802.11無線網路下具網路拓樸知覺的頻道掃描機制之設計與實作

Abstract

本論文針對IEEE 802.11 無線區域網路提出一個具網路拓樸知覺的頻道掃瞄機制， 這個機制不僅可以來縮短換手延遲時間，同時也可以更有效率地搜尋候選基地台。 近年由於行動運算技術的普及化，如今市面上許多的可攜式裝置如筆記型電腦、個 人數位助理或是智慧型手機等，皆已具備存取802.11 無線網路的能力。當使用這些具 移動性的行動裝置在802.11 網路漫遊時，將會遭遇到基地台換手的問題。尤其是使用 VoIP 等即時性應用服務，能夠快速且順暢地在基地台間換手是一項重要的需求。 802.11 網路基地台換手過可以分為基地台搜尋(discovery)與基地台連結(commit) 兩個階段。研究顯示，基地台搜尋佔據整個換手過程的絕大部分時間。如果能有效改善 基地台搜尋所需的時間，就可以降低因基地台換手造成的封包遺失和延遲等影響，進而 達到快速換手的目的。目前有相當多的研究根據此現象而致力於降低基地台搜尋的時 間。部分研究引進NG (neighbor graph)的概念，透過NG 紀錄的基地台相鄰關係與 使用頻道，行動端點在基地台搜尋階段只需要針對鄰近的基地台進行掃瞄即可，避免掃 瞄全部頻道或在一個頻道等待而造成過長的延遲時間。 然而NG 的方法需要的額外負擔太大也未善用基地台部署的「地理性資訊」，NG 的建立是藉由系統中MSs 漫遊所產生的基地台換手行為，動態紀錄並更新基地台的相 鄰關係。但是這種動態建立的方式其實是不必要的額外負擔，因為基地台的部署是一種 不會經常改變的靜態資訊。此外NG 只是紀錄基地台的相鄰關係，並未提供真實環境底 下鄰近基地台彼此位置的相對關係，因此行動裝置在基地台搜尋的過程仍會掃瞄不必要 的頻道。有鑒於此，我們提出「具網路拓樸知覺的頻道掃描機制」，讓行動裝置可以利 用基地台的網路拓樸資訊，在適當的時機量測特定鄰近基地台的訊號，觀察其衰減或增 強的變化，再根據鄰近基地台的網路拓樸資訊，判斷出使用者可能的移動方向，找出合 適的目標基地台，達到減少掃瞄所有鄰近基地台的目的，進而降低搜尋的時間。本論文 以校園大樓環境為例，在Linux 作業系統上搭配使用Atheros 無線網路卡實作此機制。 測試與分析的結果顯示，此機制的加入的確可以達到降低換手動作對即時性應用服務品 質造成的影響。

This thesis proposes a topology-aware scan approach that not only can reduce handoff latency but also scan candidate access points more intelligently in IEEE 802.11 WLAN. With the advance of wireless technology and mobile devices, nowadays many portable devices on the market such as notebooks, PDAs or smart phones, are already equipped with IEEE 802.11 wireless LAN (WLAN) adapters. However, when a device roams in IEEE 802.11 networks, it needs to perform handoff procedure and change its point of network attachment from one WLAN access point (AP) to another. The handoff procedure is very time consuming and may render real-time services infeasible in IEEE 802.11 networks. The handoff procedure consists of two phases: discovery and commit phases. Previous researches have already shown that the discovery phase contributes most of the handoff delay. Therefore, researchers have proposed various approaches to reduce the latency of the discovery phase. Among the previous proposals, Neighbor Graph (NG) is one of the most effective schemes. NG records the adjacency relations among APs and the channel used by each AP. With NG, a mobile station (MS) can determine which channels it need to scan and whether it has received all responses from the APs in a particular channel during the discovery phase. As a consequence, the number of channels probed decreases and so does the waiting time an MS staying in a channel. However, the dynamic maintenance cost of NG and the lack of the geographic information of APs in NG make the NG approach less effective in the next candidate AP discovery. Therefore, in this thesis, we propose a novel topology-aware scan mechanism where an MS can selectively scan only the APs the MS may possibly visit next according to the MS’s moving orientation and the topology information of APs. With the topology information, an MS not only can shorten the handover delay but also can reduce the impact of AP discovery phase on normal packet transmission. We use a campus building as the test environment and implement our mechanism on the Linux OS platform with WLAN cards of Atheros chipsets. Experimental result shows the proposed mechanism can achieve fast handoff without introducing extra packet jitter and delay time.