲的需求。 目前普遍實作的IEEE 802.11無線網路基地台換手過程，在邏輯上可以分離為基地台搜索與基地台連結 (association) 兩個獨立的Phases；許多研究也顯示基地台搜索佔據整個換手過程高達90%左右的時間。因此我們從改良「目標基地台搜索與量測」機制著手，讓Client端在使用過程中的適當時機，例如當服務基地台的訊號強度低於某個預先設定的臨界值時，開始預先找出目前服務基地台相鄰網路存取點的局部拓樸；接著使用Non-AP-Aligned SyncScan的技巧，在換手情況發生之前持續並且有效地對可能的目標基地台進行量測。我們所設計的換手方式，讓Client端在換手過程中不需要再進行基地台搜索，因而能縮短整個換手過程的延遲；此外換手前持續的量測數據，也提供了Client端更好的換手決策條件。 我們所設計與實作的具網路拓樸知覺無縫換手機制，可以只針對Client端進行實作層面的修改，完全符合IEEE 802.11標準，並且不需要額外的資源部署。我們使用Prism 2 chipset的無線網路卡，在Linux作業系統上，藉由修改HostAP driver的方式來實作我們所提出的機制。經由實測評估顯示我們的方法效果不錯，並且可以立即應用於已經佈署使用中的IEEE 802.11無線網路。

In this thesis, we propose a topology-aware “Target APs Search and Measurement” scheme that can reduce the handover latency of IEEE 802.11-based Wireless LANs. In recent years, 802.11-based wireless LANs, as Hot Spots, has been successfully and widely deployed in public places. Many services arise with the growth of 802.11-based wireless LAN, and VoIP and similar multimedia applications, in particular, have become the well-known services. However, most of the current implementation of 802.11-based networks only attempts a handover when a client’s service degrades to a point where connectivity is threatened. With this handover strategy, a client must perform a full search before committing a handover such that the overall handover time is far longer than what can be tolerated by real-time applications such as VoIP services. AP Discovery and AP Association are intrinsically two independent phases in IEEE 802.11-based WLANs. Furthermore, many studies have shown that AP Discovery constitutes more that 90% of the handover latency in most of the current implementation of 802.11 WLANs. Therefore we focus on improving AP Discovery process to reduce the handover latency of 802.11 WLANs and propose a topology-aware “Target APs Search and Measurement” scheme. The underlying idea of the topology-aware “Target APs Search and Measurement” scheme is that a client will discover the APs neighbor to the client’s current location and use the neighbor AP information to facilitate “Target APs Measurement.” Therefore, along with the normal communication, a client determines the local APs topology at some proper time before the handover, for example, when the serving AP’s signal strength gets below the pre-defined threshold. Later, the client measures these possible target APs regularly by using the local topology information and the proposed low-cost Non-AP-Aligned SyncScan technique. With the facility of the continuously tracking of nearby APs, a client could make better handover decision thus improve the handover delay. Finally, our approach requires only the modification at the client side without any extra deployment at the network, and is completely compatible with IEEE 802.11 standards. We have implemented our design using the Prism 2 chipset WLAN card on Linux platform with the HostAP driver. Experimental results show that our approach is very effective and is suitable for the existing 802.11 wireless networks.