This dissertation describes the design and development of systems to improve handoff performance in IEEE 802.11 wireless networks. Handoff is the process through which a client switches the associated access point. To achieve handoff faster means to provide not only higher throughput to best-effort applications, but also uninterrupted service to delay-sensitive applications on the move. The original IEEE 802.11 standard does not support it, because it was designed to provide wireless connectivity for fixed, portable, and moving clients mainly within a basic service set.

Link-layer handoff is composed of four sequential phases: detection, channel scanning, link-layer authentication and reassociation. The standard handoff incurs latency in the magnitude of hundreds of milliseconds to several seconds, and the channel scanning latency accounts for more than 90 % of that. The necessity of channel scanning phase comes from a client's inability to acquire information about nearby APs on different channels while communicating with the currently associated AP. Its large latency lies on the inefficiency that the standard mandates a client not only to scan through all the channels regardless of AP's existence on it, but also to wait for a fixed amount of time at each channel just in case more responses might arrive.

We propose two fast handoff methods to address these inability and inefficiency, respectively. As for handling inability, we aim to completely eliminate channel scanning phase by enabling clients to obtain information about the APs available in range while exchanging data frames with a currently associated AP. We further require that 1) nearby AP information should be promptly updated in a dynamically changing radio environment, and 2) the ease of deployment, which is the biggest strength of IEEE 802.11 wireless networks, should be kept. As for handling inefficiency, we aim to improve channel scanning phase by proposing a novel usage of the open system authentication phase, which became redundant with the advent of WPA. We further require that 1) no modification should be made to the current standard, 2) the handoff metrics (e.g. signal strength, traffic load, etc) should be promptly acquired during the handoff procedure, and 3) the fast handoff scheme should work with only software upgrade on the client side.

To achieve the first improvement, we have developed a fast handoff scheme, called OkScan,that integrates a shared beacon channel into IEEE 802.11 wireless networks for completely eliminating channel scanning latency. The basic idea behind OkScan is to offload management information to a second interface over a dedicated frequency band. We adopt the hardly used channel 14 as a dedicated extra channel to advertise the AP's existence,where clients can update information about geographically nearby APs by listening to it. Our proposed method enables clients to keep track of nearby APs operating on any PHY types and any channels. In this dissertation, we show the theoretical analysis on nearby AP information update rate in comparison to related work, and the effectiveness of our system through implementation and experiments.

To achieve the second improvement, we have developed a fast handoff scheme, called AuthScan, that provides a novel usage of the open system authentication phase for reducing channel scanning latency. The basic idea behind AuthScan is to exploit multiple Authentication Request / Response. AuthScan maintains a target AP list cached from a client's previous handoffs, and performs unicast scanning by transmitting not Probe Request frames, but Authentication Request frames only to the selected APs. The next AP is selected by comparing quickly acquired handoff metrics during the handoff procedure. In this dissertation, we show the theoretical handoff latency in comparison to related work, and the effectiveness of our system through implementation and experiments.

In addition, as a real case application, we extend AuthScan to increase the connected time to Wi-Fi HotSpots while moving around in Tokyo by subway. In Tokyo, approximately 97 % of subway stations are densely covered by three different service providers, but effectively utilizing network connection on the move requires a greater degree of management due to the different range of user mobility. We aim to mitigate Wi-Fi connection interruption issues arising in this environment. We investigate the commercial 802.11 HOTSPOT network deployed in Tokyo Metro to understand the target environment and, therefore, to clarify the obstacles to our goal and list up the environmental requirements. Two obstacles to our goal are: 1) a slow handoff process across overlapping coverage areas at stations, and 2) a slow connection re-establishment at the end of non-coverage area in the tunnels. Three environmental requirements are 1) client software upgrade only, derived from no right to modify the commercially deployed APs, 2) fresh handoff metrics acquisition, derived from train speed high mobility, and 3) excessive management traffic generation avoidance,derived from a power-drain concern for battery-operated mobile devices.

