802.11無線網路架構下之二維網路拓樸知覺的頻道掃描機制設計與實作

Abstract

近年來IEEE 802.11無線區域網路與可攜式產品非常迅速的發展與普及，現今市面上的可攜式設備如筆記型電腦、電子書、PDA以及智慧型手機等，皆多內建802.11無線網路的傳輸介面。使用這些具有移動特性的行動設備於802.11無線區域網路漫遊時，經常會遇到無線基地台間換手(Handoff)的狀況。如何迅速地在基地台間執行換手動作，讓行動裝置使用者透過802.11無線網路使用網路服務時，不會感受到網路中斷或是服務延遲的情況，對於行動裝置的使用者是相當重要的需求。 於802.11網路架構中，行動裝置在基地台之間漫遊的換手過程，可以分為基地台的搜尋(discovery)與基地台的連結(commit)兩個部份。據相關研究顯示，基地台搜尋的部份佔據了整個換手過程的大部分時間，所以如果能夠大幅度的減少搜尋階段所花費的時間，就能夠大大降低基地台換手過程時間過長所造成的封包遺失和延遲等現象，達成快速換手的目標。目前已有許多的研究致力於降低基地台搜尋過程的時間，以縮短換手的延遲時間。其中的部分研究使用了Neighbor Graph (NG)的概念，透過紀錄相鄰基地台的數目與使用頻道，行動裝置在基地台搜尋階段就可以只針對鄰近的基地台頻道進行掃瞄即可，避免掃瞄全部頻道產生過長的延遲時間或在一個無基地台的頻道等待Channel Waiting Time到達所造成時間浪費。 使用NG概念設計的機制需要額外付出維護NG資訊的成本，而且NG的建立是藉由行動裝置在漫遊的歷程中動態紀錄並更新基地台的相鄰關係來產生。在一個區域性的無線網路環境佈建上，此種動態建立NG資訊的方式其實是不必要的額外負擔，因為區域的基地台部署其實是一種不會頻繁改變的靜態資訊，而且NG資訊只是紀錄了基地台之間的相鄰關係，並沒辦法提供出真實環境下基地台間地理位置的相對關係，故行動裝置在基地台搜尋階段仍然會有掃瞄不必要頻道的時間延遲。因上述NG概念的方法還是存在著許多有待改善的部份，因此本論文基於IEEE 802.11無線區域網路環境提出一種具備「二維網路拓樸知覺的頻道掃瞄機制」，其係行動裝置可利用區域中基地台部署位置的網路拓樸資訊，輔以量測基地台的訊號衰減或增強的變化，研判行動裝置的移動方向，預先找出合適的換手候選基地台，避免掃瞄全部拓樸鄰近基地台，並且透過實作來驗證此機制不但可以在二維平面WLAN的布建環境下應用以減少頻道掃描的數目，也能進一步使用「Candidate AP Aware」方式有效率地在換手前選擇合適的備選基地台，進一步降低搜尋階段所需時間，縮短換手的延遲時間。 本論文在Linux作業系統平台的Notebook上搭配使用Atheros晶片的無線網卡實作此機制。經由實際的測試與分析相關數據，結果顯示此機制確實可以有效率地預先選擇到合適的漫遊候選基地台、減少搜尋掃描的次數、改善搜尋階段的時間，減低漫遊時換手延遲所造成影響達到快速換手的目的。

With the development and popularizing of 802.11 wireless technology and mobile devices in recent years, many portable devices such as notebooks、E-Book、PDA or smart phones on the market now, all many are equipped with IEEE 802.11 wireless LAN interface inside. While using a mobile device roams in IEEE 802.11 WLAN networks, it often has to perform handoff procedure from one WLAN access point to another. How to make users of portable devices do not experience the situation that the network service interrupt or service delay when they use the streaming audio/video or VOIP real-time services, it’s an important demand for the users of portable device. The handoff procedure in 802.11 network consists of two phases: the one is searching AP (discovery phase) and the other is AP association (commit phase). According to the previous researches, discovery phase use the most of time in the handoff procedure. So if we can improve and reduce a lot of search time in discovery stage, the packets lose and delay issues will be resolved and reached the goal of making handoff faster. There are many various approaches at present that are devoted to reduce the time of the discovery phase. Some approaches among them have quoted Neighbor Graph concept. Adjacency relations among APs and what channel is used by each AP are passed by NG records, and then a portable device can only scan the particular channels and wait the adjacency AP’s responses only during the discovery phase, avoid the time waste of scan all channels and the channel waiting time in a channel that doesn’t have any adjacency APs. Using the method of NG concept need to spend the extra cost to maintain NG information, and the NG information is built by updating the relationship between the adjacency APs dynamically. In a wireless local area network environment, it is unnecessary to spend the cost to build the NG information dynamically, because it is actually a kind of static information that do not changed frequently in the wireless local area network and NG information only record the adjacency relations between APs, it can not present the relative relation of geographical position between APs, so the mobile devices in the NG approach still have the time delay to scan the unnecessary channel and make less effective. Above-mentioned NG approach have a lot of part to be improved, therefore, in this thesis, we propose a 「Two Dimension Topology-aware Scan Mechanism」 , that approached an mobile device can use the topology information of APs and the RSSI signal change to find out the moving orientation and next suitable candidate AP, avoid to scan all topology neighbors, not only can reduce the numbers of channel scanning but also select a suitable candidate access points more efficiently and intelligently by 「Candidate AP Aware method」, reduce handoff delay in the IEEE 802.11 WLAN plane environment. and shorten the handover delay in discovery phase. This thesis uses the wireless network card with Atheros chipset on a Linux operating system notebook to implement this mechanism. The result of experimental data via real testing case shows this proposed mechanism can really choose suitable candidate AP efficiently, reduce the numbers of probes and improve the delay time of discovery phase, achieve the goal of fast handoff.