We have developed a system to provide a fast handoff, by an extension of AuthScan, and fast non-coverage area recognition. The basic idea behind fast non-coverage area recognition is to take advantage of the key attributes of the target environment: 1) centrally managed limited number of APs, 2) strong mobility pattern. While the behavior experienced when no preferred AP is found during scanning phase is implementation specific, our system performs passive scan only on the target channel which can be obtained from previously available nearby AP topology When a beacon frame arrives, a client executes AuthScan handoff process by comparing quickly acquired handoff metrics during the unicast channel scanning procedure. In this dissertation, we analyze the theoretical performance improvement of non-coverage area recognition, and show the effectiveness of our system through implementation and field experiments in a real subway environment.

In summary, this dissertation describes two fast handoff methods to address inability and inefficiency of the standard channel scanning phase in the IEEE 802.11 wireless networks. We also describe a real case application of AuthScan to increase the connected time to Wi-Fi HotSpots while moving around in Tokyo by subway. All the proposed methods are implemented and their performances are investigated through field experiments.

本論文は「Improving Handoff Performance in IEEE 802.11 Wireless Networks(IEEE 802.11無線ネットワークにおける高速ハンドオフ機構に関する研究)」 と題し,無線ネットワークにおいて移動端末が高速に通信を継続できる高速ハンドオフ機構の要素技術を提案し,実装に基づいて有効性を明らかにしている.

第1章は序論であり,IEEE802.11無線ネットワークの普及と802.11無線インターフェイスを塔載したモバイルデバイスの多様化,モバイル環境での多彩なアプリケーション実現への期待,ハンドオフを高速に実現することの難しさについて触れ,本論文の背景と目的について述べている.

第2章では,80211無線リンクを近隣基地局へ切り換えながら,シームレスにかつ高速な通信を継続的に可能とするハンドオフ機構について述べている.本章では,高速ハンドオフ実現において,近隣基地局探索が最も大きな課題となることから,近隣基地局探索の必要性と効率性について検討を行っている.また,次の基地局を選択する際に用いられるメトリックの重要性をも論じ,メトリックとして信号強度の有効性を明らかにしている.

第3章では,動的に変化する環境における高速ハンドオフ実現に向けて,共有ビーコンチャンネルの利用について述べている.CSMA/CAに基づく共有ビーコンチャンネルを設けることで,データ送受信の影響を受けずに高速に隣接基地局を発見できる.各基地局は共有ビーコンチャンネルで自身の存在を広告し,端末は共有ビーコンチャンネルのみを聞くことで近隣基地局探索を行う.これにより,データ送受信を停止して近隣基地局探索を行わずに,隣接基源局を高速に発見することができる.FreeBSD上に実装し,実ネットワーク環境において本手法の有効性を明らかにしている.

第4章では,複数の基地局に認証要求フレームをユニキャストすることで近隣基地局探索時間を短縮する機構について述べている.本方式では,ビーコン情報やハンドオフ履歴を端末にキャッシュし,近隣にあると推定された基地局に認証要求フレームを順次ユニキャストすることで利用可能な基地局を高速に判断する.また,認証応答フレームの信号強度等をスキャン時に取得することで,ハンドオフ時点で適切な基地局を選択する.本方式の特徴は,端末のソフトウェア更新のみで高速なハンドオフを実現できることにある.Linux上に実装し,実ネットワーク環境において本方式の有効性を明らかにしている.

第5章では,地下鉄駅などに設置されている商用802.11無線ネットワークにおいて,移動しながら高速通信を継続できる機構について述べている.無線エリアが連続である環境における高速リンク切替は,利用可能な基地局リストを事前に取得し認証要求フレームを順次ユニキャストすることで実現する.また,無線エリアが不連続な環境における高速リンク再確立は,移動パターンに基づいて推定されたチャンネルにパッシブスキャンすることにより実現する.Linux上に実装し,実ネットワーク環境において本手法の有効性を明らかにしている.

第6章は論文全体を総括しており,本論文の成果をまとめるとともに,IEEE802.11無線ネットワークにおける高速ハンドオフの実現に向けて残された課題と,今後の関連研究分野における研究開発の方向性について述べている。

以上,これを要するに本論文は,IEEE802.11無線ネットワークにおいて高速にハンドオフを行う機構を提案し,FreeBSDやLinux上での実装に基づく実験を介して有効性と実現性とを実証したものであり,情報学の基盤に貢献するところが少なくない.

よって本論文は博士(科学)の学位請求論文として合格と認められる.